Long Bone Histology and Microanatomy of Placodontia (Diapsida: Sauropterygia)

Home , Cyamodontoidea, Henodus, Psephoderma

Contributions to Zoology, 84 (1) 59-84 (2015)

Long bone histology and microanatomy of Placodontia (Diapsida: Sauropterygia)

Nicole Klein1, 5, Alexandra Houssaye2, James M. Neenan3, 4, Torsten M. Scheyer3 1Staatliches Museum für Naturkunde Stuttgart, Rosenstein 1, 70191 Stuttgart 2UMR 7179 CNRS/Muséum National d’Histoire Naturelle, Département Ecologie et Gestion de la Biodiversité, 57 rue Cuvier CP-55, 75000 Paris, France 3Paläontologisches Institut und Museum, Universität Zürich, Karl Schmid-Strasse 4, CH-8006 Zurich, Switzerland 4The Function, Evolution and Anatomy Research (FEAR) Lab; School of Environmental and Rural Science; Univer- sity of New England, Armidale NSW 2351, Australia 5 Email: [email protected]

Key words: bone mass increase, bone tissue types, extinct marine reptiles, high growth rates, PCA, swimming capabilities, Triassic

Abstract Henodus, Placodontia indet. aff. Cyamodus, non-armoured Pla- codus, and Placodontia indet. Bone tissue of Psephoderma and Placodontia, an enigmatic group of durophagous and in part Paraplacodus indicates low growth rates and a low basal meta- heavily armoured animals, were members of Sauropterygia, the bolic rate, as many modern sauropsids have such as the marine most diverse and successful group of Mesozoic marine reptiles. iguana, whereas the others grew with extremely fast growth Microanatomy and histology of long bones of several armoured rates, more typical for birds and mammals, indicating an in- and non-armoured Placodontia were studied, covering most of creased basal metabolic rate. their taxonomic breadth, to elucidate the paleoecology, physiology, and lifestyle of its members. Results reveal an unexpected and not phylogenetically or stratigraphically related disparity of microanatomical and histological features for the group. The Contents non-armoured Paraplacodus and the heavily armoured Psepho- derma grew with lamellar-zonal bone tissue type, which is typi- Introduction ...... 59 cal for modern sauropsids. In the former, the tissue is nearly Previous works on long bone histology and avascular surrounding a compacted medullary region, whereas microanatomy of placodonts ...... 63 in the latter, the lamellar-zonal bone tissue is vascularized fram- Material and methods ...... 67 ing a large open medullary cavity and a perimedullary region. Abbreviations ...... 69 Armoured Henodus and Placodontia indet. aff. Cyamodus as Results ...... 69 well as non-armoured Placodus exhibit a reduced medullary Microanatomical description ...... 69 cavity and grew with highly vascularized plexiform to radiating Histological description ...... 72 fibro-lamellar bone. Several long bones of Placodontia indet. Results of the Principal Component Analysis ...... 75 show circumferential fibro-lamellar bone and can be distin- Discussion ...... 75 guished into two groups on the basis of microanatomical fea- Microanatomical tendencies ...... 75 tures. In addition, all bones that grew with fibro-lamellar bone Ecological inferences and possible swimming styles ...... 76 show locally primary spongeous-like architecture and had sec- Histological characteristics ...... 76 ondarily widened primary osteons throughout the cortex, result- Different bone tissue types and growth strategies ...... 79 ing in a secondarily spongeous tissue. The highly vascularized Concluding remarks ...... 80 fibro-lamellar bone of these Placodontia indicates growth rates Acknowledgements ...... 80 comparable to that of open marine ichthyosaurs. Differences in References ...... 80 microanatomy and bone histology as expressed by a principal component analysis, thus clearly indicate different paleoecolo- gies, including differences in lifestyle and swimming modes and capabilities in Placodontia. This would have reduced competi- Introduction tion in the shallow marine environments of the Tethys and might be a key to their success and diversity. A certain developmental Placodontia were part of the Sauropterygia, a diverse plasticity among the studied placodonts is interpreted as re- group of diapsid marine reptiles that ranged from the sponse to different environmental conditions as is obvious from inter- and intraspecific histological variation. Most striking is late Lower Triassic until the end of the Cretaceous the difference in life history strategy in armoured Psephoderma (Rieppel, 2000a; Motani, 2009). The Triassic radiation and non-armoured Paraplacodus when compared to armoured included predominately shallow marine forms such as

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Fig. 1. Phylogenetic relationships within Diapsida, including Sauropterygia and Placodontia. The asterisks mark the his- tologically studied members of Placodon- tia. Cladogram modified from Neenan et al. (2013) and Klein and Scheyer (2014).

Placodontia, Pachypleurosauria, Nothosauria, and Pis- (Hagdorn and Rieppel, 1999; Neenan et al., 2013). tosauria; conversely, the Jurassic and Cretaceous seas Beyond the Germanic Basin they occurred along the were ruled by the open marine Plesiosauria. The latter epicontinental coasts of the western Tethys (Rieppel, had a global distribution, whereas most Triassic repre- 2000a: circummediterranean Muschelkalk localities sentatives were largely restricted to epicontinental seas in Spain, Tunisia, Israel, and Saudi Arabia) and can of the Tethys Ocean. Triassic Sauropterygia are notable also be found in the Anisian, Ladinian, and Carnian of for having a large variety of ecologies, life histories and China (Li, 2000; Li and Rieppel, 2002; Jiang et al., feeding strategies, which enabled them to live contem- 2008; Zhao et al., 2008). Placodontia vanished from the poraneously in the same habitats. fossil record of the Germanic Basin in the Lower Car- Placodonts were the sister group of Eosauropterygia nian, but survived in the Alpine Triassic realm until (Pachypleurosauria, Nothosauria, Pistosauria; Rieppel, the end of the Late Triassic (Furrer et al., 1992; Renesto 1994) (Rieppel, 2000a; Neenan et al., 2013) (Fig. 1), et al., 1995; Hagdorn and Rieppel, 1999). and they first appeared in the fossil record in the early The phylogenetic relationships within Placodontia Anisian (Lower Muschelkalk) of the Germanic Basin have repeatedly been tested in recent years, and is

Downloaded from Brill.com10/04/2021 05:33:52AM via free access Contributions to Zoology, 84 (1) – 2015 61 currently considered to be divided into a non-armoured habitant of coastal stretches and shallow seas’. An am- placodontoid grade and a heavily armoured monophyl- phibian lifestyle for Placodus was also hypothesized etic Cyamodontoidea (Rieppel, 2000a, b, 2001; Jiang et based on the analysis of humerus microstructure by al., 2008; Neenan et al., 2013). The placodontoid grade Canoville and Laurin (2010; please note that the sample contains at least three taxa (Rieppel, 2000a; Jiang et they used is not undoubtedly assignable to a certain pla- al., 2008; Klein and Scheyer, 2014). Cyamodontoidea codont taxon). However, the streamlined body, stiff were more diverse, with at least nine genera (e.g. Riep- vertebral column as shown by hyposphen and hy pel, 2000a, b, 2001; Zhao et al., 2008). pantrum processes, as well as the long and laterally Placodonts shared some unique morphological fea- flattened tail of Placodus clearly indicates certain tures such as a massive, akinetic skull, and a reduced swimming capabilities (Vogt, 1983; Westphal, 1988; but specialized dentition, enabling a durophagous diet Rieppel, 1995). Additionally, Neenan and Scheyer (summarized in Scheyer et al., 2012; Neenan et al., (2012) concluded that the alert head position of Placo- 2014). According to Hagdorn and Rieppel (1999), all dus was ideally suited for feeding in an aquatic envi- placodonts with the exception of Henodus Huene, 1936 ronment. Vogt (1983) and Westphal (1988) considered are exclusively found in marine sediments, and their Placodus to be an axial swimmer whereas Braun and occurrence was strongly related to the availability of Reif (1985) identified Placodus as a subcarangiform shelly prey items. Most Cyamodontoidea showed nu- swimmer. Despite divergences concerning its mode of merous armour plates (osteoderms) fused into one or locomotion, there is a consensus that Placodus was in two dorsal shields, and in some cases, an additional any case a slow and not very active swimmer. Based on ventral armour shield (Rieppel, 2002; Scheyer, 2007, morphology, Westphal (1988) concluded long diving 2010). Conversely, the placodontoid Paraplacodus Pe- capabilities for Placodus due to movable ribs and a yer, 1931 lacked any kind of armour and Placodus large ribcage volume. This was later confirmed by other Agassiz, 1833 showed only a single row of osteoderms authors who studied bone histology and microanatomy along its vertebral column (Rieppel, 1995, 2000a, b; Ji- in a placodont humerus; documenting that bone mass ang et al., 2008). While the postcranial anatomy of increase would have counteracted an increased oxygen Paraplacodus and Placodus is reasonably well known store (e.g. Buffrénil and Mazin, 1992; Taylor, 1994; (e.g. Drevermann, 1933; Rieppel, 1995, 2000a, b, 2002; Houssaye, 2009). Jiang et al., 2008), the postcranial morphology of many The heavily armoured cyamodontoids have a dorso- cyamodontoids is only partly understood due to ob- ventrally flattened and laterally extended body, remi- scuring dermal armour, and the fact that they are com- niscent of the shape and morphology of flat aquatic monly embedded in large slabs of rock matrix. turtles, such as snapping turtles (Chelydridae) or the Placodus is the best-known placodontoid so far. Due mata mata (Chelus fimbriatus). Another hypothesis to its cylindrical, sea cow-like body shape, Placodus states that they buried themselves in sand or gravel on has always been interpreted as a slow bottom walker the seafloor, like rays (Mazin and Pinna, 1993; Renesto that needed a heavy skeleton to act as ballast, allowing and Tintori, 1995) or soft shelled turtles (Ernst and it to walk/stay on the seafloor when feeding (e.g. Vogt, Barbour, 1989). The cyamodontoid armour and the tur- 1983; Westphal, 1988; Rieppel, 1995; Scheyer et al., tle shell evolved convergently and are only superficially 2012). Despite its occurrence in solely marine sedi- similar (Scheyer, 2010), because the armour of placo- ments (Hagdorn and Rieppel, 1999) some authors con- donts is not connected to the underlying postcranial sider the lifestyle of Placodus as not being fully marine skeleton. In Cyamodus hildegardis Peyer, 1931 for ex- (aquatic) but as amphibious, similar to that of the extant ample, the armour can be divided into a main carapace, marine iguana Amblyrhynchus cristatus Bell, 1825 a pelvic shield, and tail armour (Scheyer, 2010). from the Galapagos Islands (Westphal, 1988) that only The study of bone tissues of extinct vertebrates re- enters the sea for feeding. The main reason for this is veals insights into growth rates and thus into their me- the lack of gross morphological specialization for an tabolism and physiology (e.g. Bakker, 1980; Ricqlès, aquatic life in the Placodus skeleton (Carroll, 1988; 1983, 1992; Padian and Horner, 2004; Padian et al., Westphal, 1988). Placodus limbs are not specially 2004; Montes et al., 2007). From the preserved growth adapted for swimming and it is assumed that Placodus record, growth patterns and life history strategies can was still able to walk on land (Vogt, 1983; Carroll, be deduced (e.g. Castanet et al., 1993; Buffrénil and 1988). Rieppel (1995: 38) stated that ‘the skeletal struc- Castanet, 2000; Sander and Klein, 2005; Sanchez et al., ture of Placodus clearly indicates a durophagous in- 2010; Hugi and Sánchez-Villagra, 2011; Köhler et al.,

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Fig. 2. Photographs of cross sections of sampled bones. A, Cross section of humerus and B, Femur of Paraplacodus broilii (PIMUZ T5845). C, Cross section of humerus (PIMUZ A/III 1476) and D, Femur (PIMUZ A/III 0735) of Psephoderma alpinum. E, Photograph of a breakage at midshaft of Henodus (GPIT/RE/7289). F-M, Cross sections of Placodontia indet. aff. Cyamodus sp. humeri. F, SMNS 59831. G, MHI 2112-6. H SMNS 15891. I, SMNS 54569. J, SMNS 15937. K, SMNS 54582. L, MHI 697. M, MHI 1096. N, Cross section of a humerus of the marine reptile Horaffia kugleri (MHI 2112-4). O, Micro-CT image of the cross section of a humerus (SMNS 59827) assigned to Placodus gigas (Vogt, 1983; Rieppel, 1995). P-X Cross sections of humeri and femora of Placodontia indet. P, humerus MB.R. 454. Q, humerus IGWH 9. R, femur SMNS 84545. S, femur IGWH 23. T, Micro-CT image of the cross section of placodont femur MB.R. 965. U, femur MB.R. 961. V, femur MB.R. 814.2. W, femur MB.R. 812. X, femur SMNS 54578. For locality information and stratigraphic age see Table 1.

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Fig. 3. Microanatomical clusters obtained by Principal Component Analysis (PCA). Graphs showing the distribution of the variance in the humerus and femur sections of modern and extinct aquatic taxa for which bone profiler values were obtained (see S3). The comparative taxa are included to observe, in addition to the placodont diversity, the distribution of Placondontia within aquatic taxa. A, based on humeri. B, based on femora. Abbreviations for the taxa in the PCA graphs as in (S3).

2012; Chinsamy Turan, 2012; Griebeler et al., 2013). marine environments) and of the Alpine Triassic (shal- The two main trends in life history strategies in extant low to open sea environments), can histology be linked vertebrates can be summarized—though largely sim- to habitat preferences? Can histological characteristics plified—as lamellar-zonal bone tissue vs. fibro-lamellar of isolated long bones be used for taxonomic assign- bone tissue. Both tissue types seem to be linked to a ment to a group? If so, this would increase the morpho- certain metabolism (poikilotherms vs. homoeotherms; logical knowledge of placodonts as was done before for see e.g. Houssaye, 2012). Bone microstructure (i.e., bone other Sauropterygia (e.g. Klein, 2010, 2012; Sander et compactness, occurrence and size of the medullary al., 2013). cavity, presence of perimedullary region, spongeous- or trabecular-like organization, vascularity, etc.) has Previous works on long bone histology and micro- shown to reflect lifestyle (e.g. Canoville and Laurin, anatomy of placodonts 2010; Houssaye et al., 2010, 2014, Dumont et al., 2013; Quemeneur et al., 2013) and can indicate ecological Placodont long bone tissue has been previously studied preferences (e.g. Buffrénil et al., 1987, 1990; Laurin et by Buffrénil and Mazin (1992). The humerus sampled al., 2004, 2007, 2011; Germain and Laurin, 2005; and described therein was assigned to Placodus, al- Hayashi et al., 2013; Houssaye et al., 2013). though this assignment cannot be verified. The humerus It is the aim of the current paper to describe and that was figured by Drevermann (1933) in his descrip- compare long bone histology and microanatomy of ar- tion of Placodus gigas was later identified to belong to moured and non-armoured placodont taxa and to ad- a nothosaur (Rieppel, 1995). The skeleton of the com- dress and compare their lifestyles in a phylogenetic plete P. inexpectatus is still embedded in sediment, framework. The following questions will be addressed: which makes it difficult to address and compare all since our placodont sample spans most of the strati- morphological features of both humeri of that indi- graphical range of the group, can evolutionary stasis or vidual (Jiang et al., 2008; NK, pers. obs.). Thus, an un- changes/trends be identified? As the samples originate equivocal taxonomical assignment to a certain placo- from various localities of the Germanic Basin (shallow dont taxon of the humerus thin sectioned by Buffrénil

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Downloaded from Brill.com10/04/2021 05:33:52AM via free access Contributions to Zoology, 84 (1) – 2015 67 and Mazin (1992) is not possible. Buffrénil and Mazin and Sander, 2007) and yield the strongest ecological (1992) characterized the bone tissue as highly vascular- signal (Canoville and Laurin, 2010). A morphological ized fibro-lamellar bone with a mainly plexiform and description for each bone is given in the supplementary reticular vascularization that is in some parts essen- information (S1). The material includes armoured and tially radial. According to Buffrénil and Mazin (1992) non-armoured Placodontia and spans most of the strati- inner resorption is very limited. Ricqlès and Buffrénil graphical range of the group from the early Anisian (2001: 292) also referred to the placodont sample of (Middle Triassic) to the Rhaetian (Late Triassic) (Table Buffrénil and Mazin (1992) and characterized the bone 1). Paleogeographically, the sample focuses on the west- as having a ‘pachyostotic-like condition [resulting] from ern Tethys i.e., on localities from the Germanic Basin an absence, or a very poor development of the marrow and the Alpine Triassic. For further information on lo- cavity, instead of a true hyperplasy of periosteal corti- calities, as well as the paleogeographical and strati- ces’. Among a sample of various Triassic Sauropterygia graphical ranges of placodont taxa, please see Rieppel Klein (2010) studied two bones assigned to Placodontia and Hagdorn (1997), and Hagdorn and Rieppel (1999). indet. that are also included into the current study A complete list of the bones sampled in this study, with (IGWH 9, IGWH 23; Table 1) and found that these two associated measurements, is given in Table 1. bones have the highest growth rates within Sauroptery- The humeri and femora were photographed, mea gia. In their quantitative study of humeral microanato- sured, and described using traditional paleontological my and lifestyle in amniotes Canoville and Laurin methods (Table 1; S1-2). When possible, bones were (2010) included the placodont sample of Buffrénil and sampled exactly at midshaft by cutting an entire cross Mazin (1992) and postulated that this specimen had an section. The importance of exact midshaft samples for amphibious lifestyle. However, they admitted that their comparability was pointed out before (e.g. Konietzko- inference required to be interpreted with caution, be- Meier and Klein, 2013). However, in some specimens, cause of an ‘apparent contradiction in the various sig- sampling location is slightly proximal or distal to the nals’ (Canoville and Laurin, 2010: 401). Houssaye midshaft on account of bone preservation (see Table 2). (2012), in a review on bone histology of aquatic reptiles The thin sections were produced following standard with inferences on secondary adaptations to an aquatic petrographic methods (Klein and Sander, 2007). Thin life, concluded that the sampled placodont bone from sections were then studied and photographed with a the study of Buffrénil and Mazin (1992), together with Leica® DMLP compound polarizing microscope, some ichthyosaurs and plesiosaurs, showed the highest equipped with a digital camera (Leica® DFC 420C) growth rates within aquatic reptiles. and a Leica® DM 750P compound polarizing micro- Klein and Hagdorn (2014) described a new marine scope equipped with a digital Leica® ICC50HD cam- reptile taxon, Horaffia kugleri, from the Ladinian of era. Cross-sections were scanned with an Epson V740 southwest Germany. Its bone histology is described and PRO high-resolution scanner (Fig. 2). The bone histo- compared to that of cf. Cyamodus (herein assigned to logical terminology follows Francillon-Vieillot et al. Placodontia indet. aff. Cyamodus) humeri from the (1990). The size of the medullary cavity/region was same horizon. Humeri of Horaffia kugleri are pachyos- measured along the pre-postaxial width of the bones totic whereas those of cf. Cyamodus are not. Humerus (Table 2). histology is essentially similar in both taxa and can Some samples were micro-CT scanned with a be summarized as highly vascularized, plexiform to v|tome|xs by GE phoenix|x-ray at the Steinmann Insti- radiating fibro-lamellar bone tissue with a poorly de- tute of Geology, Mineralogy and Paleontology (StIPB) veloped medullary cavity or region, resulting in osteo- (Table 2). Image visualization was performed using sclerosis. The samples of Horaffia kugleri and cf. Cya- VGStudio MAX 2.0 software (Volume Graphics modus described by Klein and Hagdorn (2014) are in- GmbH) and Adobe Photoshop. CT slices were used to cluded in the PCA of the current study. trace vascularity and bone tissue type. However, con- trast between bone and the infilling sediment, and resolution were not always sufficient to precisely determine Material and methods these features. Scanned pictures of thin sections were transformed For the current study humeri and femora of placodonts into black (vascular spaces) and white (bone) pictures were sampled (Fig. 2). These bones usually have most of (S4) that were analyzed with the program Bone Profiler the primary periosteal bone tissue preserved (e.g. Klein (Girondot and Laurin, 2003) (Table 2; Fig. 3; S3). We

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examined five parameters provided by this software to Results characterize the bone density distribution: C, P, S, Min and Max. C is the global bone compactness for the Microanatomical description whole sectional area. P, is the relative distance from the center of the section to the point where the most abrupt The humerus and femur of Psephoderma alpinum both change in compactness is observed, which is propor- have a central free medullary cavity that is surrounded tional to the size of the medullary cavity. S, is the recip- by scattered large erosion cavities (in a perimedullary rocal of the slope at the inflection point, generally re- region), which reach far into the cortex (Table 1; Figs flects the width of the transition zone between the corti- 2C, D; 3C, D). The erosion cavities are mainly roundish cal bone and the perimedullary region. Min and Max to oval shaped. In the humerus, they are randomly or- are, respectively, the minimum and maximum asymp- ganized whereas those in the femur are arranged in cir- totes and thus represent the minimum and maximum cumferential rows. Vascular density is low (Table 2, 3). bone compactness values in a section. A Principal The photograph of the midshaft cross section of the Component Analysis (PCA) was performed on the Henodus humerus indicates a relatively large medullary Bone Profiler parameters cited above using the statistic region consisting of large cavities and randomly organ- software R (R Development Core Team, 2008) in order ized trabecles (Table 1; Fig. 2E) surrounding a rather to explore the distribution of the different taxa in mor- small medullary cavity. The vascular system is domi- phospace while optimizing the variance. Various ex- nated by a radial organization and contains longitudinal tant and fossil aquatic amniotes (S3) were added to our and radial canals as far as observable. In general, micro- sample in the PCA. structure of Henodus seems to be comparable to that of Placodontia indet. aff. Cyamodus humeri (see below). Abbreviations Midshaft samples of humeri of Placodontia indet. aff. Cyamodus usually show a small free medullary IGWH Institute of Geosciences of the Martin- cavity (Figs 2F-M; 4E-L) that is lined by a thin layer Luther-University Halle-Wittenberg, of endosteal bone, those that were not taken exactly Germany at midshaft have a medullary region consisting of MfN (MB.R.) Museum of Natural History, Leibniz In- small cavities, endosteal bone deposits, and scattered stitute for Research on Evolution and pockets of calcified cartilage (Figs 4F, J). The med- Biodiversity at the Humboldt University ullary region is then surrounded by a sharp line, Berlin, Germany which consists of a very thin layer of endosteal bone. MHI M uschelkalkmuseum Ingelfingen, Ger- Vascular density in the humeri is poor in the inner- many most cortex but always high in the middle and outer PIMUZ Paleontological Institute and Museum cortex (Table 2; Figs 2F-M; 4E-L). Contrary to the of the University of Zurich, Switzerland condition in other vertebrates vascular density in- SMNS Stuttgart State Museum of Natural His- creases towards the outer cortex in Placodontia in- tory, Germany det. aff. Cyamodus humeri. Long and thick, mainly radial vascular canals forming a primary trabecle- like architecture dominate locally the inner to middle cortex of the pre- and postaxial bone sides. The entire cortex of humeri MHI 1096, SMNS 15937, and SMNS 59831 is spongeous-like in organization: al-

Fig. 4. Details of medulla and inner cortex of Placodontia long bones. A, Humerus in polarized light and B, Femur in normal light of Paraplacodus broilii (PIMUZ T5845). The medullary region consists of endosteal deposits with pockets of calcified cartilage in between. In the femur large erosion cavities occur in the centre of the section. C, Humerus (PIMUZ A/III 1476) and D, Femur (PIMUZ A/III 0735) of Psephoderma alpinum both in polarized light. Note that the medullary cavities are surrounded by a distinct perimedullary region. E-L, Placodontia indet. aff. Cyamodus humeri all in polarized light. Except for SMNS 54569 (Fig. 3), which represents a more proximal sample and MHI 697 all midshaft samples exhibit a small free cavity. SMNS 54569 (Fig. 3) and MHI 697 (Fig. 3F) show a medullary region infilled by secondary trabecels, sediment filled cavities, and pockets of calcified cartilage. E, MHI 1096. F, MHI 697. G, SMNS 59831. H, SMNS 15891. I, SMNS 54582. J, SMNS 54569. K, MHI 2112/6. L, SMNS 15937. M-O, Humeri of the marine reptile Horaffia kugleri. Horaffia kugleri has a small free cavity surrounded by a medullary region at midshaft. M, MHI 2112/4. N, SMNS 84816. O, MHI 2112-1.

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Fig. 5. Details of medulla and inner cortex of Placodontia indet. humeri and femora. All bones have a large medulla consisting of secondary trabecles and large cavities. A, Medulla of humerus MB.R. 454 in normal light. B, Medulla of humerus IGWH 9 in polarized light. C, Medulla of femur SMNS 84545 in normal light. D, Medulla of femur IGWH 23 in polarized light. though the surface occupied by bony tissues repre- pers. obsv.). A medullary cavity in the center of the sents much more than 50% of the cortex surface, the cross section cannot be identified and the organiza- high amount of primary osteons with large vascular tion seems to be rather loose, most likely indicating a spaces confers to the cortex a spongeous aspect (Ta- medullary region (Fig. 2O). The vascular system re- ble 2, 3; Figs 2, 7). In the other samples, the inner sembles that of humeri of Placodontia indet. aff. Cya- cortex can be locally spongeous-like and/or trabecu- modus. The outer cortex clearly shows large vascular lar-like. All samples share a spongeous-like organi- canals (Fig. 2O). zation of the outer cortex (Fig. 7). The spongeous- The humerus and femur of Paraplacodus broilii like organization is the result of locally primary both have a medullary region that is densely packed spongeous and trabecular bone and secondarily wid- with endosteal deposits and pockets of calcified car- ened primary osteons. tilage (Figs 2A, B; 4A, B). The bone tissue is nearly The midshaft of a humerus (SMNS 59827), which avascular (Table 2; Figs 6C, D). is thought to represent aff. Placodus (Vogt 1983; The medullary region of humerus MB.R. 454 (Pla- Rieppel 1995) was micro-CT-scanned. The resolu- codontia indet.) is very large (Figs 2P, 5A). The area tion of the resulting pictures is poor, which is how- consists of thin secondary trabecles surrounding in- ever often the case for Muschelkalk samples (NK, tertrabecular spaces. Humerus IGWH 9 (Placodontia

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Fig. 6. Histological details of armoured Psephoderma and non-armoured Para- placodus. A, Detail of lamellar-zonal bone tissue of Psephoderma humerus (PIMUZ A/III 1476) and B, femur (PIMUZ A/III) in normal light. Note the large erosion cavities of the perimedullary region in both bones. C, Detail of lamellar-zonal bone tissue of Paraplaco- dus humerus (PIMUZ T5845) in normal light and D, in polarized light. E, Detail of lamellar-zonal bone tissue of Parapla- codus femur (PIMUZ T5845), normal light and F, in polarized light.

indet.) also shows a large medullary region and a have an off-centered free medullary cavity, which trabecle-like architecture at the postaxial bone side corresponds to a medullary region in the more distal (Figs 2Q, 5B). The medullary region is filled by en- section SMNS 54578 (Fig. 2W). The medullary re- dosteal deposits. Vascular density is moderate gion in non-midshaft sample SMNS 54578 consists (MB.R. 454) or high (IGWH 9) (Table 2). Bone com- of secondary trabecles. Vascular density in all speci- pactness increases towards the outer cortex in both mens is high (Table 2). The vascular canals have un- samples. dergone resorption resulting in the formation of large Femora IGWH 23 and SMNS 84545 (both Placo- cavities. All samples show locally (mainly at the dor- dontia indet.) both have a large medullary region that sopostaxial bone sides) a radial or irregularly formed consists of several generations of secondary trabecles trabecle-like architecture and locally spongeous-like (Fig. 5C, D). Please note that the cortex is incomplete tissue. Vascular density decreases towards the outer in IGWH 23 at the dorsal and preaxial bone sides due cortex. One placodont femur (MB.R. 965) was mi- to preservation (Fig. 25). Ventrally, large irregularly cro-CT-scanned. The quality of the resulting pictures shaped cavities are aligned in circumferential rows. documents a similar microstructure as was described The cortex is in both scattered with randomly shaped for the femora above: most of the cross section shows and oriented large cavities, resulting in a spongeous- a large medullary region followed by spongeous-like like tissue (Fig. 2R, S). tissue in the inner cortex (Fig. 2T). Locally a trabe- Midshaft sections of placodont femora MB.R. cle-like architecture is visible. Only the outer cortex 814.2, MB.R. 961, and MB.R. 812 (Figs 2U, V, X) region is compact.

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Fig. 7. Histological details of humeri of Placodontia indet. aff. Cyamodus. A, De- tail of the postaxial bone side of humerus MHI 1096 in normal light and B, in polarized light. The inner cortex shows a spongeous tissue and a radial trabecle- like structure. In the middle cortex vascular canal organization changes to longitudinal and mainly irregular vascular spaces. The spongeous tissue continues until the outermost cortex. Note the fibro- lamellar bone tissue between the radial trabecular spaces. C, Detail of spongeous inner cortex with radial, longitudinal, and irregularly formed vascular canals of humerus SMNS 54582 in normal light and D, in polarized light. E, Detail of outer cortex with secondarily widened longitudinal primary osteons of humerus SMNS 54569 in normal light and F, in polarized light. G, Detail of humerus SMNS 15937 in normal light and H, in polarized light. The middle to outer cortex shows a trabecle-like structure with radial, circumferential, reticular, and irregular vascular canals. Vascular canals are connected by anastomosis. The entire cortex contains secondarily widened vascular canals.

Histological description few longitudinal vascular canals in their outer cortex that are thinly lined by lamellar bone, and so corre- The bone tissue of the humerus and femur of Psepho- spond to primary osteons (Fig. 6A). In the humerus derma alpinum is altered by diagenesis in both samples they occur at the postaxial side and in the femur at the and its nature is obscured under cross-polarized light. ventral side (Fig. 6B). The cortex is divided by growth Scattered erosion cavities, which are thinly lined by la- marks (Klein et al., unpubl. data). The bone tissue type mellar bone, indicate remodelling. Primary bone tissue can be summarized as lamellar-zonal bone. seems to consist solely of lamellar bone and did not The cortex of Henodus shows mainly longitudinal show any osteocyte lacunae. Both bones show only a and radial vascular canals that are arranged in a plexi-

Downloaded from Brill.com10/04/2021 05:33:52AM via free access Contributions to Zoology, 84 (1) – 2015 73 form to radiating system (Table 2), resembling that of higher organized and less vacularized bone tissue) and Placodontia indet. aff. Cyamodus humeri. Details of SMNS 59831, which is the largest sampled bone and bone tissue cannot be established due to the poor reso- the only one that shows lamellar bone in the outermost lution of the micro-CT-scan. cortex. The bone tissue type of Placodontia indet. aff. Cya- Details of bone tissue of the humerus assigned to modus can be summarized as radiating to plexiform Placodontia indet. aff. Placodus (SMNS 59827) cannot fibro-lamellar bone. Woven bone is typically comprised be addressed due to the poor resolution of the micro by lamellar bone. However, the core of woven bone can CT-scan. Growth marks are visible in the outer cortex. correspond to parallel-fibred bone based on the extinc- The primary bone tissue of the humerus and femur tion pattern under polarized light that is made of coarse assigned to Paraplacodus broilii is in the inner cortex and thick fibers, which overlay each other. However, the highly organized parallel-fibred bone that, towards the large, round, and numerous osteocyte lacunae resemble outer cortex, grades into lamellar bone (Figs 6C, D). the chaotic arrangement seen in typical woven bone. In Simple longitudinal and radial vascular canals domi- the outer cortex of SMNS 59831 osteocyte lacunae are nate the tissue but immature primary osteons also oc- almost flat, and thus more in accordance with typical cur. The entire bone tissue is interspersed by regularly parallel-fibred bone. All kinds of vascular canals are sized, small but round osteocyte lacunae. The cortex is present: radial, longitudinal, reticular, and circumfer- divided by growth marks (Klein et al., unpubl. data). ential ones. Vascular canals often show a tendency to The bone tissue type can be summarized as lamellar- anastomose. Radial vascular canals in the outer cortex zonal bone. are often connected to the bone surface. Longitudinal The bone tissue of humeri MB.R. 454 and IGWH 9 vascular canals tend to be lined up in rows whereas (both Placodontia indet.) consists of coarse parallel- radial canals can cross growth marks. Onset of remod- fibred bone, intermixed with woven bone. Primary elling processes is indicated by local cross cutting and osteons are surrounded by lamellar bone, resulting in cementing lines are sometimes visible (Fig. 7). In two fibro-lamellar bone tissue (Figs 8A, B). Numerous thick samples (SMNS 54582, MHI 2112-6) infilled (mature) osteocycte lacunae are accumulated in the areas of wo- secondary osteons (sensu Currey, 2002) can occur in ven and coarse parallel-fibred bone. Both samples the innermost cortex. In the inner cortex, vascular ca- share an increase in organization towards the outer- nals are often not or only incompletely lined by lamel- most cortex. Vascular canals are radial, longitudinal, lar bone representing neither simple vascular canals and reticular. They are arranged in a mainly plexiform nor fully developed primary osteons (Fig. 7, S5A-C), system and canals are locally lined up in rows. The in- which we identify as immature primary osteons (Klein, ner cortex is dominated by immature primary osteons, 2010). The number of fully lined primary osteons in- the outer cortex by fully developed primary osteons. creases generally towards the outer cortex. The prima- The cortex of MB.R. 454 and IGWH 9 is divided by ry osteons are throughout the entire cortex secondarily growth marks (Klein et al., unpubl. data). widened by successive resorption processes. A thick In femora IGWH 23 and SMNS 84545 (both Placo- layer of lamellar bone surrounds the widened primary dontia indet.) the bone tissue consists of fibro-lamellar osteons then, which are however, far away from being bone with coarse parallel-fibred and woven bone (Figs infilled, resulting in incomplete or immature secondary 8C, D). The organization of the bone tissue increases osteons (S5, 6). The development of these osteons can towards the outer cortex. In SMNS 84545, tissue or- impede the observation of the nature of the collagenous ganization is generally higher and the outer cortex is wave of the surrounding bone (Figs 7E-H). Bone tissue solely made of avascular lamellar bone, except for a can locally be obscured by diagenesis (i.e. bacteria and/ few, mainly radial canals that open into the surface. or fungal growth) or by a dark permineralization of the Osteocyte lacunae are small but numerous and accu- bone. Placodontia indet. aff. Cyamodus humeri show mulated in the areas of woven and coarse parallel-fi- an alternation of broad, highly vascularized layers of bred bone. In both, the vascular canals are mainly ir- fibro-lamellar bone (zones) and thin, avascular layers regularly/reticularly formed. Some represent imma- made of highly organized parallel-fibred bone or la- ture, others mature primary osteons. In SMNS 84545 mellar bone (annuli) (Klein et al., unpubl. data). No vascular canals are largely eroded into irregularly clear increase in bone tissue organization from the in- formed cavities. The cortex is in both samples divided ner to the outer cortex is visible except in the medium by growth marks. Infilled (mature) secondary osteons sized humerus SMNS 54582 (that has in general a are rare but can occur in the inner cortex.

Downloaded from Brill.com10/04/2021 05:33:52AM via free access 74 Klein et al. – Histology and microanatomy of Placodontia

Fig. 8. Histological details of Placodontia indet. humeri and femora. All samples are in normal light. A, Detail of cortex of humerus MB.R. 454. B, Detail of cortex of humerus IGWH 9. C, Detail of cortex of femur SMNS 84545. D, Detail of cortex of femur IGWH 23. The cortex is spread with irregularly formed large vascular canals. E, Detail of cortex of femur SMNS 54578. F, Detail of cortex of femur MB.R. 812. G, Detail of cortex of femur MB.R. 961. H, Detail of cortex of femur MB.R. 814.2. All samples have secondarily widened primary osteons.

The bone tissue in the Placodont indet. femora vascular canals occur (longitudinal, radial, circum- (MB. R. 814.2, MB.R. 961, MB.R. 812, SMNS 54578) ferential, and reticular) but longitudinal and radial is fibro-lamellar with woven bone or coarse parallel- ones dominate. Locally, vascular canals are lined up fibered bone and primary osteons surrounded by la- in circumferential rows. Most of the vascular canals mellar bone (Figs 8E-H). The organization of the have been developed into primary osteons but imma- bone tissue increases towards the outer cortex as the ture primary osteons also occur. Details of the bone number of vascular canals decreases. In the two larg- tissue of femur MB.R. 965 are not visible in micro- est sampled femora, MB.R. 814.2 and MB.R. 961, the CT-pictures. All samples show growth marks through- outer cortex is made of lamellar bone. All kind of out the cortex (Klein et al., unpubl. data). All Placo-

Downloaded from Brill.com10/04/2021 05:33:52AM via free access Contributions to Zoology, 84 (1) – 2015 75 dontia indet. bones show locally primary spongeous sampled (among which no Placodontia indet. aff. Cya- and trabecular bone and a secondary widening of modus) display a rather restricted distribution distinct osteons (Fig. 8; S5, 6). from the other taxa except Plesiosaurus. It must be specified that, as Bone Profiler was origi- Results of the Principal Components Analysis nally conceived for tubular structures, some peculiar microanatomical organizations obtain aberrant values The PCA of the humeris (see Fig. 3A) shows that the in the program. It was the case for Paraplacodus bones, two main axes explain 66.7% of the variance (46.1 and notably because compactness was higher in the core 20.9% respectively). C is the only parameter that nega- than in the outer surface of the bone, so that there was tively contributes to the first axis. Taxa on the first axis no sigmoid adapted to describe the structure. This was essentially discriminate based on C and P (projections corrected based on an estimate for the analysis. As a values of -0.21 and 0.24, respectively). On the second result, Paraplacodus should have been much further axis, they essentially discriminate based on Max and away from the other taxa in the analysis (Fig. 3). C (projections of 0.20 and 0.16, respectively). Only P negatively contributes to the second axis. The graph shows that very efficient swimmers (orange rectangle Discussion in Fig. 3A including cetaceans, ichthyosaurs, Progna- thodon [Mosasauridae], Dermochelys [leatherback Microanatomical tendencies turtle], and the elephant seal Mirounga), together with the nothosaur humerus MB.R. 269, group together no- Within our placodont sample, five main distinct types tably as a result of their relatively lower compactness. of microanatomical organization are observed: 1) ex- All specimens of Placodontia indet. aff. Cyamodus tremely compact humerus (CI=94.7%) and femur and Horaffia show a restricted distribution that high- (CI=97.9%) with no medullary cavity in Paraplaco- lights the strong ‘intraspecific’ similarity and, more dus; 2) strongly compact humeri (78.5% pachypleurosaur An- mean value=83.1%) with a small open medullary cav- arosaurus and the Pistosaurus specimens. These taxa ity in Placodontia indet. aff. Cyamodus (no femur clearly distinguish from other less efficient swimmers, was available); 3) strongly compact humerus notably based on the relatively lower compactness of (CI=91.2%) with a small medullary cavity surround- the cortex and the sharp transition between the com- ed by a spongiosa and a compact outer cortex, and a pacta and the medullary cavity (with almost no rather compact femur (CI=78.2%) with a reduced spongeous transition zone). Paraplacodus and Pse- medullary cavity surrounded by a thick spongiosa, phoderma are among the taxa showing the highest with almost no outer compact cortex, in Psephoder- compactness. Together with the two Placodontia indet. ma; and 4) humeri and femora with a large medullary humeri MB R 454 and IGWH 9, they group with the region but no open medullary cavity and 5) femora taxa showing a wider transition zone. with an open medullary cavity surrounded by a rather The PCA of the femora (see Fig. 3B) results in two large spongiosa. main axes jointly explaining 72.4% of the variance The extremely compact bones of Paraplacodus show (49.2 and 23.2%, respectively). Only the parameter C strong osteosclerosis. Remodelling occurs in the core of positively contributes to the first axis. On this axis, taxa the sections and is characterized by abundant endosteal essentially discriminate based on P (projection of bone deposits so that the medullary area is compacted. -0.28) and, to a lesser extent, C and Min (projections of The humeri of Placodontia indet. aff. Cyamodus show a +/-0.24). Both S and P positively contribute to the sec- strong inhibition of primary bone resorption and re- ond axis. On the second axis, taxa essentially discrimi- modelling process is initiated although only incom- nate based on Max (projection of 0.264) whereas P pletely. They are similar to those of the diapsid Horaffia does almost not contribute to this axis. Because of its kugleri. However, if the latter are characterized by high compactness and absence of medullary cavity, pachyosteosclerosis (Klein and Hagdorn, 2014), those Paraplacodus is isolated from the other taxa sampled. of Placodontia indet. aff. Cyamodus, with no morpho- The other placodonts generally display an open medul- logically observable thickening, only display osteoscle- lary cavity or highly spongeous medullary region and a rosis (S2). As a result of the PCA Placodontia indet. aff. rather large transition zone. Despite variation in the Cyamodus and the diapsid Horaffia kugleri along with values of the corresponding parameters, the placodonts Pistosaurus and the pachypleurosaur Anarosaurus are

Downloaded from Brill.com10/04/2021 05:33:52AM via free access 76 Klein et al. – Histology and microanatomy of Placodontia well separated from both the remaining placodonts on Thus, based on sediments and microanatomy an the one hand and sustained swimmers on the other aquatic lifestyle in shallow marine habitats can be in- (Fig. 3). ferred for all. Due to their specialized durophageous The Psephoderma femur is characterized by peri- dentition all placodonts have fed predominately on osteal bone resorption in form of a perimedullary re- hard shelled mollusks that moved only slowly or were gion containing large erosion cavities. This is to a less- sessile. Thus, there was no need for placodonts to be er degree also visible in the much smaller and more fast swimmers. However, the discussion of swimming gracile humerus. Periosteal remodelling is initiated by styles and capabilities remains difficult, because the deposition of a thin layer of lamellar bone around among modern aquatic vertebrates only sirenians dis- the large erosion cavities. The femur organization of play BMI, which makes interpretation of the placodont Psephoderma rather resembles that of the femur, hu- data difficult. Additionally, morphological, histologi- merus and rib of the diapsid Claudiosaurus (Buffrénil cal, and microanatomical inferences contradict each and Mazin, 1989) that was an anguilliform swimmer other in some taxa. (Houssaye, 2012). Some humeri and femora from our BMI is most intense in Paraplacodus. Importantly sample, referred to as Placodontia indet. have a large this taxon lacks any kind of armour, which could oth- medullary region consisting of secondary trabecles. erwise also serve as a source of body mass increase The occurrence of open medullary cavities in other (e.g. Scheyer, 2007). If it was an inefficient swimmer, femora of Placodontia indet. suggests different func- achieving long dives close to the bottom, a high in- tional requirements (swimming styles) and slow swim- crease in bone mass would have been advantageous to ming skills. control buoyancy. BMI is generally concentrated in the anterior portion of the body when it is also involved in Ecological inferences and possible swimming styles body trim control (Houssaye, 2009). The occurrence of strong BMI in Paraplacodus humeri and femora According to Ricqlès and Buffrénil (2001), increase in would be in accordance with a bottom-walker rather skeletal density and mass is a clear advantage for poor- than shallow-swimmer ecology for this taxon. ly active aquatic tetrapods because the resulting addi- Psephoderma alpinum also shows high bone com- tional ballast allows a hydrostatic (passive) control of pactness among the sample in spite of resorption pro- body trim in water and counteracts lung buoyancy. As cesses in the cortex. Psephoderma was armoured with a result it facilitates diving and extended underwater a dorsal shield consisting of a main carapace and a stays and improves stability in rough water (Taylor, pelvic shield, which increases body mass, too. Its over- 1994). Conversely, this skeletal specialization increases all habitus and largely inflexible body axis would sug- the inertia of the body and induces limitations of the gest a comparable swimming style to modern freshwa- swimming speed and capabilities to perform rapid ma- ter turtles that is a combination of bottom-walk and neuvers (Ricqlès and Buffrénil, 2001). A different pat- rowing with the limbs. However, Psephoderma has tern is a spongeous general organization as a result of slender and short (reduced) limb bones making rowing the combined absence of medullary cavity with an in- with effective strokes unlikely. Microanatomy is simi- crease in cortical porosity due to the development of lar to Claudiosaurus that was an anguilliform swim- erosion bays, which are not entirely filled up by second- mer. However, an anguilliform swimming mode for ary bone (Ricqlès and Buffrénil, 2001). In general one Psephoderma is unlikely due to a largely inflexible could say that taxa that lived in shallow water display body axis (dorsal armour). Although in modern inter- an increase in bone compactness (Laurin et al., 2004), pretations the dorsal armour is divided at the pelvic whereas species that lived in open marine habitats tend region anguilliform movements are still restricted. to have spongy bone (Ricqlés, 1977), as is obvious in The best interpretation is so far that Psephoderma has e.g. several ichthyosaurs (Buffrénil and Mazin, 1990; had a passive lifestyle sitting most of the time on the Talevi and Fernandez, 2012; Houssaye et al., 2014). bottom/ground (or was maybe buried in analogy to All placodonts sampled share a thick cortex and some flat turtles). For feeding it slowly walked the clearly display bone mass increase (BMI) via osteo- ground. Certain propulsion by wriggling movements sclerosis. However, the processes involved are dispa- of the pelvic and tail region is conceivable. Higher re- rate, as are the same global swimming mode. All pla- sorption in the femur than in the humerus further indi- codonts are known from marine sediment except for cates more active movements of the limbs and thus Henodus that was found in lagoonal or lake sediments. might support more active swimming when compared

Downloaded from Brill.com10/04/2021 05:33:52AM via free access Contributions to Zoology, 84 (1) – 2015 77

, , - d d p ) s o e u n s B r to Paraplacodus (that has a similar high BMI). a o fi , e

Z i r l c , i c c g L 2 l t

s . Humeri of Placodontia indet. aff. Cyamodus also ( . a o

t 4 o t c e e 1 e s

, u t d show BMI although to a lesser extend when compared 8 o s s

F n . y

s i o ) ) ) . i

h R ) t y n n n n n n o to Paraplacodus and Psephoderma. The humerus as- a .

c i i i i i i c i h n i e a s s s s s s t

B c s a a a a a a n p n a s = signed to aff. Placodus, the humerus of Henodus, and o

o M B B B B B B a p d

b n d , r

c c c c c c

n = , i i l i i i i o the humeri of the diapsid Horaffia all share a similar 2

a T s . c a

n n n n n n

1 e a e a a a a a a o e n 8 l

y s e n g microanatomy and histology. Thus, similar compact- o

t i m m m m m m t P r i z R

r r r r r r s = p . . - a s

e e l e e e e l r o g o

Placodus Henodus B ness values are assumed for aff. and y a a G G A G G G G n

p l = . ( ( ( ( ( ( ( i p l

t M

e ; e e e e e e e p d n m (although they are both not tested in the PCA due to the , s

l n n n n n n n e l i

1 m i i i i i i i , t s s m a 6 r r r r r r r a i e e n l o n a a a a a a lack of a thin section). The lower BMI indicates more a 9

r v e r

o : w i . p e m m m m m m m s s e o

l m

R s

efficient swimming in these taxa when compared to s p g c . n l w w w w w w w a w s a y o l a B o o o o o o o t o r o / l l l l l l l

m l i n l l l Paraplacodus and Psephoderma, because less BMI l l l l

e e s e M v a t

a a a a a a a d k

u s c n h h h h h h h = a a n s

o e l s s s s s s s

suggests more maneuverability. r r s

a i

o a w t = l e s

. l m Cyamodus hildegardis u was armoured with a dorsal g e l

s r e c

c n f , s a w w s

o l g t s s a

shield consisting of a main carapace and a pelvic shield o l l b s

n t v

s s = i

, w w o

/ /

w w

s s s d , s s D

Cyamodus r (Rieppel, 2002; Scheyer, 2010). Other spe- m w w w w c c

l l e

m e . H l J b b a a b s s b n f

o . i : e cies most likely carried armour as well (as evidenced in e n o r n d

w i n o s

= i

C. kuhnschnyderi; see Nosotti and Pinna, 1996) but w

s = e

y p n

v y /

, n n n i

e l u

t s I ? y n ? y y y n i their exact configuration on the body is not known. In , o c e r s m r i a y a e

g d y d Cyamodus hildegardis

addition, also carried a heavily e

. = . r n

b . . = o o

e o

l l y . . . . .

Henodus c c l l c armoured tail (Scheyer, 2010). was heavily l l l d m y . t m

i t s s c c c c c e

n . s l t t s s s s s t i s n

s s o = t t t t t

n e

armoured as well, with a closed dorsal and ventral , s s s s s a o o a s e i A H ? o o ? o o o p w E t s

s : r . e shield (Huene, 1936). Thus, body mass was increased n s c e r p o a

n n p

, d

, o o e

r by armour in both taxa. Cyamodus has long and mas- i o x t n b e n

c e

/ a t o k a r i G y n y y n y y y l r l

sive humeri, Henodus humeri are tiny (reduced). Based b a v

a o l P e l

d c r . w

on its morphology, lifestyle and swimming capabilities 5 b e r

4 m b h m b t a n 5 o

fi /

l A

Henodus

of are interpreted as similar to those de- t 4 -

- n t F ? n y ? y n n y . l i 8 o

s o e

r t l e b scribed above for Psephoderma, making Henodus l S l

b r u a fi = u N s r

t e

mainly a bottom walker. No propulsion by wriggling a c g

r M

p e

b E ? y y ? n y y y S t l i

i e t : seems possible due to its entirely enclosed body. a s d h a e c i

l i c n a r y d L L L Humeri of Placodontia indet. aff. Cyamodus are a t

a a o s B B B

o 3 c L L L

t F F F B B g 2 ,

o Cyamodus B B B k long and massive (as it is the case in all a

t n i p Z p p Z C F F F

n i / / / l

H D r L r r L c c c - . a m spp.), allowing strong strokes and making a swimming m e o o r l W r n c o m

c G i f i style by rowing with the limbs possible. [Please note e = i I

b w x m a n a s

r r l , that it is unknown if the feet of any placodont were , d t C ? n y ? n y y y o J s

n ,

e . a m y o G L webbed.] Additionally, the divided dorsal shield would e

, n . f l B

t =

F = n d c have allowed some (minimal) propulsion by wriggling

y r

e i

/ n : s B y y y n n ? ? ?

g a p t ,

o with the rear and tail. s n , r l 4 ) e e p o 5

s y L t

4 e Placodus s has only one single row of osteoderms =

i r = .

y F h p s s s R

m m m m p y

, . A s s l l s l l l along its vertebral column that has not much contrib- , r ( . t l

t B u n a e n o e c e uted to body mass. It was classified as a subcarangi- n i M s

s m o

g e e r d s r b o r

a l u form swimmer (Braun and Reif, 1985), which is sup- n p

p o , a d

t a

h o

l B o 9 g l ) ) ported by its laterally compressed tail (Drevermann, p

n . m e n r

B A i d a

H o l = m e y

l 1933) and the supposed BMI. p p a c e m n u u W C l

u d - o o : . f G t r r d f o We did not know if armour was present or not for o o I f f g g r e

a a ( ( i r m b y

/ r . . . h e r r

the taxa represented by humeri and femora assigned fi t t t e s r

a e

e e e i l l d

d r m d d d i a

m . a

i e

to Placodontia indet. All bones show a high compact- n u n n n s e l 8 t a i i i s m e t h m u

g 7 n l

u s t m a a a d u s e 5 u i i i o d a r ness and have a large spongeous medullary area of a o r t o t t

t S k 4

e o

d e c n n n e s . 5 s c f d a n n

a o o o g u r 3 l comparable size to that of some otariids (but com- a o fi a e

a S d d d l d e

m f

p n l h , e f t o o o o i a P u l I N a o p r e

c c c r n c r . .

bined with a much higher compactness in Placodon- b r e x r o a a a r f

e o s s a M l l l a a i a f h i A T s S T s c c H P P a P tia indet.). In addition, some femora show an open P P H

Downloaded from Brill.com10/04/2021 05:33:52AM via free access 78 Klein et al. – Histology and microanatomy of Placodontia medullary cavity. Thus, microanatomy indicates cer- The absence of large amounts of mature secondary os- tain, but not very fast, swimming capabilities. teons (Francillon-Vieillot et al., 1990; Currey, 2002) in It is also unknown whether the diapsid Horaffia was the placodont samples expressing fibro-lamellar bone is armoured. Its humeri were large and pachyostotic atypical. Ricqlès (1976) hypothesized a possible rela- (Klein and Hagdorn, 2014). Microanatomy clearly doc- tion between fibro-lamellar bone and remodelling (i.e., uments osteosclerosis (Klein and Hagdorn, 2014; cur- the presence and number of secondary osteons), which rent study). Both identify this taxon as a shallow ma- is true for many extant and extinct vertebrates such as rine inhabitant most likely a bottom walker or slow large herbivorous mammals, large birds, dinosaurs, and swimmer but due to the lack of a complete skeleton this ichthyosaurs (e.g. Klein and Sander, 2008; Houssaye et remains rather speculative. al., 2014). All placodonts, thanks to an inhibition of bone re- Histological characteristics modelling, have a good and fairly complete growth record preserved, which is the subject of another study Placodonts share several histological features. Numer- (Klein et al., unpubl. data). ous primary osteons occur in all bones that grew with Paraplacodus, one humerus of Placodontia indet. fibro-lamellar bone. Primary osteons are not developed aff. Cyamodus, and some humeri of Horaffia show in Psephoderma alpinum and they are rare in Parapla- pockets of calcified cartilage at midshaft. Retention of codus broilii, which both grew with lamellar-zonal calcified cartilage at midshaft is often coupled with os- bone. All placodonts share immature (i.e. incompletely teosclerosis and is recognized in many secondarily lined) primary osteons that predominately occur in the aquatic vertebrates, e.g. in pachypleurosaurs and plesio- inner cortices. Incompletely lined primary osteons are saurs (Buffrénil et al., 1990; Ricqlès and Buffrénil, also described for other Sauropertygia such as a pachy- 2001; Hugi et al., 2011; Krahl et al., 2013). It is also de- pleurosaurs and a pistosauroid (Klein, 2010), and for scribed in some armour plates of placodonts (Scheyer, young Alligator mississippiensis (Woodward et al., 2007). 2014). There thus might be a correlation between Two major histological groups can be distinguished aquatic lifestyle (and an inferred increased growth rate among placodonts, which do not follow the classical when compared to terrestrial forms; see White, 2011) phylogenetic distinction into armoured vs. non-ar- and incompletely lined primary osteons. moured Placodontia. The armoured Psephoderma and Resorption occurs in all placodont samples. Psepho- the non-armoured Paraplacodus both grew with com- derma humerus and femur exhibit a perimedullary re- pact, low vascularized or avascular lamellar-zonal gion in which periosteal bone is resorbed and partially bone, indicating a slow growth rate and rather low ba- replaced (remodelled). The entire medullary region of sal metabolic rate comparable to that of modern am- Paraplacodus is made of secondary (endosteal) bone, phibians and reptiles. The armoured Henodus, non-ar- resulting in a very compact centre. Some Placodontia moured aff. Placodus, and the armoured Placodontia indet. show an extended medullary region filled by sec- indet. aff. Cyamodus grew with plexiform to radiating ondary trabecels. In fact, the Placodontia indet. sample fibro-lamellar bone tissue, indicating high growth rates can be divided into two groups based on the presence and a high basal metabolic rate. For Placodontia indet. of a medullary cavity in some femora (MB.R. 961; and for Horaffia it is unknown if the individuals had MB.R 812; MB.R 814.2; SMNS 54578) and the pres- carried an armour or not, also grew with fibro-lamellar ence of a large medullary region, respectively (IGWH bone tissue but here the organization is more circum- 9, 23; SMNS 54585; MB.R. 454). The presence of a free ferential, indicating somewhat lower growth rates cavity in some Placodontia indet. aff. Cyamodus and (Margerie et al., 2004). Fibro-lamellar bone tissue is Placodontia indet. is not typical for aquatic tetrapods known from modern birds, dinosaurs, most synapsids, (e.g. Quemeneur et al., 2013; Hayashi et al., 2013). and from other extinct marine reptiles such as ichthyo- In those samples that grew with fibro-lamellar bone saurs and plesiosaurs (e.g. Chinsamy-Turan 2005, 2011; remodelling is always only initiated, which means that Houssaye et al., 2014; Wiffen et al., 1995). Thus, fibro- secondarily widened (eroded) vascular canals or pri- lamellar bone tissue clearly originated several times mary osteons are surrounded by a layer of circumferen- within different vertebrate lineages. tially deposited lamellar bone but stay widely open and The fibro-lamellar bone of placodonts has the typical are not infilled. Mature secondary osteons are very rare scaffolding of woven bone surrounded by lamellar bone. and are restricted to the inner cortex of few samples. However, sometimes the woven bone component is re-

Downloaded from Brill.com10/04/2021 05:33:52AM via free access Contributions to Zoology, 84 (1) – 2015 79 placed or grades into parallel-fibred bone with specially Long bone histology allows the identification and thick and coarse fibers. Both always contain a high assignment of isolated bone fragments to Placodontia. amount of thick and numerous osteocyte lacunae. A re- Neither the combination of the avascular to low vascu- placement or modification of the woven bone component larized lamellar-zonal bone tissue of Psephoderma and by parallel-fibred bone in the fibro-lamellar tissue was Paraplacodus nor the special plexiform to radiating described before for archosaurs (Ricqlés et al., 2003), fibro-lamellar bone tissue of aff. Placodus, Placodontia the ornithopod dinosaur Gasparinisaura (Cerda and indet. aff. Cyamodus, and Henodus or circumfertential Chinsamy, 2012), and the titanosaur dinosaur Ampelo- fibro-lamellar bone of Placodontia indet. has so far saurus (Klein et al., 2012). An atypical parallel-fibred been reported in the here described combinations in bone was mentioned for mosasaurs (Houssaye et al., any long bone of any other marine reptile (e.g. Sander, 2013) and for the temnospondyl Plagiosaurus (Konietz- 1990; Wiffen et al., 1995; Pellegrini 2007; Klein, 2010; ko-Meier and Schmitt, 2013), which both did not grow Hugi, 2011; Hugi et al., 2011; Krahl et al., 2013; Hous- with fibro-lamellar bone. It also occurs within the fibro- saye et al., 2014). lamellar bone of ichthyosaurs (Houssaye et al., 2014). Locally, a high amount of radial vascular canals oc- Different bone tissue types and growth strategies curs within the fibro-lamellar bone of some placodonts. Radial bone tissue reveals according to Margerie et al., As summarized in Table 3, long bone histology indi- (2004:869) in the king penguin chick the highest cates two major groups, which do not follow the classi- growth rates. In ichthyosaurs, a radial arrangement of cal phylogenetic distinction into armoured vs. non-ar- vascular canals was interpreted as a consequence of the moured Placodontia (Fig. 1). The armoured Psepho- insertion of Sharpey’s fibers and thus linked to me- derma and the non-armoured Paraplacodus both grew chanical reasons (Houssaye et al., 2014). The local ra- with lamellar-zonal bone tissue indicating slow growth dial trabecle-like architecture in the inner cortex of rates and a low basal metabolic rate, comparable to some placodont samples may also indicate mechanical modern reptiles. Henodus, aff. Placodus, Placodontia properties in addition to high overall growth rates. The indet. aff. Cyamodus, and Placodontia indet. grew with fibro-lamellar bone tissue in combination with the fibro-lamellar bone tissue combined with a high, plexiform to radiating organization indicates for Placo- though extremely variable, vascular density, indicating dontia indet. aff. Cyamodus the highest growth rates high growth rates and an increased (sustained high) ba- among placodonts but also among other Triassic Sau- sal metabolic rate. Both tissue types represent two ropterygia and is comparable to that of ichthyosaurs completely different growth strategies in these closely (Buffrénil and Mazin, 1990; Houssaye et al., 2014). related taxa. Differences in growth rate and basal meta- In all placodonts that grew with fibro-lamellar bone, bolic rate as well as the somewhat lower BMI in Heno- primary tissue is in general highly vascularized. Lo- dus, Placodontia indet. aff. Cyamodus, aff. Placodus, cally it is even spongeous-like or trabecular-like. Pri- and Placodontia indet. could point to a more active life- mary osteons are secondarily widened by successive style when compared to Psephoderma and Paraplaco- resorption processes, resulting in an overall secondary dus. Perhaps a more active lifestyle also includes sus- spongeous tissue, which is similar to some ichthyosaurs tained swimming (?migration) over long distances for (Houssaye et al., 2014). aff. Placodus, Placodontia indet. aff. Cyamodus, and In spite of certain inter- and intraspecific variability Placodontia indet. but not for Henodus. within the placodont sample that grew with fibro-lamel- That the two different growth strategies are the re- lar bone, the primary bone tissue is in general very sim- sult of gross differences in habitat/environment or cli- ilar to that of ichthyosaurs. Differences include the lack mate can be excluded. All bones exhibiting fibro-la- of a medullary cavity, high endosteal and periosteal re- mellar bone originate from localities within the Ger- modelling, and an overall more spongeous organization, manic Basin, whereas Psephoderma and Paraplacodus in ichthyosaurs (Houssaye et al., 2014). The similarities samples originate from localities settled in the Alpine are notable, because ichthyosaurs were efficient and sus- Triassic realm. All localities were interpreted to repre- tained swimmers that lived in the open marine sea with sent shallow marine environments. During the Middle a comparable body shape and lifestyle to dolphins or tu- Triassic, climate was subtropical warm with alternating nas. Placodonts have a cylindrical, sea cow-like body dry and wet seasons (megamonsoonal intervals) (Par- shape (non-armoured forms) or resemble the shape and rish et al., 1982; Röhl et al., 2001). Accordingly, water morphology of flat aquatic turtles (armoured forms). temperatures of the Muschelkalk Sea and the Alpine

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Triassic lagoonal settings have been correspondingly In conclusion, all placodonts sampled followed an high and less metabolic effort was needed for marine essentially or probably exclusive aquatic lifestyle. How- reptiles to sustain a high growth- and basal metabolic ever, we observe distinct differences in histology and rate in both realms. Thus, strong differences in pale- microanatomy that lead to different growth strategies oclimate can be excluded as explanation why some pla- and life (swimming) styles in Placodontia independent codonts grew with fibro-lamellar bone whereas others from phylogenetic relationships, ontogenetic stages or grew with lamellar-zonal bone. the presence or absence of armour. The differences im- High predatory pressure that makes faster growth ply a high inter- as well as intraspecific variability, necessary, although conceivable as a trigger for the most likely depending on the environment each indi- presence of fibro-lamellar bone in those placodonts vidual lived in (developmental plasticity), and diverse from the Germanic Basin, appears very unlikely since lifestyles among the group. predators also occurred in the environments of the Al- pine Triassic. Predatory pressure might even be higher in environments of the Alpine Triassic due to the pres- Acknowledgements ence of a higher number of possible predators such as ichthyosaurs, large nothosaurs, and thalattosaurs. O. Dülfer (StIPB) and Ch. Wimmer-Pfeil (SMNS) are thanked So far we can only observe that Henodus, Placodon- for the production of thin sections. We are grateful to H. Furrer (PIMUZ), H. Hagdorn (MHI), N. Hauschke (IGWH), R. Schoch tia indet. aff. Cyamodus, aff. Placodus, and Placodon- (SMNS), and D. Schwarz-Wings (MfN) who kindly gave per- tia indet. followed a different life history strategy when mission for the histological sampling of placodont bones. P. Hav- compared to Psephoderma and Paraplacodus the ex- lik (GPIT) is acknowledged for the photo of the cross section of act reasons for this have yet to be fully understood. the Henodus humerus. M. Sander (StIPB) gave allowance to use the technical equipment at the StIPB and is acknowledged for always inspiring discussions. G. Oleschinski (StIPB) is acknowl- Concluding remarks edged for some of the photographs. D. Konietzko-Meier and Chris Shelton (both StIPB) contributed constructive comments The study of the morphology, long bone microanatomy, on the thin sections. R. Baumann, A. Kupfer, M. Rasser, and FX. and histology of Placodontia clearly shows that extinct Schmidt (all SMNS) are thanked for technical support. The fol- taxa are not ‘simply similar’ to modern taxa but that they lowing persons provided samples for the PCA analyses: H. Fur- rer (PIMUZ), H. Hagdorn (MHI), N. Hauschke (IGWH), A. could have had a variety of features, which are in certain Krahl (StIPB), D. Nieweg (TWE), H. Oosterink (Winterswijk), combinations sometimes without a modern analogy so M. Sander (StIPB), R. Schoch (SMNS), and S. Hajashi (OMNH). that the possibilities for interpretation are limited. The study was partly funded by the Swiss National Science Clear evolutionary trends within Placodontia re- Foundation (grant no. 149506 to TMS). AH acknowledges finan- garding microanatomical and histological features are cial support from the ANR-13-PDOC-0011. We are thankful to the constructive comments of I.A. Cerda (Museo de Geologıa y not observable but identification might be hampered Paleontologıa, Universidad Nacional del Comahue, Buenos due to still limited sample size. 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Downloaded from Brill.com10/04/2021 05:33:52AM via free access 84 Klein et al. – Histology and microanatomy of Placodontia

Online supplementary material

S1. Morphological description of sampled humeri and femora

S2. Photographs of sampled bones.

S3. Table with Bone Profiler values.

S4. Cross sections of bones transformed into black and white pictures.

S5. Histological details of Placodontia.

S6. Histological details of Placodontia.

Downloaded from Brill.com10/04/2021 05:33:52AM via free access S1. Morphological description of sampled humeri and femora

The classical bone bed localities from the Germanic Basin have largely produced isolated bones. As a result, alpha taxonomy of Sauropterygia is often essentially based on skull morphology and taxa are only incompletely known (Rieppel, 2000). Postcranial morphology of Triassic Sauropterygia is highly conservative and uniform. As a consequence taxonomical assignment of isolated bones is generally difficult. Previous studies suggested that, in sauropterygians, bone histological features can be used to support taxonomical assignment (e.g., Klein, 2010, 2012; Sander et al., 2013). Bones of placodonts are rarer than those of eosauropterygians such as Nothosauria and Pachypleurosauria. Additionally, isolated bones are often incompletely preserved or highly weathered, making taxonomic assignment still more problematic. The here described humeri and femora are assigned to Placodontia because their morphology (summarized in Rieppel, 2000) and histology (Buffrénil and Mazin, 1992; Klein, 2010) differs from that of Eosauropertygia (e.g., Sander, 1990; Klein, 2010; Hugi, 2011; Hugi et al., 2011; Krahl et al., 2013). Placodont humeri are more massive and have a simpler morphology with less features than eosauropterygian ones. This means that they are more adapted to an aquatic lifestyle when compared to Eosauropterygia humeri. Femora are more curved (heads twisted), and display distinct condyli with a well-developed fourth trochanter. However, proximal and midshaft fragments of placodont humeri and femora are often hard to taxonomically identify. The assignment of the sampled bones is—if not mentioned otherwise—based on morphological features. The here described bones were compared in detail to published material (see references below). Their assignment is however, often only tentatively due to the lack of diagnostic and/or comparable material. Both the humerus and femur of Psephoderma experienced distal taphonomic compaction. The humerus of Psephoderma PIMUZ A/III 1476 is long and slender with a laterally expanded and curved rectangular proximal head (S2C, D). The distinctly constricted narrow/slender midshaft has a postaxially curved and preaxially straight margin. The distal end is broad and flattened, the natural condition being undoubtedly amplified by the taphonomic compaction. The femur PIMUZ A/III 0735 is proximally incomplete and has a similar shape to the humerus except for a straight midshaft region and a less expanded proximal head. Both cross sections are round-oval with a slightly pointed preaxial side. For further morphological details and a comparison of the here included bones see Meyer (1858), Pinna and Nossotti (1989), and Renesto and Tintori (1995). The humerus of Henodus belongs to one (GPIT/RE/7289) of the seven known specimens (Huene, 1936, 1938). The humerus is preaxially straight and postaxially concave with a constricted midshaft. The massive proximal head is thick and round to oval. The broad and flat distal end is twisted. The humerus is bent in a ventral direction and dorsoventrally flattened (S2E). The cross section is oval with a slightly convex dorsal margin and a straight ventral margin. Both lateral margins are convex with the preaxial side pointed. For further morphological details see Huene, 1936, 1938). In the Muschelkalk, no Cyamodus skull has yet been found together with a humerus. Isolated humeri can thus only tentatively be assigned to Cyamodus (Vogt, 1983; Rieppel, 1995; Klein and Hagdorn, in press). Several humeri that share certain morphological and histological features where combined as Placodontia indet. aff. Cyamodus. However, this assignment is only tentatively and the group might include different taxa as is indicated by the study of the growth record (Klein et al., unpubl. data). Humeri of Placodontia indet. aff. Cyamodus are morphologically variable (S2F-M), which could be the result of inter-specific variation, intraspecific variation or ontoegentic variation. They have a thick proximal head, a curved and slender midshaft, and a fan-shaped, flat distal end. The proximal head has a triangular shape with the dorsopreaxial margin forming the peak of the isolescent triangle. The proximal head is twisted ventrally in relation to the midshaft. The most distinct and unique character of humeri of Placodontia indet. aff. Cyamodus is a concavity at the ventropostaxial side of the proximal head (Klein and Hagdorn, in press). The distal end has an ectepicondylar groove, a capitellum and an entepicondyle. SMNS 59831 is a large, fan-shaped, and flat distal end of a humerus with a deep ectepicondylar groove and an entepicondyle. The preserved midshaft is constricted with

Downloaded from Brill.com10/04/2021 05:33:52AM via free access straight margins (S2F). SMNS 15937 is a complete dorso-ventrally flattened humerus with poorly preserved proximal and distal ends. The flat proximal head is shorter than the half-round and flat distal end. The proximal head is ventro-preaxially flat/straight rather than concave (S2J). MHI 2112-6 is a large, slender, and slightly curved humerus. The broad proximal head is ventro-postaxially slightly concave. The shaft is slender and constricted. The distal end is broad and flat but due to preparation morphological details are obscured (S2G). Humerus SMNS 54569 has at the broad proximal head the characteristic concavity at the ventropostaxial side. Although the distal end of SMNS 54569 is missing the curvature of the slender shaft is obvious (S2I). SMNS 15891 is only poorly preserved and confidently establish its morphology is not possible. The humerus SMNS 15891 has a dorso-ventrally flattened shape; the midshaft is slightly curved, and it has a symmetrical broadened half-round distal end (S2H). The distal humerus fragment MHI 697 is only poorly preserved, consisting of a constricted, slender and curved shaft. The distal end is flattened; fan shaped, and has an ectepicondylar groove (S2L). SMNS 54582 is the distal half of a humerus. The shaft is constricted and postaxially slightly curved. The distal end is fan-shaped with striations on the ventral and dorsal sides. An ectepicondylar groove is developed. The entire bone is dorsoventrally very flat (S2K). MHI 1096 is the proximal half of a humerus with a slightly curved midshaft. The broad proximal head is incomplete, poorly preserved, and the characteristic proximal concavity described for Placodontia indet. aff. Cyamodus (Klein and Hagdorn, in press) is not visible. The postaxial half is flat rather than concave (S2M). MHI 1096 is similar to the much larger humerus fragment SMNS 54569. The overall shape of the midshaft cross sections is oval with a slightly convex dorsal margin and a straight ventral margin that becomes postaxially convex. Both lateral margins are convex and the preaxial side is pointed. In SMNS 54582 and MHI 697 the cross section is dorsoventrally flattened, whereas that of MHI 1096 is rounder, when compared to the others. In the Muschelkalk, so far no Placodus skull has been found together with a humerus. The humerus associated with the nearly complete Placodus skeleton described by Drevermann (1933) belongs to a nothosaur (Rieppel 1995). Isolated humeri can thus only tentatively be assigned to Placodus (Vogt, 1983; Rieppel, 1995; Klein and Hagdorn, in press). Femur morphology of Placodus was described by Drevermann (1933) and Rieppel (1995). It has a slender and straight midshaft region. The proximal head is characterized by a well-developed trochanter. Humeri assigned to aff. Placodus possess a rectangular shaped proximal head, which means that no proximal margin is protruding. The postaxial side is covered with striations. The proximal head is not twisted in relation to shaft or distal end, resulting in a general dorsoventrally flattened shape. The midshaft is constricted, postaxially curved but preaxially nearly straight. The symmetrical broadened distal end of aff. Placodus humerus is divided into an entepicondyle, an ectepicondylar groove, and an ectepicondyle (Rieppel, 1994). In humerus SMNS 59827 a partially broken off entepicondylar foramen is also identified (S2O). The shape of the midshaft cross section resembles that of humeri of Placodontia indet. aff. Cyamodus. Humerus PIMUZ T 5845 and femur PIMUZ T 5845 are assigned to Paraplacodus based on their morphology (see Peyer, 1935, Rieppel, 2000a, b). Both bones are distally extremely flattened as a result of taphonomic processes. The midshaft region underwent very slight compaction in the humerus and little to no compaction in its proximal part. The humerus also experienced proximally some surface erosion. The humerus of Paraplacodus (PIMUZ T 5845) has a postaxially concave and preaxially straight margin. The proximal head is rectangular with a straight postaxial surface and a well set off margin (S2A, B) and not distinctly broadened. The constricted midshaft leads to a broad and flat distal end. The femur of Paraplacodus (PIMUZ T 5845) has a thick and massive proximal end, a slender constricted shaft. The broad proximal articular surface carries at the postaxial portion a distinct concave indentation. Postaxially, the bone is shorter and more curved than the preaxial side. It has an oval midshaft cross section. The flat distal end is less broad than the proximal head and the roughly equally sized condyli are not easy to distinguish. The femur is dorsoventrally curved. Due to bone ratio and find situation both bones can well belong to the same individual. For further morphological details on humerus and femur morphology of Paraplacodus see Peyer (1935) and Rieppel (2000). The humerus (PIMUZ T5845) cross section is elliptical with a convex dorsal, a concave

Downloaded from Brill.com10/04/2021 05:33:52AM via free access ventral side and a preaxially more pointed side than postaxially (S4A). The femur (PIMUZ T 5845) cross section is round-oval (S4B). The following humeri and femora have placodont affinities due to their morphology and histology but did not match any described taxon and are thus assigned to Placodontia indet. Due to morphological, histological and microanatomical diferecnes these bones represent more than one taxon (see also Klein et al., unpubl. data). Humerus MB.R. 454 (S2P) and IGWH 9 (S2Q) share a distinct midshaft cross section with a slightly convex dorsal margin and a divided ventral margin. The preaxial half of the ventral margin is straight but the postaxial half convex. Both lateral margins are convex with a pointed preaxial side. IGWH-9 is a complete left humerus, which was already figured and described in Klein (2010). It has a simple morphology with a roughly crescent-shaped form. Although poorly preserved, the proximal head seems to have been originally rectangular. The incomplete distal end is broad and flat and carries an ectepicondylar groove. The shaft is only minimally constricted. The postaxial margin is curved the preaxial one straight and the entire humerus dorsoventrally flattened. The midshaft cross section has a convex dorsal margin, preaxially the ventral margin is straight, but it becomes postaxially convex. Both lateral margins are convex with a pointed preaxial side. Humerus MB.R. 454 has a simple rectangular proximal head, a postaxial striation pattern, a postaxially curved midshaft, and a dorso-ventrally flattened general shape. The distal end is not symmetrical broadened which, however, can be obscured due to preservation and preparation artifacts. Femora SMNS 84545 (S2R) and IGWH 23 (S2S) share an obconical shaped cross section. The ventral margin is long and slightly convex; the dorsal margin is shorter and straight (minimal convex in IGWH 23). The lateral margins of SMNS 84545 are both in their dorsal portion concave but in the ventral half convex that of IGWH 23 are both convex. IGWH 23 is the incomplete proximal end of a right femur, which was already figured and described in Klein (2010). The proximal head is rectangular; the shaft is straight. The midshaft cross section has a rather untypical form. The ventral margin is long and concave; the dorsal one short and convex. Both lateral margins are convex. SMNS 54578 is only incompletely preserved and the morphology of the proximal and distal ends cannot be established. The bone represents a femur, because the proximal head is longer than wide (Table 1), the midshaft cross section is round-oval, and the midshaft has straight margins. SMNS 84545 is a ?proximally poorly preserved and ?distally incomplete femur. The midshaft area however, is well preserved and exhibits a very special cross section, comparable to that of IGWH 23. Femora MB. R. 814.2 (S2T), MB.R. 961 (S2U), SMNS 54578 (S2V), MB.R. 812 (S2W) share a round oval cross section. MB.R. 814.2 is a poorly preserved and broken, hollow/cavernous midshaft. Femora MB.R. 961 and MB.R. 812 both have only incompletely preserved proximal and distal ends and the morphology cannot be established. Due to their size, their straight and round midshaft regions taxonomical affinities to pachypleurosaur or nothosaur can be excluded (but not those to pistosaurs). Their assignment to Placodontia and their identification as femora is solely based on the shape of their cross sections and their bone histology (see below). The cross section of femur MB.R. 965 (S2X) is largely round but with a distinct peak preaxially, indicating the position of the 4th trochanter. The taxon Horaffia kugleri is only based on five humeri, most likely representing a growth series (Klein and Hagdorn, in press). Humeri of H. kugleri are pachyostotic and have a dorsopreaxially elongated margin, a massive and ventrally protruding triangular proximal head as well as a preaxially slanted asymmetrical distal end. They are easy to distinguish from Cyamodus, Placodontia indet. aff. Cyamodus, aff Placodus and Placodus humeri and all other here described long bones (S2N) However, although their phylogenetic relationship and possible placodont affinities remain unresolved their bone histology and microanatomy largely resembles those of aff. Cyamodus (Klein and Hagdorn, in press), which is the reason why Horaffia is included into the current study.

Downloaded from Brill.com10/04/2021 05:33:52AM via free access References

Buffrénil de V, Mazin J-M. 1992. Contribution de l’histologie osseuse à l’interpretation paléobiologique du genre Placodus Agassiz, 1833 (Reptilia, Placodontia). Revue Paléobiology 11: 397-407. Drevermann FR. 1933. Die Placodontier. 3. Das Skelett von Placodus gigas Agassiz im Senckenberg- Museum. Abhandlungen der Senckenbergischen Naturforschenden Gesellschaft 38: 319-364. Huene v F. 1936. Henodus chelyops, ein neuer Placodontier. Palaeontographica A 84: 99-148. Huene v F. 1938. Der dritte Henodus, Ergänzungen zur Kenntnis des Placodontiers Henodus chelyops Huene. Palaeontographica A 89: 105-114. Hugi J, Scheyer TM, Sander PM, Klein N, Sánchez-Villagra MR. 2011. Long bone microstructure gives new insights into the life history data of pachypleurosaurids from the Middle Triassic of Monte San Giorgio, Switzerland/Italy. Comptes Rendus Palevol 10: 413-426. Hugi J. 2011. The long bone histology of Ceresiosaurus (Sauropterygia, Reptilia) in comparison to other eosauropterygians from the Middle Triassic of Monte San Giorgio (Switzerland/Italy). Swiss Journal of Palaeontology 130(2): 297-306. Klein N. 2010. Long Bone Histology of Sauropterygia from the Lower Muschelkalk of the Germanic Basin Provides Unexpected Implications for Phylogeny. PLoSOne 5(7): e11613. doi:10.1371/journal.pone.0011613. Klein N. 2012. Postcranial morphology and growth of the Winterswijk pachypleurosaur Anarosaurus heterodontus (Sauropterygia) from the Lower Muschelkalk of Winterswijk, The Netherlands. Paläontologsiche Zeitschrift 86(4): 389-408. Klein N, Hagdorn H. in press. Humerus morphology and histology of a new marine reptile (Diapsida) from the Muschelkalk-Keuper-Grenzbonebed (Middle Triassic, Ladinian) of Southwest Germany. Paleodiversity. X: X-X. Krahl A, Klein N, Sander PM. 2013. Evolutionary implications of the divergent long bone histologies of Nothosaurus and Pistosaurus (Sauropterygia, Triassic). BMC Evolutionary Biology, 13: 1-23. Meyer v H. 1858. Psephoderma Alpinum aus dem Dachsteinkalke der Alpen. Neues Jahrbuch für Mineralogie, Geognosie, Geologie und Petrefaktenkunde 1858: 646-650. Peyer, B. (1935). Die Triasfauna der Tessiner Kalkalpen. VIII. Weitere Placodontierfunde. Abhandlungen der schweizerischen Palaontologischen Gesellschaft 55: 1-26. Renesto S, Tintori A. 1995. Functional morphology and mode of life of the Late Triassic placodont Psephoderma alpinum Meyer from the Calcare di Zorzino (Lombardy, N Italy). Rivista Italiana di Paleontologia e Stratigrafia 101: 37-48. Rieppel O. 1994. Osteology of Simosaurus gaillardoti, and the phylogenetic interrelationships of stem-group Sauropterygia. Fieldiana Geology, n. s. 28: 1-85. Rieppel O. 1995. The genus Placodus: Systematics, morphology, paleobiogeography, and paleobiology. Fieldiana (Geology), n.s.. 31: 1-44. Rieppel O. 2000a. Sauropterygia I. Pp 1-134 in: Wellnhofer P, ed., Encyclopedia of Paleoherpetology, Volume 12A. München: Friedrich Pfeil Verlag. Rieppel O. 2000b. Paraplacodus and the phylogeny of the Placodontia (Reptilia: Sauropterygia). Zoological Journal of the Linnean Society, London 130: 635-659. Sander PM. 1990. Skeletochronology in the small Triassic reptile Neusticosaurus. Annales des Sciences Naturelles, Zoologie (13)11: 213-217. Sander PM, Klein N, Albers PCH, Bickelmann C, Winkelhorst H. 2013. A skeleton of a basal Pistosauroidea from the Lower Muschelkalk of the Germanic Basin (Winterswijk, The Netherlands) and a reworked and extended phylogenetic analysis of Triassic Sauropterygia. Paläontologische Zeitschrift 88: 55-71. Vogt C. 1983. Evolutive Palökologie der Placodontier (Placodus, Henodus; Euryapsida, Trias). PhD Dissertation, Eberhard-Karls-Universität, Tübingen, Germany. p. 1-99.

Downloaded from Brill.com10/04/2021 05:33:52AM via free access S2. Photographs of sampled bones

A, Humerus and B, Femur of Paraplacodus broilli (PIMUZ T5845) from the Anisian/Ladinian (Besano Formation, Grenzbitumenhorizont) of the locality of Monte San Giorgio (Switzerland/Italy). C, Humerus (PIMUZ A/III 1476) and D, Femur (PIMUZ A/III 0735) of Psephoderma alpinum from the Rhaetian (Kössener Formation) of Tinzenhorn (Switzerland). E, Humerus of Henodus cheylops (GPIT spec. no. 1) from the Carnian of Tübingen-Lustnau (Baden Württemberg, Germany). F-M, Humeri of Placodontia indet. aff. Cyamodus. from the early Ladinian (Upper Muschelkalk) of southern Germany (see Table 1 for locality information). F, SMNS 15937. G, Humerus MHI 2112/6. H, Humerus SMNS 15891. I, proximal part of humerus SMNS 54569. J, distal part of humerus SMNS 59831. K, distal part of humerus SMNS 54582. L,

Downloaded from Brill.com10/04/2021 05:33:52AM via free access distal part of humerus MHI 697. M, proximal part of humerus MHI 1096. N, Humerus MHI 2112-4 of the marine reptile Horaffia kugleri from the middle Ladinian (Upper Muschelkalk/Keuper Grenzbonebed) of Obersontheim-Ummenhofen (Crailsheim, Baden Württemberg, Germany). O, Humerus (SMNS 59827) assigned to Placodus gigas (Vogt, 1983; Rieppel, 1995) from the early Ladinian (Upper Muschelkalk) of Hegnabrunn (Franconia, Bavaria, Germany). P-X Humeri and femora of Placodontia indet. P, Placodont humerus MB.R. 454 from the early Anisian (Lower Muschelkalk) of Ohrdruf (Górny Śląsk, Poland). Q, Placodont humerus IGWH 9 from the Anisian (Lower/Middle Muschelkalk) of Freyburg (Unstrut river valley, East-Germany). R, Proximal part of a placodont femur SMNS 84545 from the middle Ladinian (Upper Muschelkalk/Keuper) of Hegnabrunn (Franconia, Bavaria, Germany). S, Placodont femur IGWH 23 from the Anisian (Lower/Middle Muschelkalk) of Freyburg (Unstrut river valley, East-Germany). T, Midshaft region of placodont femur MB.R. 814.2 from the early Anisian (Lower Muschelkalk) of Górny Śląsk (Poland). U, Midshaft region of placodont femur MB.R. Ladinian (UpperMuschelkalk) of Bayreuth (Germany), Midshaft region of placodont femur SMNS 54578 from the early Ladinian (Upper Muschelkalk) of Hegnabrunn (Franconia, Bavaria). W, Midshaft region of placodont femur MB.R. 812 from the early Anisian (Lower Muschelkalk) of Opatowitz (Górny Śląsk, Poland). X, Femur MB.R. 965 from the early Anisian (Lower Muschelkalk) of Górny Śląsk (Poland).

Downloaded from Brill.com10/04/2021 05:33:52AM via free access S3. Bone profiler values of the analysed humeri and femora

C is the global bone compactness for the whole sectional area. P is the relative distance from the centre of the section to the point of inflection, where the most abrupt change in compactness is observed. P is thus proportional to the size of the medullary cavity. S is the reciprocal of the slope at the inflection point and generally reflects the width of the transition zone between the cortical bone and the medullary region. Humerus abbr. C S P Min Max Placodontia Fig. 8 Psephoderma alpinum Ps 91.2 0.2522054 0.100823 0.3284395 0.9974383 PIMUZA/III 1476 Placodontia indet. - 82.3 0.0192668 0.0516515 0.0000011 0.8267046 aff. Cyamodus SMNS 59831 Placodontia indet. - 79.9 0.0588768 0.080027 1.00E+00 0.8125375 aff. Cyamodus SMNS 15937 Placodontia indet. - 84.7 0.0149849 0.0350636 0.000000967 0.8487779 aff. Cyamodus MHI 2112-6 (part) Placodontia indet. - 87.1 0.0147433 0.0549787 0.685 0.874075 aff. Cyamodus SMNS 54569 Placodontia indet. - 84.7 0.0232605 0.155462 0.154 0.87024 aff. Cyamodus SMNS 54582 Placodontia indet. - 81.8 0.0130419 0.1688655 0.000000148 0.8424392 aff. Cyamodus MHI 697 Placodontia indet. - 85.9 0.0077368 0.0881896 0.000000244 0.8661181 aff. Cyamodus SMNS 15891 Placodontia indet. - 78.5 0.0111719 0.1417196 0.000000433 0.8014756 aff. Cyamodus MHI 1096 Paraplacodus broilii Pa 94.7 0.0971625 0.000001 0.3456165 0.9467067 PIMUZ T 5845 Placodontia indet. P 82.4 0.0796261 0.4757301 0.4843481 0.9367116 MB.R. 454 Placodontia indet. P 82.7 0.2593411 0.0695503 0.000000897 0.9402868 IGWH 9 Pachypleurosauria Anarosaurus heterodontus - 82.5 0.0369192 0.3300958 0.000000775 0.9329377 TWE 48000023 Anarosaurus heterodontus - 78.8 0.054676 0.2635428 0.000000341 0.8574109 Wijk08-183 Neusticosaurus sp. Ne 92 0.051431 0.000001 0.00000261 0.9237458 PIMUZ T 3455-2

Downloaded from Brill.com10/04/2021 05:33:52AM via free access Nothosauria Nothosaurus sp. - 76.2 0.3090958 0.2595539 0.00000197 0.999999 SMNS 84772 Nothosaurus ?giganteus - 51.4 0.0665729 0.8226691 0.293 0.999999 MB.R. 269 Nothosaurus sp. - 70.9 0.1215368 0.4458386 0.0000000876 0.940332 MHI 754 Simosaurus gaillardoti - 81.7 0.175219 0.0483883 0.000000754 0.8711621 SMNS 18698 Simosaurus gaillardoti - 75.9 0.1203213 0.3304208 0.0418088 0.8910345 SMNS 52095 Pistosauria Cymatosaurus sp. - 91.8 0.2037634 0.0689963 0.909758 0.999999 IGWH 19 Cymatosaurus sp. - 84.7 0.1150202 0.2282219 0.000000764 0.934409 Wijk11-39 Pistosaurus sp. 91.4 0.0207013 0.0517014 0.000000817 0.9180916 Plesiosauria Ichthyosauria Ichthyosaurus sp. Ich 68 0.1273568 0.1281216 0.0232191 0.7214996 StIPB R 222 Stenopterygius sp. St 54.2 0.0307268 0.9294553 0.471013 0.9945984 SMNS A Mosasauria Dallasaurus turneri Da 89.8 0.0516712 0.4467431 0.5179167 0.999999 SMU 76386 Prognathodon sp. Pr 46.5 0.1007096 0.8509145 0.251967 0.999999 IRScNB 1624 Crocodylia Crocodylus sp. Cr 82.3 0.0470759 0.3662778 0.00000026 0.9601293 Stenopterygius sp. 72.6 0.0486055 0.5723891 0.5651298 0.8091993 Testudines Chelonoidea Ch 72.6 0.1031421 0.6081272 0.349319 0.9818974 StIPB R 409 Dermochelys coriacea De 56.7 0.0441581 0.9192567 0.4853482 0.999999 OMNH unnumbered Diapsida indet. Horaffia kugleri - 86.3 0.0077258 0.0654034 0.000000722 0.8672099 SMNS 84816 Horaffia kugleri - 87.2 0.0719306 0.0648492 0.00000505 0.887713 MHI 2112-1 Horaffia kugleri - 83.8 0.0460113 0.1610575 0.000000494 0.8664546 MHI 2112-2 Horaffia kugleri - 82.7 0.0720729 0.000001 0.000000851164 0.8326156 MHI 2112-4 (part) Mammalia

Downloaded from Brill.com10/04/2021 05:33:52AM via free access Paleoparadoxia sp. Pap 99.1 0.050096 0.2083801 0.8632984 0.9976097 AMP AK0011 Otariidae StIPB unnumbered Ot 75.6 0.0916312 0.3795253 0.000000196 0.9150631 Otariidae StIPB unnumbered Ot 72.2 0.1288439 0.4659179 0.2728689 0.887505 Phocoena phocoena Pp 42 0.0750872 0.8726646 0.233615 0.999999 MNHNAC 1881-232 Leptonychotes weddellii Le 76.3 0.0970976 0.7085641 0.548983 0.999999 NSM M 29643 Trichechus manatus Tm 84.7 0.0495546 0.3919578 0.071788 0.9989915 ZFMK 73223 Mirounga leonine Ml 34.8 0.102654 0.8723616 0.1207943 0.999999 Laurin et al. (2011) Delphinus delphis Dd 32.9 0.1941883 0.7910332 0.0264792 0.8089076 MNHN 1880-1310 Tursiops truncatus Tt 46.2 0.0944516 0.7790845 0.2733049 0.7728642 MNHNAC 1978-09 Femora Placodontia Psephoderma alpinum Ps 78.2 0.16647 0.502521 0.327674 1 PIMUZ A/III0735 Paraplacodus broilii Pa 97.9 0.02887 0.348015 0.935841 0.98691 PIMUZ T 5845 Placodontia indet. P 75.7 0.12631 0.222099 1.01E-07 0.83904 MB.R. 812 Placodontia indet. P 79.3 0.2023 0.295706 6.85E-07 0.98121 MB.R. 961 Placodontia indet. P 72.8 0.09033 0.443579 0.209233 0.87531 SMNS 54578 Placodontia indet. P 75.1 0.111339 0.317635 1.99E-07 0.874517 MB.R. 814.2 Placodontia indet. P 83.5 0.14266 0.408616 0.420981 0.95714 IGWH 23 Placodontia indet. P 77.5 0.0766466 0.1650459 9.96E-07 0.8117076 SMNS 84545 Eosauropterygia Anarosaurus heterodontus - 85.4 0.028206 0.249154 2.17E-07 0.914454 Wijk-HO-A568 Neusticosaurus sp. - 94.1 0.020479 0.100766 8.70E-07 0.951613 Nothosaurus sp. - 85.8 0.114946 0.243863 2.94E-07 0.954017 Wijk05-11 Nothosaurus sp. - 70 0.03141 0.53516 3.91E-08 0.990768 MHI 279 Nothosaurus sp. - 83.5 0.063216 0.358611 2.54E-07 0.975223 MHI 756 Nothosaurus sp. - 82.5 0.020176 0.361545 5.09E-07 0.952129 MHI 1987 Ceresiosaurus calcagnii 91 0.0095684 0.0977743 4.44E-07 0.9195684

Downloaded from Brill.com10/04/2021 05:33:52AM via free access from Hugi 2011

Simosaurus gaillardoti 74.2 0.0266378 0.4645042 1.16E-07 0.9519623 SMNS 18689 Simosaurus gaillardoti 87.3 0.0624372 0.2260992 3.49E-07 0.9331059 SMNS 18038 Cymatosaurus sp. - 83.9 0.019728 0.325621 5.08E-07 0.941592 IGWH 24 Pistosaurus sp. Pi 89.8 0.00532 0.051613 1.22E-06 0.90021 SMNS 84825 Pistosaurus sp. Pi 73.5 0.087861 0.543595 0.1861428 0.997228 StIPB R 90 Plesiosaurus sp. Pl 74.7 0.0657891 0.2948545 0.000000148 0.8323505 StIPB R90 Ichthyosauria Ichthyosaurus sp. Ich 68.1 0.107999 0.866764 0.563628 0.999999 LO 11904t Crocodylia Steneosaurus sp. Ste 80.9 0.08959 0.306159 3.89E-07 0.919983 SMNS unnumbered Metriorhynchus sp. Me 78.9 0.038623 0.386662 1.72E-07 0.935684 SMNS 9427 Teleosaurid indet. Te 83.3 0.074591 0.364303 9.77E-08 0.983314 BHN 2R 883 Alligator mississipiensis Al 89.1 0.075638 0.310733 6.95E-02 0.999999 SMNS 10481 Testudines Dermochelys coriacea De 64.3 0.061635 0.779624 0.4257667 0.999999 OMNH unnumbered Mosasauria Dallasaurus turneri Da 65.1 0.022499 0.591683 0.0194296 0.999999 SMU 76529

Downloaded from Brill.com10/04/2021 05:33:52AM via free access S4. Cross sections of sampled bones transformed into black (medulla and vascular cavities) and white (compact bone) pictures

A, Black and white picture of humerus and B, Femur of Paraplacodus broilli (PIMUZ T5845) from the Anisian/Ladinian (Besano Formation, Grenzbitumenhorizont) of the locality of Monte San Giorgio (Switzerland/Italy). Note the lack of a medullary cavity and the low number of vascular canals in both samples. C, Black and white picture of humerus (PIMUZ A/III 1476) and D, Femur (PIMUZ A/III 0735) of Psephoderma alpinum from the Rhaetian (Kössener Formation) of Tinzenhorn (Switzerland). Note the large medullary cavities surrounded by a distinct and large perimedullary region and the nearly avascular cortex. E-

Downloaded from Brill.com10/04/2021 05:33:52AM via free access L, Black and white pictures of humeri of Placodontia indet. aff. Cyamodus from the early Ladinian (Upper Muschelkalk) of southern Germany (see Table 1 for locality information). E, Black and white picture of humerus SMNS 59831. F, Black and white picture of humerus MHI 2112-6. G, Black and white picture of humerus SMNS 15891. H, Black and white picture of humerus SMNS 54569. I, Black and white picture of humerus SMNS 15937. J, Black and white picture of humerus SMNS 54582. K, Black and white picture of humerus MHI 697. L, Black and white picture of humerus MHI 1096. M, Black and white picture of humerus MHI 2112-4 of the marine reptile Horaffia kugleri from the middle Ladinian (Upper Muschelkalk/Keuper Grenzbonebed) of Obersontheim-Ummenhofen (Crailsheim, Baden Württemberg, Germany). Note the small medullary cavities and the high vascularization in humeri of cf. Cyamodus and Horaffia kugleri. N-U Black and white picture of humeri and femora of Placodontia indet. N, Black and white picture of placodont humerus MB.R. 454 from the early Anisian (Lower Muschelkalk) of Ohrdruf (Górny Śląsk, Poland). O, Black and white picture of placodont humerus IGWH 9 from the Anisian (Lower/Middle Muschelkalk) of Freyburg (Unstrut river valley, East-Germany). P, Black and white picture of placodont femur SMNS 84545 from the middle Ladinian (Upper Muschelkalk/Keuper) of Hegnabrunn (Franconia, Bavaria, Germany). Q, Black and white picture of placodont femur IGWH 23 from the Anisian (Lower/Middle Muschelkalk) of Freyburg (Unstrut river valley, East-Germany). Note the medullary and perimedullary regions that are merged into each other in humeri MB.R. 454, IGWH 9 and femora SMNS 84545 and IGWH 23. R, Black and white picture of placodont femur MB.R. 961 from the Ladinian (Upper Muschelkalk) of Bayreuth (Germany). S, Black and white picture of placodont femur MB.R. 814.2 from the early Anisian (Lower Muschelkalk) of Górny Śląsk (Poland). T, Black and white picture of placodont femur MB.R. 812 from the early Anisian (Lower Muschelkalk) of Opatowitz (Górny Śląsk, Poland). U, Black and white picture of placodont femur SMNS 54578 from the early Ladinian (Upper Muschelkalk) of Hegnabrunn (Franconia, Bavaria). Note the de-central medullary cavity and the mainly postaxially oriented trabecular-like bone.

Downloaded from Brill.com10/04/2021 05:33:52AM via free access S5. Histological details of Placodontia

Downloaded from Brill.com10/04/2021 05:33:52AM via free access Histological details of Placodontia in normal (left), polarized (middle), and polarized with gypsum light (right). A-C, Immature (incompletely lined) primary osteons in a matrix of coarse parallel-fibred bone in femur MB.R. 814.2 (Placodontia indet.). D-F, Primary and secondary osteons in a matrix of coarse parallel- fibred bone in humerus SMNS 54569 (Placodontia indet. aff. Cyamodus). Note the distinct resorption line of the secondary osteons when compared to primary osteons. G-I, Primary osteons in humerus SMNS 54582 (Placodontia indet. aff. Cyamodus). J-L, Erosion of primary osteons by secondary osteons in femur MB.R. 814.2 (Placodontia indet.) in a matrix of coarse parallel-fibred bone. A similar pattern of remodelling was described by Redelstorff et al. 2014 in a long bone of a dinosaur. M-O, Secondarily widened primary osteons in femur SMNS 84545 (Placodontia indet.). Abbreviations: ipo = immature (not completely lined) primary osteons; po = primary osteons; so = secondary osteons; swpo = secondarily widened osteons.

Reference

Redelstorff R, Sander PM, Galton PM. 2014. Unique bone histology in partial large bone shafts from Upper Triassic of Aust Cliff, England: An early independent experiment in gigantism. Acta Palaeontologica Polonica 59: 607–615.

Downloaded from Brill.com10/04/2021 05:33:52AM via free access S6. Histological details of Placodontia

Histological details of Placodontia in normal (left), polarized (middle), and polarized with gypsum light (right). A- C, Primary osteons in humerus SMNS 59831 (Placodontia indet. aff. Cyamodus). Note the different appearance of primary osteons when compared to those figured in Suppl. Fig. 3G-I. D-F, Secondarily widened primary osteons in femur MB.R. 961 (Placodontia indet.). G-I, Outer ring of secondarily widened primary osteons in humerus SMNS 54569 (Placodontia indet. aff. Cyamodus). J-L, Enlargment of the same section. M, Secondarily widened primary osteons in humerus SMNS 15937, in humerus MHI 1096, and in humrus MHI 2112-6 (all (Placodontia indet. aff. Cyamodus).

Downloaded from Brill.com10/04/2021 05:33:52AM via free access