Biological Journal of the Linnean Society, 2013, 108, 3–21. With 4 figures


Bone histology of aquatic : what does it tell us about secondary to an aquatic life?


Steinmann Institut für Geologie, Paläontologie und Mineralogie, Universität Bonn, Nussallee 8, 53115 Bonn,

Received 29 May 2012; revised 5 July 2012; accepted for publication 5 July 2012

Aquatic reptiles are very diversified in the record. The description and pooling of certain histological features (collagenous weave and vascular network) of the various groups of aquatic reptiles highlight what this histological information can tell us about the process of secondary adaptation to an aquatic life. Notably, they show the absence of interaction between these histological features on the one hand and body size, mode of swimming, type of microanatomical specialization and phylogeny on the other. These histological features in aquatic reptiles seem to essentially provide information about the growth rate and metabolic rate of these taxa. The growth rate seems to have been rather high in most marine reptiles, when compared with terrestrial ectotherms. Moreover, distinct metabolic abilities are suggested. Indeed, various groups probably displayed a peculiarly high body temperature, and some show trends towards endothermy. This study also emphasizes the crucial need for homologous comparisons in histology and shows that much remains to be done to better understand the relationship between histological features, growth rate and metabolism in extant taxa in to make inferences in the fossil groups. © 2012 The Linnean Society of London, Biological Journal of the Linnean Society, 2013, 108, 3–21.

ADDITIONAL KEYWORDS: collagenous weave – growth rate – metabolism – vascular network.

INTRODUCTION 2005; Erickson et al., 2009; Sanchez et al., 2010). His- tological features have been analysed in several Extant aquatic reptiles are rather scarce, especially if aquatic taxa (see below). However, these we consider only the essentially (spending much of studies have generally focused on a single group (see their time in water), or even exclusively, aquatic taxa, below). It is of particular interest to review these consisting of the highly aquatic and snakes various data in order to better understand what bone (the being essentially terrestrial). histology can tell us about the process of secondary However, they were much more diverse in the fossil adaptation to an aquatic life. This is the object of this record, especially during the Mesozoic. Indeed, study, which focuses on two main histological fea- various groups of reptiles illustrating distinct mor- tures: (1) the organization of the collagenous weave phologies and degrees of adaptation to an aquatic life [to distinguish between lamellar, parallel-fibred and were secondarily adapted to aquatic environments fibrous (woven-fibred) bone]; and (2) the organization (cf. Mazin, 2001; Fig. 1). of the vascular network. The terminology follows Bone histology is one of the major sources of infor- Francillon-Vieillot et al. (1990). The histological fea- mation about life history traits (e.g. Sander & Klein, tures described for the various aquatic reptiles (based on adult specimens, except when precised; cf. Fig. 1) are listed in order to be interpreted, based on phylo- *E-mail: [email protected] genetic, functional and physiological perspectives.

© 2012 The Linnean Society of London, Biological Journal of the Linnean Society, 2013, 108, 3–21 3 4 A. HOUSSAYE

BONE HISTOLOGY OF AQUATIC REPTILES made up of lamellar bone with lines of arrested MESOSAURIDAE growth (LAGs); it is poorly vascularized with longi- tudinally oriented simple vascular canals and a few Histological data on mesosaurs are rather scarce. primary osteons (Fig. 2A, B), which were described as They are based on the analysis of thin sections and organized in concentric rows by Nopcsa & Heidsieck fragments of a few ribs (Nopcsa & Heidsieck, 1934; (1934). The medullary region is occupied by a spon- Kaiser, 1960; de Ricqlès, 1974), and of one long bone giosa made of rather thick, irregularly oriented (? tibia), carpals and tarsals (de Ricqlès, 1974). In trabeculae of lamellar bone and of intertrabecular both the ribs and the long bone, the cortex is thick (as remains of calcified cartilage (de Ricqlès, 1974; a result of pachyosteosclerosis). Periosteal bone is Fig. 2A). In carpals and tarsals, the microstructure is

Figure 1. Consensual phylogeny illustrating the relationships between the various groups of marine reptiles. From Modesto & Anderson (2004), Tsuji & Müller (2009) and Scheyer, Klein & Sander (2010).

᭤ Figure 2. A, Mesosaurus brasiliensis. Rib transverse section (TS) in natural light (NL). From de Ricqlès (1974). B, Mesosaurus rib TS in NL. From Kaiser (1960). In both (A) and (B), the cortex is at the top and the medullary cavity is at the bottom. C, germaini. Rib TS in polarized light (PL) (left) and NL (right) (Personal photograph). Note the clear limit between the compact primary cortex and the medullary cavity. D, E, () adult TS (D) and detail of the transition between the cortex (top) and the medullary region (bottom) in NL (E). From de Buffrénil et al. (1990). F, humerus TS in NL. Cortex. From de Buffrénil & Mazin (1990). G, Ichthyosaurus humerus TS in PL. Cortex. (Personal photograph) H, I, Dermochelys coriacea. H, Tibia TS. From Kriloff et al. (2008). I, Femur cortex. From de Ricqlès et al. (2004). Abbreviations: cc, calcified cartilage; ds, dense spongiosa; fb, fibrous bone; lb, lamellar bone; lzb, lamellar-zonal bone; po, primary osteon; sb, secondary bone; svc, simple vascular canal.

© 2012 The Linnean Society of London, Biological Journal of the Linnean Society, 2013, 108, 3–21 BONE HISTOLOGY OF AQUATIC REPTILES 5 similar, except that the cortex is thin and avascular. CLAUDIOSAURUS A few secondary osteons occur in the perimedullar region of the long bone as a result of remodelling (de de Buffrénil & Mazin (1989) described the bone his- Ricqlès, 1974). tology of limb (humerus, femur and tibia), ribs

© 2012 The Linnean Society of London, Biological Journal of the Linnean Society, 2013, 108, 3–21 6 A. HOUSSAYE and vertebrae of this taxon. Their primary periosteal the ontogenetic stage, as a result of additional sec- bone consists of lamellar bone with LAGs and with a ondary lamellar bone deposits during remodelling, limited vascular supply made up of thin simple vas- which fill the cavities and intertrabecular spaces in cular canals, mainly longitudinally oriented (Fig. 2C). Simoedosaurus (de Buffrénil et al., 1990). These bones are characterized by osteosclerosis caused by excessive deposits of secondary lamellar bone during intense remodelling, which mainly occurs HUPEHSUCHIA in the medullary region of long bones and ribs, and in No data on the histology of Hupehsuchia are the core of the vertebral centrum, as well as in the available. inner cortex (Fig. 2C). Secondary osteons are numer- ous in these regions. Histological features of ichthyosaurs are known from the study of limb bone, and rib sections of Within Younginiformes (although the of five taxa: , , Ichthyo- the group is questioned; Bickelmann et al., 2009), two saurus, and (Kiprijanoff, taxa are adapted to a marine environment and are 1881–1883; Fraas, 1891; Seitz, 1907; Gross, 1934; de thus taken into consideration here: and Buffrénil, Mazin & de Ricqlès, 1987; de Buffrénil & (Carroll, 1988). No histological data are Mazin, 1990; Lopuchowycz & Massare, 2002; Kolb, known for these taxa. Sánchez-Villagra & Scheyer, 2011). Periosteal bone is spongious with a zonal organization of circumferen- tial rows of numerous simple vascular canals and CHORISTODERA primary osteons, with a mainly longitudinal, but also The histology of the vertebrae, femora and ribs of radial and sometimes oblique, orientation, giving Champsosaurus and Simoedosaurus has been studied the bone a plexiform aspect (de Buffrénil & Mazin, (Nopcsa & Heidsieck, 1934; de Ricqlès, 1976a; de 1990; Kolb et al., 2011; Fig. 2F). Periosteal bone cor- Buffrénil et al., 1990; Katsura, 2010). In these bones, responds to fibrous bone at rapid stages of growth the cortex is essentially made up of parallel-fibred (Fig. 2G) and to parallel-fibred bone much later in bone – associated with lamellar bone in the vertebrae ontogeny. In large specimens, the periphery of the – with LAGs and a few simple vascular canals cortex consists of a thin layer of compact bone dis- (Fig. 2D, E). Fibrous bone generally occurs in the core playing a few, longitudinally oriented simple vascular of the bones. Vascular canals are very scarce and canals and primary osteons (de Buffrénil & Mazin, radially oriented in the vertebrae. In femora and ribs, 1990; Kolb et al., 2011). In the cortical region, the vascularization is more extensive and displays a lon- trabeculae consist of a core of fibrous bone covered by gitudinal orientation in the deep cortex; vascular platings of lamellar bone (Fig. 2G). In the medullary density decreases in the periphery, where the canals region, trabeculae are rather thin and mainly formed are rather radially oriented. The medullary region of by secondary lamellar bone deposits, as a result of the ribs and femora, which is much larger in adults intense remodelling. Intertrabecular spaces (in both than in juveniles, exhibits a dense spongiosa with cortical and medullary regions) are rather large thick trabeculae of lamellar bone (Fig. 2D, E); small (Fig. 2F, G), except in the outer cortex. Remodelling islands of calcified cartilage occur in juvenile and of the inner cortex is characteristically imbalanced, adult femora and in juvenile ribs. A small medullary as bone resorption is more intense than bone recon- cavity occurs in the centre of juvenile femora struction (de Buffrénil & Mazin, 1990); secondary (Fig. 2D), unlike in ribs. The general compactness of osteons have only been described in the cortex of the femora increases during ontogeny, as the amed- Omphalosaurus femur (de Buffrénil et al., 1987). A ullar but more porous condition of adult femora is small medullary cavity is observed in Omphalosaurus more compact than the juvenile condition, character- long bones; it is absent in the other taxa. Seitz (1907) ized by a thick cortex and a small medullary cavity and Gross (1934) have described distinct zones and (Katsura, 2010). Vertebral centra consist entirely of a annuli in the cortex of ribs of Ichthyosaurus and spongiosa. Trabeculae are long and longitudinally Stenopterygius. A cyclical growth pattern has also oriented in the periosteal territory, whereas they been described in Omphalosaurus long bones (de Buf- are shorter and randomly oriented in the denser frénil & Mazin, 1990), and clear growth marks in endosteo-endochondral territory, where they consist of Mixosaurus long bones and other skeletal elements a core of calcified cartilage with endosteal platings of (gastral ribs; Kolb et al., 2011), but not in the fast- lamellar bone. Bone compactness is always greater in growing post- ichthyosaurs (de Buffrénil & Simoedosaurus than in Champsosaurus, whatever Mazin, 1990). Local remains of calcified cartilage

© 2012 The Linnean Society of London, Biological Journal of the Linnean Society, 2013, 108, 3–21 BONE HISTOLOGY OF AQUATIC REPTILES 7 occur in the medullary region of long bones of absent. Periosteal bone is described as generally Mixosaurus (Kolb et al., 2011). highly vascularized (especially in its inner part) with very large longitudinal canals (mainly simple vascu- lar canals but also a few osteons) arranged in con- centric rows and a few radially oriented ones, which No data on the histology of Thalattosauria are gives the tissue a plexiform organization (de Buffrénil available. & Mazin, 1992; Fig. 3A). The primary bone tissue consists of fibrous bone in the zones of rapid growth and of parallel-fibred bone when growth slows down TESTUDINES (de Buffrénil & Mazin, 1992; Klein, 2010). In the de Ricqlès (1976a: 126) wrote: ‘as already noted medullary region, the spongiosa is dense. It is made by Foote (e.g. 1916) the bones of chelonians have up of randomly oriented, short and thick trabeculae more complex and varied histological patterns than of lamellar bone. Remodelling appears to be rather does any other living reptilian order’. Dermal plates, limited. carapaces and plastra generally display an inner Scheyer (2007) described in placodont dermal armor spongious structure surrounded by periosteal layers the formation and retention of cartilaginous tissue of compact bone (de Ricqlès, 1976a). The latter gen- (which has never been observed in any other ) erally corresponds externally to metaplastic bone and the absence of secondary bone remodelling. (interwoven structural fibre bundles of the dermis) and internally to parallel-fibred bone with a variable degree of vascularization (Scheyer & Sánchez- PACHYPLEUROSAURIA Villagra, 2007; Scheyer & Sander, 2007). Scheyer & Klein (2010) and Hugi et al. (2011) recently Sanchez-Villagra (2007) showed that the shell bone described in detail the histology of histology of the marine Archelon differs strik- long bones (which had been investigated previously ingly from that of other turtles in that there is by, for example, Nopcsa & Heidsieck, 1934; Zangerl no clear distinction between a peripheral layer of & Peyer, 1935; de Ricqlès, 1976a; Sander, 1989). The compact bone and an inner spongiosa, but rather a cortex is thick (as a result of pachyosteosclerosis; homogeneous spongiosa. The cortex is described as Fig. 3B). primary cancellous with cyclical growth marks. Klein (2010) described a histotype that she Remodelling then causes the enlargement of the assigned to (see also Klein, in press). In intertrabecular and vascular spaces (Scheyer & the latter, periosteal bone essentially consists of Sánchez-Villagra, 2007). incipient fibrolamellar bone (i.e. parallel-fibred bone Long bones display a medullary region with no or with a large amount of woven bone, and with some with a rather small medullary cavity (Laurin et al., simple vascular canals and not only primary osteons; 2011). In long bones, primary bone generally con- sensu Klein, 2010; Fig. 3C) with a high vascular sists of parallel-fibred bone tissue with LAGs density, when compared with that observed in other and various degrees of vascularization (generally (cf. Hugi et al., 2011; Fig. 3D, E). simple vascular canals longitudinally oriented and The vascular network displays a dominant radial arranged into concentric rows) within a single orientation (Fig. 3C). Three subtypes were defined on bone and between bones (de Ricqlès, Castanet & the basis of distinct growth patterns. Subtype 1 is Francillon-Vieillot, 2004; A. H, pers. observ.). Haver- characterized by a continuous, unordered vascular sian reconstruction can be observed in the inner network until the outer cortex, which consists of cortex (de Ricqlès, 1976a; de Ricqlès et al., 2004). lamellar-zonal bone. Subtype 2 displays distinct Few histological data are available for aquatic LAGs throughout the cortex. In subtype 3, vascular turtles. The long bones of the Dermoche- canals are organized in rows, but true growth marks lys are entirely spongious (Fig. 2H). Vascular canals do not occur. All display a small medullary cavity are numerous, longitudinally oriented and organized lined by a thin layer of endosteal bone. in concentric rows (Fig. 2H, I). In the other pachypleurosaurs studied, the core of the medullary region contains abundant remains of calcified cartilage which are partially (or even entirely) replaced by endosteal lamellar bone during The histological features of the humerus of ontogeny (Hugi et al., 2011; Fig. 3B). Periosteal bone have been described by de Buffrénil & Mazin (1992), consists of a mixture of woven-fibred and parallel- and those of the humeri and femora of placodonts fibred bone at an early ontogenetic stage, and later indet. by Klein (2010). The cortex is very thick (as a only of lamellar-zonal bone (either parallel-fibred result of osteosclerosis) and the medullary cavity is or lamellar bone; Sander, 1989; Hugi et al., 2011;

© 2012 The Linnean Society of London, Biological Journal of the Linnean Society, 2013, 108, 3–21 8 A. HOUSSAYE

Figure 3. A, Placodus humerus transverse section (TS) in natural light (NL). From de Buffrénil & Mazin (1992). B, (Pachypleurosauria) femur TS in NL. From de Ricqlès & de Buffrénil (2001). C, Anarosaurus heterodontus (Pachypleurosauria) humerus. Fibrolamellar bone in NL. From Klein (2010). D, mirigiolen- sis (Pachypleurosauria) long bone TS in NL. From Hugi (2011). E, Neusticosaurus edwardsii (Pachypleurosauria) long bone TS in NL. From Hugi (2011). F, femur. Detail of the cortex (incipient fibrolamellar bone with simple vascular canals) in NL. From Klein (2010). G, Nothosaurus long bone TS in NL. Detail of the cortex. From Hugi (2011). H, I, Cryptocleidus () rib TS (personal photographs). H, External cortex showing a peripheral (limit of the bone shown as broken line) layer of parallel-fibred bone. I, Cortex with secondary osteons. Abbreviations as in Figure 2; iflb, incipient fibrolamellar bone; lc, longitudinal canal; rc, radial canal; so, secondary osteon; tc, thick cortex.

© 2012 The Linnean Society of London, Biological Journal of the Linnean Society, 2013, 108, 3–21 BONE HISTOLOGY OF AQUATIC REPTILES 9

Fig. 3D, E). Vascularization consists of simple vascu- CYMATOSAURIDAE lar canals and only a few primary osteons, which are Klein (2010) described a histotype that she assigned to radially and longitudinally oriented and show a mod- or to a closely related taxon. Primary erate density (Fig. 3D, E). Remodelling is rather periosteal bone essentially consists of parallel-fibred limited (Hugi et al., 2011). bone with LAGs and with a moderate to high vascular density (lower than in the pachypleurosaur Anarosau- rus, but higher than in ) made up of simple vascular canals and primary osteons (whose number No data about the histology of Simosaurus are increases with ontogeny). In the inner cortex, parallel- known. fibred bone with mainly longitudinally oriented vascu- lar canals indicates a relatively slow initial phase of growth. In the middle cortex, the dominant radial NOTHOSAURIA orientation of the vascular network and the occurrence de Ricqlès (1976a) described the microanatomical of distinct layers of incipient fibrolamellar bone char- and histological features of nothosaurs (in fact, he acterize a faster growth phase. The outer cortex gen- included both nothosaurs and pachypleurosaurs) as erally contains large amounts of lamellar bone and a rather similar to those of mesosaurs. This was based dominance of longitudinally oriented vascular canals. on the presence of pachyostosis in the ribs, the per- Sometimes, a second phase of faster growth occurs. sistence of calcified cartilage in the middle part of the The medullary region is completely filled or thickly shaft and the presence of lamellar periosteal bone lined by endosteal bone. with numerous longitudinally oriented primary osteons arranged in concentric circumferential rows. This zonal pattern was also described by Gross PISTOSAURIDAE (1934). Two main histotypes occur within nothosaurs. In Krahl et al. (2009) described lamellar-zonal bone in histotype 1, as illustrated by , perio- the femur and fibrolamellar bone in the humerus of steal bone in long bones mainly consists of lamellar to . The vascular pattern also varies between parallel-fibred bone with LAGs, although a large these two bones. It consists of longitudinally and amount of woven-fibred bone occurs at an early radially oriented simple vascular canals and primary ontogenetic stage (Hugi, 2011). The vascular network osteons that are abundant in the humerus, but much shows a low to moderate density of radially and scarcer in the femur (N. Klein, Bonn Universität, longitudinally oriented simple vascular canals and Germany pers. comm.). primary osteons. The medullary region is filled with calcified cartilage remains and lamellar bone deposits (Hugi, 2011). Remodelling is rather limited (Hugi, PLESIOSAURIA 2011). The bone histology of various plesiosaurs has been In histotype 2, illustrated by Nothosaurus, primary studied by, for example, Kiprijanoff (1881–1883), bone essentially consists of parallel-fibred bone. Wiffen et al. (1995), Fostowicz-Frelik & Gazdzicki Irregular intercalation of layers of incipient fibrola- (2001), Salgado, Fernandez & Talevi (2007) and mellar bone in parallel-fibred bone occur (Krahl, Street & O’Keefe (2010). Pachyosteosclerosis was Sander & Klein, 2009; Klein, 2010; Hugi, 2011; described in juveniles, as opposed to a spongious Fig. 3F, G). Lamellar bone is observed in zones of (osteoporotic-like) inner structure in adults (Wiffen slower growth. Vascularization is moderate and et al., 1995). Periosteal bone in long bones, ribs and essentially consists of longitudinally oriented simple vertebrae of adults consists of woven-fibred bone with vascular canals and primary osteons (Fig. 3F, G). a plexiform-like or radiating-like vascular network Within Nothosaurus, four morphotypes are known during the rapid phases of growth, and of moderately (Klein, 2010). Two are distinguished on the basis of vascularized parallel-fibred zonal bone tissue when histology, more precisely the growth pattern: morpho- growth slows down (at a relatively late ontogenetic type II displays alternating phases of fast and slow stage; Fig. 3H; Wiffen et al., 1995). A rather dense growth, whereas growth speed decreases continuously Haversian system occurs in most of the cortex as a in morphotype IV (Klein, 2010). result of strong remodelling (Wiffen et al., 1995; The thickness of the cortex varies widely between Fig. 3H, I). In the inner cortex, trabeculae are mainly taxa and with ontogeny. The medullary cavity is made up of endosteal lamellar bone, but sometimes much larger in Nothosaurus (Hugi, 2011). It is lined show a core of primary woven-fibred bone (Salgado by a thin irregular layer of endosteal bone with et al., 2007); in the medullary region (or endosteo- remains of calcified cartilage (Klein, 2010). endochondral territory in vertebrae), they mainly

© 2012 The Linnean Society of London, Biological Journal of the Linnean Society, 2013, 108, 3–21 10 A. HOUSSAYE consist of secondary lamellar bone with only a few (2007) noted that secondary osteons occur in the remains of calcified cartilage persisting (Wiffen et al., trabeculae of the spongiosa in some mosasauroid long 1995). bones.

RHYNCHOCEPHALIA [= SPHENODONTIA] HELVETICOSAURUS A few adapted to an aquatic life, No data about the histology of this taxon, whose and are thus taken into consideration in this review: phylogenetic position remains uncertain, are Ankylosphenodon (Reynoso, 2000) and, although available. without strong evidence, Sapheosaurus. No histologi- cal data regarding these taxa are available. PHYTOSAURIA The histological features of femora have been described by de Ricqlès, Padian & Horner (2003) The gross bone histological features of squamates are and Bronowicz (2009). They display a free medullary now relatively well known, as a result of the detailed cavity. The cortex is dominated by lamellar-zonal analysis of their long bones (e.g. Castanet, 1974; de bone (essentially parallel-fibred bone) with zones dis- Ricqlès, 1976a; de Buffrénil, Houssaye & Böhme, playing a moderate network of longitudinally oriented 2008; Hugi & Sánchez-Villagra, in press) and verte- primary osteons (with scarce anastomoses; subplexi- brae (de Buffrénil & Rage, 1993; Houssaye et al., form organization) and with nearly avascular annuli. 2010). Squamate periosteal bone basically consists of In the zones, woven bone occasionally occurs; this parallel-fibred or lamellar bone with LAGs. Vasculari- bone tissue is thus considered here as incipient fibro- zation is only present in large (e.g. Python, lamellar bone sensu Klein (2010). Secondary osteons Eunectes, Varanus) as scarce radially oriented simple are frequent and show that remodelling is intense, vascular canals (Fig. 4A). In long bones, the medul- especially in the deep cortex (de Ricqlès et al., 2003). lary cavity is surrounded by some endosteal deposits Phytosaur display a trabecular inner of lamellar bone. This is the general trend observed in organization surrounded by a thick cortex of lamellar- all extant groups, even in the partially or exclusively zonal bone (Scheyer & Sander, 2004). aquatic taxa (e.g. Amblyrhynchus cristatus, Hydro- phis sp., Pelamis paltura; Houssaye et al., 2010). However, some aquatic fossil groups show peculiari- ties on the histological level in addition to the pres- In extant crocodylomorphs, which belong to the ence of pachyostosis and osteosclerosis (A. H, pers. Eusuchia, periosteal bone in long bones essentially observ.): a high degree of vascularization is observed consists of lamellar-zonal bone (Fig. 4D), with only the in vertebrae of some shallow water forms within zones being vascularized (de Ricqlès et al., 2003; Klein, varanoid and snakes, such as Pachyvaranus Scheyer & Tütken, 2009). In the latter, the limited (de Buffrénil et al., 2008), Carentonosaurus (Houssaye vascular network consists of longitudinally oriented et al., 2008; Fig. 4B), plesiopelvic (with terrestrial-like simple vascular canals (with some few anastomoses) pelvis) mosasauroids (Houssaye, 2008, in press), Pal- arranged in concentric rows (Fig. 4D). Primary osteons aeophis (A. H, pers. observ.) and Simoliophis (de occur essentially in the deep cortex (de Ricqlès et al., Buffrénil & Rage, 1993; ‘Pachyvaranus-like’ histotype 2003). Fibrolamellar bone has also been described with in Table 1). No long bone of these taxa has been a reticular vascularization pattern during the initial analysed. Moreover, a distinct pattern of vasculariza- rapid phase of growth (Woodward, Horner & Farlow, tion occurs in large hydropelvic (absent ) 2011), but also beyond (Tumarkin-Deratzian, 2007), in mosasauroids: although radially oriented simple vas- some captive and wild specimens. cular canals are locally observable in adults and occur Osteoderms display a diploe structure with compact in juveniles (Houssaye & Tafforeau, 2012), vasculari- cortical bone surrounding a spongiosa. The cortex zation generally consists of longitudinally oriented essentially consists of parallel-fibred bone (few lamel- (feebly radially and circumferentially anastomosed) lar bone) with LAGs and a few, scattered simple primary osteons or simple vascular canals in consecu- vascular canals. The spongiosa is highly remodelled tive layers (Houssaye & Bardet, 2012; Fig. 4C). This and consists of lamellar bone with only a few remains pattern, described in vertebrae, also occurs in long of interwoven structural fibre bundles (Klein et al., bones (A. H, pers. observ.). In both juvenile and adult 2009). hydropelvic mosasauroids, most of the section con- In vertebrae of the dyrosaurid Dyrosaurus, perio- sists of a spongiosa and the compact layer of cortical steal bone consists of poorly vascularized lamellar- bone is rather thin. de Ricqlès (1976a) and Pellegrini zonal bone and Haversian remodelling is active

© 2012 The Linnean Society of London, Biological Journal of the Linnean Society, 2013, 108, 3–21 BONE HISTOLOGY OF AQUATIC REPTILES 11

Figure 4. A, Varanus niloticus femur transverse section (TS) in polarized light (PL). Cortex (lamellar-zonal bone). From de Buffrénil et al. (2008). B, Carentonosaurus mineaui (varanoid ) vertebral TS in natural light (NL) showing an extremely thick and highly vascularized compact cortex. (Personal photograph) C, Clidastes propython (hydropelvic mosasauroid) vertebral TS in NL. Cortex with circumferential rows of longitudinal canals. Modified from Houssaye & Bardet (2012). D, mississipiensis femur TS in NL. Cortex with circumferential rows of longitudinally oriented simple vascular canals. From de Ricqlès et al. (2004). E, Steneosaurus sp. () humerus TS in NL. Cortex (outer cortex at the top) showing the increase in vascularization towards the inner cortex and the numerous secondary osteons. From Hua & de Buffrénil (1996). Abbreviations as in Figures 2 and 3. pfb, parallel-fibred bone; ra, radial anastomosis.

(Buffetaut et al., 1982). The inner spongious organi- of long bones as a result of remodelling (Hua & de zation is made up of thin and scarce trabeculae (Buf- Buffrénil, 1996; Fig. 4E). fetaut et al., 1982). Two histologies were observed in teleosaurid (Tha- In thalattosuchian long bones, ribs and vertebrae, lattosuchia) osteoderms: (1) a ‘small and thin osteo- periosteal bone consists of parallel-fibred bone with derm histotype’, characterized by poorly vascularized, LAGs and with a medium to dense vascular network parallel-fibred periosteal bone with LAGs surround- of both simple vascular canals and primary osteons ing a highly remodelled spongiosa; and (2) a ‘large (Fig. 4E). Secondary osteons occur in the deep cortex and massive histotype’, characterized by

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Table 1. Summary of the main features cited for the various groups at the adult stage HOUSSAYE A.

Taxon CW VD VMO TS SM MS Bone Milieu

Mesosauridae LZB (LB) P L/CR 0.3 to 1 m (Carroll, 1988) A BMI T? SM Claudiosaurus LZB (LB) P Mainly L ~60 cm (Carroll, 1988) A BMI H, F, T SM Younginiformes X X X Up to 1.5 m (Carroll, 1988) A BMI X Amph/SM Choristodera LZB (mainly PFB) P R or L Up to 3 m (Mazin, 2001) SC BMI F Fresh 02TeLnenSceyo London, of Society Linnean The 2012 © Hupehsuchia X X X < 1 m (Carroll & Dong, 1991) SC BMI X SM FLB H L/CR < 1mto> 20 m (Motani, 2005) SC T OLP H, F, T SM/OM (plexiform) Thalattosauria X X X Up to 4 m (Mazin, 2001) A No/BMI X SM Extant aquatic turtles LZB (PFB) M/H L/CR Up to 2 m P No/OLP F Fresh/SM/OM Placodus FLB H L/CR Up to 2 m (Klein, 2010) SC BMI H, F SM (plexiform) Pachypleurosauria (most) LZB (PFB & LB) M R or L 0.2 to 1.2 m (Klein, 2010) A BMI H, F SM Anarosaurus LZB (PFB) & iFLB H Mainly R Up to 50–60 cm (N. Klein, A BMI H, F SM (Pachypleurosauria) pers. comm.) Simosaurus X X X 3–4 m (Rieppel, 1994) A BMI X SM Nothosaurus (Nothosauria) LZB (PFB) & iFLB M L Up to 6 m (Klein, 2010) A? OLP H, F OM? Ceresiosaurus (Nothosauria) LZB (LB to PFB) L to M R & L Up to 3 m (Hugi, 2011) A BMI H, F SM Cymatosauridae LZB (PFB) & iFLB M to H R & L Up to 3 m? (N. Klein, pers. P BMI H, F OM? comm.) ilgclJunlo h ina Society Linnean the of Journal Biological Pistosauridae FLB LZB H/M R,R&L 3to4m(Klein, 2010) P X H F OM? Plesiosauria FLB High L/CR 3 to 15 m (Mazin, 2001) P BMI/OLP H SM/OM (plexiform) Aquatic Rhynchocephalia X X X X A BMI X SM Extant aquatic squamates LZB (LB to PFB) No/L R X A No H, F Fresh/SM/OM Pachyvaranus-like histotype LZB (PFB) H R > 1 m to 2 m (Houssaye, in A BMI V SM press) Hydropelvic mosasauroids LZB (PFB) H L/CR 3 to 15 m (Houssaye & SC OLP H OM (plexiform) Tafforeau, 2012) Helveticosaurus XXX~2 m (Carroll, 1988) A No X SM Phytosauria LZB & iFLB M L (subplexiform) Up to 5 m (Mazin, 2001) SC No F SM Thalattosuchians LZB (PFB) M to H L Up to 5 m (Young et al., 2011) C No H, T Amph/SM Dyrosaurus LZB L X Up to 6.5 m (Jouve, 2004) SC No V Amph/SM Eusuchia LZB (PFB) L to M L Up to 12 m SC No F Fresh/Amph/OM

2013, , The data especially concern long bones. CW, type of collagenous weave (FLB, fibrolamellar bone; iFLB, incipient fibrolamellar bone; LB, lamellar bone; LZB, lamellar-zonal bone; PFB, parallel-fibred bone); VD, vascular density (H, high; L, low; M, moderate; P, poor); VMO, vascular main orientation (CR, concentric rows; L, longitudinal; R, radial); TS, taxon size; SM, swimming mode (A, anguilliform; SC, subcarangiform; C, carangiform; T, thunniform; P, paraxial); MS, microanatomical specialization 108 (BMI, bone mass increase; OLP, osteoporotic-like pattern); for MS and Milieu, see references in Houssaye (2009); H, humerus; F, femur; T, tibia; V, vertebra; 3–21 , Amph, amphibious; Fresh, freshwater; OM, open marine; SM, shallow marine; X, no data available. BONE HISTOLOGY OF AQUATIC REPTILES 13 woven-fibred, highly vascularized periosteal bone sur- the outer cortex (de Buffrénil et al., 1990). However, it rounding a spongiosa almost deprived of secondary must also be noted that the bone growth rate is only deposits. one factor among others explaining the vascular dis- Woven-fibred bone with a dense plexiform vascular tribution. Indeed, it has been suggested that blood network occurs in the of juvenile Metriorhyn- supply and bone size may also be important factors chus, unlike in adults (Hua & de Buffrénil, 1996). (Montes, Castanet & Cubo, 2010). Fibrous bone is observed in almost all aquatic reptiles at an early ontogenetic stage, but it is its occurrence and clear prevalence over LZB (mainly DISCUSSION restricted to the outermost cortex) in later ontogenetic BONE TISSUES, GROWTH RATE AND SIZE stages that characterize taxa that mainly deposit As stated by de Ricqlès (1976b), two main types of FLB. Various patterns of the collagenous weave and primary periosteal bone can be distinguished: vascular network are observed in aquatic reptiles. lamellar-zonal bone (LZB; cf. ‘type I’ of Foote, 1916; They illustrate a wide range of growth rates after the ‘zonarer Periostknochen’ of Gross, 1934) and fibrola- rapid early phase of growth. Concerning the growth mellar bone (FLB; cf. ‘type II’ of Foote, 1916; ‘lamin- rate, more importance is generally given to the colla- arer Periostknochen’ of Gross, 1934). de Ricqlès genous weave (LZB depositing more slowly than (1976b) observed that LZB – either lamellar or FLB) and, only secondarily, to the vascular network parallel-fibred bone – is generally poorly vascularized (density and organization; de Margerie et al., 2002; with scarce (or even absent), scattered simple vascu- Starck & Chinsamy, 2002). lar canals and rare primary osteons, whereas vascu- Comparisons of the features of the collagenous larization is dense or very dense with numerous weave and vascular network in the diverse groups of osteons in FLB – with a fibrous collagenous weave. aquatic reptiles should enable their growth rate to be This statement is consistent with that observed in compared. However, growth rate is known to vary ichthyosaurs, for example (see above). However, some between bones – as illustrated by the differences taxa, notably hydropelvic mosasauroids, display observed between the humerus and femur of pisto- (except at an early ontogenetic stage) LZB with a high saurs – and even within a single bone. Moreover, degree of vascularization and numerous primary Starck & Chinsamy (2002) and de Margerie et al. osteons. (2004) have observed that the relationship between a Bone histological features, notably the organization given bone tissue and its growth rate may vary of the collagenous weave and the pattern of vascu- between different bones of the skeleton. larization, inform about the rate of bone deposition, Histological data concerning aquatic reptiles are so and thus of growth (Amprino, 1947; de Margerie, far too scarce to perform precise homologous compari- Cubo & Castanet, 2002). Growth rate increases with sons at a large scale. For example, histological data of the degree of vascularization. However, it decreases long bones are missing for many fossil varanoid with the degree of organization of the collagenous lizards and the only data available essentially fibres (from fibrous to lamellar bone). LZB, which is concern vertebrae. Despite this difficulty, the combi- encountered in extant terrestrial reptiles (cf. Hous- nation of the histological data listed above enables saye et al., 2010), deposits much more slowly than some trends to be observed within aquatic reptiles, FLB (Amprino, 1947; de Margerie et al., 2002), which which must nevertheless be taken with caution for is generally encountered in rapid-growing mammals the reasons cited above. and , in extinct synapsids, and ptero- Long bones are the bones for which data are the saurs (e.g. Enlow & Brown, 1956, 1957, 1958; de most abundant. Their analysis suggests that the Ricqlès, 1976b; Padian, de Ricqlès & Horner, 2001; highest growth rate within aquatic reptiles is Ray, Botha & Chinsamy, 2004; Chinsamy, Codorniú & observed in some ichthyosaurs, plesiosaurs and pla- Chiappe, 2009). It is considered that tissues with codonts (Placodus), which are characterized by FLB longitudinal canals organized in concentric rows indi- with a dense vascular network displaying a plexiform cate a relatively slower growth than tissues with organization. The humerus of pistosaurs displays more radial canals (de Ricqlès, 1976b; de Margerie similar histological features. The pachypleurosaur et al., 2004). This is consistent with the variations in Anarosaurus, the Nothosaurus, cymatosau- the vascular pattern observed in hydropelvic mosas- rids and show LZB associated with incipi- auroids during ontogeny (Houssaye & Tafforeau, ent FLB (iFLB), which indicates slower, although 2012), although it does not seem in accordance with relatively high, growth rates. Among them, various the observations made on Champsosaurus femora, i.e. vascular patterns occur: Anarosaurus displays a the occurrence of longitudinally oriented vascular dense network of essentially radially oriented vascu- canals in the deep cortex, as opposed to radial ones in lar canals, whereas cymatosaurids, Nothosaurus and

© 2012 The Linnean Society of London, Biological Journal of the Linnean Society, 2013, 108, 3–21 14 A. HOUSSAYE phytosaurs display a moderate to high density of both istics in these taxa. The faster growth rates of Der- radial and longitudinal (cymatosaurids) and of essen- mochelys and Archelon, as a potential advantage tially longitudinal (Nothosaurus, phytosaur) canals. relative to a protracted slower growth, might have Some late shallow marine squamates been linked to environmental or ecological factors/ (with the Pachyvaranus-like histotype) display (only) pressures (see Heupel, Carlson & Simpfendorfer, LZB – indicating a lower growth rate – with a dense 2007; Heithaus et al., 2008; Jaffe, Slater & Alfaro, network of radially oriented vascular canals. As these 2011), such as the occurrence of predators in the life data are based on vertebrae and not long bones, and environment and the pelagic lifestyle. as vertebrae appear to grow more slowly than limb bones (A. H, pers. observ.), they illustrate a minimal growth rate for these taxa in the framework of this BONE TISSUES AND RESTING METABOLIC RATE comparison. Dermochelys and hydropelvic mosasau- Bone growth rate is considered to be partially linked roids display a dense network of longitudinally to the resting metabolic rate (Montes et al., 2007). It oriented vascular canals. This is also the case of also appears that the maximum possible bone growth thalattosuchians, although their vascular network is rate is constrained by the resting metabolic rate less dense. Most pachypleurosaurs and the nothosaur (Cubo et al., 2008). Numerous studies have evoked Ceresiosaurus display a lower vascular density with this link between bone histology and metabolic rate both radial and longitudinal canals. The remaining (e.g. de Ricqlès, 1992; Padian et al., 2001). However, taxa (Mesosauridae, Claudiosaurus, Choristodera, as this link is considered to be indirect, several extant aquatic squamates and Eusuchia) are charac- researchers have specified that, although reasonable terized by a poorly (or not) vascularized LZB with inferences can be made, caution should be taken either longitudinally (Mesosauridae, Claudiosaurus, when using bone tissues to infer about the thermal Eusuchia) or radially (extant aquatic squamates) ori- physiology and metabolic rate (Reid, 1987; Chinsamy- ented vascular canals, or both (Choristodera). This Turan, 2005). poor vascularization, which corresponds to that The various histological patterns (collagenous observed in extant terrestrial ectotherms (e.g. Hous- weave and vascular network) observed in aquatic saye et al., 2010), yields the slowest growth rates reptiles should thus in part reflect different metabolic among aquatic reptiles. It is interesting to note that rates between these organisms. For example, FLB all freshwater forms, although data are missing for occurs in various aquatic reptiles, including some – freshwater turtles, display this terrestrial ectotherm- but maybe not all (especially concerning placodonts) – like pattern. However, the growth rate seems to be ichthyosaurs, plesiosaurs and placodonts. This tissue, significantly higher in most marine reptiles, a trend which indicates a high growth rate (see above), has that has not been highlighted in mammals and birds also been considered to indicate a high body tempera- (see, for example, de Margerie et al., 2004; Sander & ture and, possibly, a relatively rapid metabolism in Andrassy, 2006; de Buffrénil et al., 2010). these taxa, based on its occurrence in mammals and Large size can be the result of either faster or birds (e.g. Padian et al., 2001). FLB and iFLB (sensu protracted growth. It clearly appears that, within Klein, 2010) – although the presence of interrupted aquatic reptiles in general, growth rate does not cor- FLB would suggest a slower metabolism than its relate with taxon size (cf. Table 1) For example, both uninterrupted occurrence (Chinsamy-Turan, 2005) – small and giant ichthyosaurs display FLB, whereas are also encountered in other sauropterygians [e.g. even giant hydropelvic mosasauroids do not; Anaro- Anarosaurus, cymatosaurids, Pistosaurus (but only in saurus and Claudiosaurus, which are of similar size, its humerus)] and have been interpreted to be related display clearly distinct histological features. This to higher metabolic rates in these taxa (Klein, 2010); absence of correlation between size and growth rate this would have enabled them to spread through in rather unrelated groups is not surprising, as it is colder seas, and favoured the rise of plesiosaurs also unclear in a given clade (cf. Cubo et al., 2012). (Klein, 2010). A high metabolic rate has also been Rhodin (1985), based on the analysis of the suggested on the basis of histological data in ichthyo- chondro-osseous development of long bones in marine saurs, and has been interpreted as being related to turtles, has shown that the vascularization pattern, ‘some sort of endothermic, gigantothermic or incipi- and thus the mode of growth, of Dermochelys is diver- ently endothermic physiology’ (de Buffrénil & Mazin, gent from that of other chelonians. Similar features 1990). However, gigantothermy is defined as the were also observed in the giant Cretaceous Archelon maintenance of constant high body temperature by ischyros, but not in the giant Tertiary Stupendemys means of large body size, low metabolic rate and the geographicus, which lived in freshwater environ- use of peripheral tissues as insulation (Paladino, ments. Giant sizes are thus illustrated by distinct O’Connor & Spotila, 1990). This term is thus associ- collagenous and vascular bone histological character- ated with a rather low metabolic rate and disagrees

© 2012 The Linnean Society of London, Biological Journal of the Linnean Society, 2013, 108, 3–21 BONE HISTOLOGY OF AQUATIC REPTILES 15 with the inference made about ichthyosaurs, although Finnerty, 1994), and might have predisposed to heat the ‘endothermic’ or ‘incipiently endothermic’ physi- conservation (Block & Finnerty, 1994). This can be ologies remain possible. achieved in three ways: by an increase in body size, FLB is observed in all extant endotherms. However, change in shape (more cylindrical) or the use of low- it is also encountered in ectotherms (e.g. some speci- conductance tissues as insulation (Block & Finnerty, mens of crocodilians) and in considered as 1994). The first two characteristics are clearly such (e.g. dicynodonts and gorgonopsians; observed as trends in various groups of marine rep- Chinsamy-Turan, 2005). It thus appears that FLB tiles (e.g. ichthyosaurs, mosasaurs). Behaviour can does not require a high metabolic rate for its forma- also be used to enable the body temperature to tion, and therefore that its occurrence does not nec- remain elevated, for example by returning to the essarily imply endothermy (Chinsamy-Turan, 2005), surface to bask in warmer waters and warm up, as in although the reverse seems true. swordfishes (Block & Finnerty, 1994), or by shuttling An isotopic study has proposed a high metabolic either to cooler (to remove the excess heat produced rate in some marine reptiles (Bernard et al., 2010). It during active swimming) or warmer (to reduce heat has been suggested that ichthyosaurs, plesiosaurs loss) waters, in the tropics and in temperate waters, and mosasaurs were able to maintain a constant and respectively, as observed in Dermochelys (Wallace & high body temperature. Their body temperatures Jones, 2008). were estimated to be between 35±2°Cand39±2°C Two key factors have been considered to influence (Bernard et al., 2010). The authors suggested homeo- the level of resting metabolism: body temperature thermy for ichthyosaurs and plesiosaurs and at least and lifestyle. Analyses on teleost fishes have shown partial homeothermy in mosasaurs. Although Motani that species with more active lifestyles have higher (2010) raised hypotheses about a potential bias in the resting metabolic rates for a given water temperature results, he considered that it did not affect the con- (Morris & North, 1984, Zimmermann & Hubold, clusions of the authors. 1998). Although it does not appear possible to dem- It thus appears highly probable that several of onstrate this in reptiles, it might be a general result these marine reptiles were able to maintain a high (Clarke & Pörtner, 2010). body temperature. This, however, does not mean that From a locomotory perspective, as sprint capabili- they were true endotherms. It is considered that ties are comparable in mammals and reptiles endothermy is acquired gradually in several (at least (Bennett & Ruben, 1979; Bennett, 1991; Clarke & two) steps: first, with the ability to maintain a more Pörtner, 2010), endothermy would ‘only’ be advanta- or less constant high body temperature (homeo- geous to enable sustained aerobic activity. Dermoche- thermy) and, second, with the evolution of a higher lys and hydropelvic mosasauroids display similar metabolic rate and body temperature (Crompton, histological features (see above), suggesting a rela- Taylor & Jagger, 1978). Bennett & Ruben (1979) tively low metabolism. Gigantothermy, which is inter- have suggested that enhanced aerobic activity to preted as an ability to fulfil thermoregulatory needs allow increasingly sustained locomotor activity is a without a high metabolic rate (Paladino et al., 1990), key factor for the evolution of endothermy. Both has been suggested for Dermochelys and might also maximum oxygen consumption and absolute aerobic apply to hydropelvic mosasauroids, as suggested by scope are greater at higher body temperature, which Motani (2010). The metabolism of Dermochelys hatch- is therefore interpreted as a secondary consequence of lings (data are not available for adults), although still selection for enhanced aerobic scope (Bennett & rather low, was estimated to be three times that Ruben, 1979). Partial endothermy has evolved in measured in some cheloniid sea turtle (green and scombroid fishes (tuna, mackerel) and lamnid sharks loggerhead turtle) hatchlings (Zug & Parham, 1996), (and probably also Alopiidae) (Westneat et al., 1993; probably because Dermochelys hatchlings, contrary to Block & Finnerty, 1994; Graham & Dickson, 2000). It the latter, swim constantly (Zug & Parham, 1996; is considered that the centralization of red muscles, Eckert, 2002). Indeed, Dermochelys appears to con- related to the evolution of a stiffer bodied swimming stantly and uniformly swim at a moderate speed style, might have secondarily become a source of (Eckert, 2002). Hydropelvic mosasauroids are gener- metabolic heat in tuna (Block & Finnerty, 1994). ally considered to be active ambush predators, able to Selection for endothermy would only have been pos- perform quick accelerations, but not to swim at sible in animals that already had a mechanism for sustained high performance levels (Cowen, 1996). retaining metabolic heat (Clarke & Pörtner, 2010). Neither Dermochelys nor hydropelvic mosasauroids The ability to reduce heat loss to the environment is are thus pursuit predators. This is consistent with the considered to be an essential factor contributing to absence of endothermy in these taxa, as only endo- the initial retention of metabolic heat (Neill & thermy would have conferred the ability for sustained Stevens, 1974; Neill, Chang & Dizon, 1976; Block & swimming. However, sustained swimming has been

© 2012 The Linnean Society of London, Biological Journal of the Linnean Society, 2013, 108, 3–21 16 A. HOUSSAYE suggested in ichthyosaurs and in pliosauroids (e.g. which suggests no phylogenetic signal in these histo- Massare, 1988; Motani, 2010), which is consistent logical features. with the occurrence of uninterrupted FLB evoking a high metabolism. However, rather slow swimming has been suggested for plesiosauroids (Massare, RELATIONSHIP TO THE SWIMMING MODE 1988). Placodonts, which are considered to be Mesosaurs, Claudiosaurus, pachypleurosaurs, notho- slow shallow divers and bottom walkers (Scheyer saurs and several aquatic squamates (except for et al., 2012), do not display the ability for sustained hydropelvic mosasauroids) are anguilliform swim- swimming. The histological features of these taxa, mers (Braun & Reif, 1985; Carroll, 1985). Hydropelvic although suggesting a high growth rate, do not seem mosasauroids, Triassic ichthyosaurs, Crocodyli- to be associated with endothermy. formes, choristoderes and the placodont Placodus Differences in metabolic rates probably enabled are considered to be subcarangiform swimmers some taxa to extend to cooler waters, as suggested by (Braun & Reif, 1985), whereas metriorhynchids Klein (2010), so that the various histological patterns () are interpreted as carangiform analysed might illustrate variations in the tolerance swimmers (Braun & Reif, 1985; Carroll, 1985; to water temperature. If these histological features Massare & Callaway, 1990) and post-Triassic ich- are clearly not correlated with a shallow vs. open- thyosaurs as thunniforms (Lingham-Soliar, 1998). marine habitat (see Table 1), they might be correlated Conversely, plesiosaurs are considered to be pectoro- with the tolerance to cooler waters, via an extension pelvic oscillatory swimmers and aquatic turtles as of the taxon niche, either at the surface of the globe or pectoral oscillatory swimmers (Braun & Reif, 1985; at depth. Carpenter et al., 2010). Therefore, it is clear that there is no link between the histological features analysed here and the mode of swimming in aquatic PHYLOGENETIC SIGNIFICANCE reptiles (cf. Table 1). Interestingly, it is assumed that Although numerous terrestrial taxa could be taxa among ichthyosaurs and plesiosaurs, which included at various positions in the phylogeny pre- show similar histological features, adapted to sus- sented in Figure 1, it remains of interest to test tained swimming and are possible endotherms (see whether a phylogenetic signal is reflected in the above). However, these groups display distinct modes bone histology of these aquatic taxa, and therefore of swimming. The association between a thunniform in the histological adaptation to a secondary aquatic mode of swimming and a trend towards endothermy life within . has already been highlighted in fishes (Block & In order to do so, random tree generation was used Finnerty, 1994; Donley et al., 2004). This raises the in MESQUITE (Maddison & Maddison, 2006) follow- question of whether endothermy (or a trend towards ing the method described in Germain & Laurin endothermy) could be associated with several swim- (2005). Three characters were considered: (1) domi- ming modes. nant type of collagenous matrix (state 0 for LZB, 1 for LZB with iFLB and 2 for FLB); (2) degree of vascu- larization (0 for low, 1 for moderate, 2 for high); and RELATIONSHIP WITH THE (3) main orientation of the vascular canals (0 for MICROANATOMICAL SPECIALIZATIONS longitudinal, 1 for both longitudinal and radial, 2 for Two microanatomical osseous specializations are radial), with the characters states being ordered observed in aquatic reptiles: (1) bone mass increase 0 → 1 → 2. The taxa for which the histological fea- corresponding to an increase in bone volume and/or tures are known (except pistosaurids because of the compactness involved in a hydrostatic control of buoy- distinct patterns of the humerus and femur) were ancy and body trim (Houssaye, 2009); and (2) an incorporated into a consensual (con- ‘osteoporotic-like’ pattern corresponding to a spong- sistent with Fig. 1). The number of steps for the three ious inner organization of the bone without any med- characters was analysed and compared with that ullary or large cavity (de Ricqlès & de Buffrénil, obtained for 9999 trees generated by the randomiza- 2001). If the first is generally encountered in slow tion of terminal taxa. The number of trees (random shallow-water swimmers, the second occurs in active and reference), at least as short as the reference tree swimmers requiring high speed and manoeuvrability divided by 10 000, gives the probability that the char- (de Ricqlès & de Buffrénil, 2001; Houssaye, 2009). acter analysed does not show any phylogenetic signal Bone mass increase has been observed in, for (H0). H0 is rejected when this number is fewer than example, mesosaurs, Claudiosaurus, choristoderes, 5% and the phylogenetic signal is therefore consid- Placodus, pachypleurosaurs, nothosaurs, some plio- ered to be significant. Indices of 0.35, 0.30 and 0.29, saurs, stem-ophidians and plesiopedal mosasau- respectively were obtained for the three characters, roids (Houssaye, 2009), whereas an ‘osteoporotic-like’

© 2012 The Linnean Society of London, Biological Journal of the Linnean Society, 2013, 108, 3–21 BONE HISTOLOGY OF AQUATIC REPTILES 17 pattern characterizes, for example, ichthyosaurs, larly grateful to N. Klein (Bonn Universität, Dermochelys, plesiosaurs and hydropelvic mosasau- Germany), S. Sanchez (Uppsala University, Sweden), roids (de Ricqlès & de Buffrénil, 2001; cf. Table 1). T. Scheyer (Paläontologisches Institut und Museum, Although both microanatomical and histological fea- Zürich, ) and to an anonymous reviewer tures deal with the inner bone structure, and could for fruitful comments that improved the manus- thus be considered to be correlated, there is clearly no cript, and to J. A. Allen (University of Southamp- link between the microanatomical specializations and ton, UK) and S. Moore (Wiley-Blackwell, UK) for the histological features analysed here in aquatic editorial work. This research was supported by reptiles (cf. Table 1). a postdoctoral fellowship of the -Foundation. CONCLUSION

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