Vertebrate Skeletal Histology and Paleohistology

28 Nonmammalian Synapsids Jennifer Botha and Adam Huttenlocker CONTENTS Introduction to Nonmammalian Synapsids .................................................................................................................................. 550 Nontherapsid Synapsids ................................................................................................................................................................551 Caseasauria ..............................................................................................................................................................................551 Eupelycosauria .........................................................................................................................................................................551 Nonmammalian Therapsids ......................................................................................................................................................... 553 Basal Therapsids and Dinocephalians .................................................................................................................................... 553 Anomodonts ............................................................................................................................................................................ 554 Gorgonopsians ........................................................................................................................................................................ 556 Therocephalians ...................................................................................................................................................................... 556 Nonmammaliaform Cynodonts ............................................................................................................................................... 558 Summary of Nonmammalian Synapsid Histology ...................................................................................................................... 560 Acknowledgments ........................................................................................................................................................................ 560 References .................................................................................................................................................................................... 560 therapsids (Huttenlocker and Rega 2012, Reisz 2014). These Introduction to Nonmammalian Synapsids early synapsids were the dominant terrestrial vertebrates during the late Carboniferous to early Permian, but by the The fossil record of Permo-Triassic Synapsida has been the middle Permian they had essentially been replaced by the source of important data for studying the origin and evolu- mammal-like therapsids. The Permo-Triassic Therapsida tion of mammalian traits (e.g., Kemp 1982, 2005, Angielczyk represent one of the most iconic evolutionary transitions in 2009), terrestrial community structure (e.g., Roopnarine and the fossil record. The first mammals evolved from therapsids Angielczyk 2015, Codron et al. 2017), survival from mass by the Early Jurassic as tiny shrew-sized creatures. Despite extinctions (e.g., Huttenlocker 2014, Smith and Botha-Brink some degree of ecological diversity, mammals remained rel- 2014, Day et al. 2015, Botha-Brink et al. 2016) and macro- atively small (less than a few kilograms) until the extinction evolution (e.g., Sidor 2001, Laurin 2004, Roopnarine and of the nonavian dinosaurs some 66 million years ago (Ma) Angielczyk 2012, 2016, Didier et al. 2017). Importantly, (Kielan-Jaworowska et al. 2004). this group also produced many physiological innovations, The first researchers to document and describe the osteo- providing some of the earliest evidence for rapidly forming histology of extinct synapsids included Moodie (1923), Gross fibrolamellar bone tissues, systematic skeletal remodeling, (1934), Enlow and Brown (1957), Enlow (1969) and Ricqlès and, perhaps, endothermic thermometabolism. Synapsida (1969, 1972, 1974a, b). However, the last two decades have includes all amniotes more closely related to mammals than seen a profusion of studies within a phylogenetic context to reptiles (Angielczyk 2009). The nonmammalian synapsids (including, but not limited to those by Botha-Brink, Chinsamy- classically comprised two major radiations: a paraphyletic Turan, Huttenlocker, Jasinoski, Laurin, Ray, Shelton, among assemblage of basal synapsids commonly referred to as others; see below for detailed references), which have greatly “pelycosaurs” and the more recent, monophyletic Therapsida. improved our understanding of synapsid histology, growth, Synapsids arose during the Late Carboniferous and origi- and physiology. These studies are summarized below in their nally included two major clades: the extinct Caseasauria phylogenetic context. and the more diverse Eupelycosauria, which includes the 550 Non-Mammalian Synapsids 551 sometimes recurved, ziphodont teeth (Evans et al. 2009). Few Nontherapsid Synapsids varanopid taxa have been examined histologically, including the varanodontine Varanops (Ricqlès 1974b, Huttenlocker Caseasauria and Rega 2012, Shelton 2015), anecdotal observations from Caseasaurs included small to very large-bodied (>200 kg) gen- the varanodontine Watongia (Ricqlès 1976, Bennett and eralists and herbivores characterized by a recumbent snout with Ruben 1986) and a mycterosaurine from the lower Permian enlarged nares, some with an absurdly small skull compared of Oklahoma (Huttenlocker and Rega 2012, Shelton 2015). to their body length (Olson 1968). During the Carboniferous- Histologically, varanopid taxa show some degree of variation Permian transition (ca. 300 Ma), the caseids were among in tissue texture, vascularity and growth zone formation. In the first diverse group of large-bodied vertebrate herbivores, Varanops, the cortices of the radius and femur were generally along with diadectomorphs (anamniotes) and edaphosaurids thin and dominated by lamellar primary bone with occasional (eupelycosaurs). They also included some of the latest surviv- growth marks. Mycterosaurine femora were slightly thicker ing pelycosaur-grade synapsids, with some caseids recovered walled with combinations of parallel-fibered bone transition- from middle Permian-aged rocks. Although their lifestyle is ing to lamellar bone (Figure 28.1A), and numerous growth somewhat controversial, with some researchers suggesting marks and growth zones incorporating mainly longitudinal a possible aquatic habitus (Lambertz et al. 2016), they have vasculature arranged in radial rows. Any transition to slowed mainly been interpreted as terrestrial (Olson 1968, Felice and growth into adulthood was subtle as there is little evidence Angielczyk 2014). of distinctive, avascular outer circumferential lamellae. The The diversity of skeletal tissues in caseids is poorly known, bone wall was generally dense and compact, but with a distinct as they have largely been neglected in histological studies. medullary cavity sometimes bounded by well-developed inner Our first insights into caseid histology were offered by Ricqlès (endosteal) circumferential lamellae. The well-differentiated (1974b), who sectioned and described the midshaft histology medullary cavity of varanopids contrasts with some other of a femur of a subadult caseid, Ennatosaurus tecton from the early tetrapods, which tended to exhibit bones with a medul- middle Permian of Russia. The short, robust femur showed a lary region invaded and partly occluded by a dense spongiosa. thin cortical wall and a poorly developed free medullary cav- The relative lightening of the skeleton in varanopids, and the ity; most of the medullary area is occupied by a spongiosa, well-defined tubular architecture of their bones, may be more flanked by occasional perimedullary erosion cavities. The consistent with a more efficient terrestrial locomotion. thin cortex was dominated by inconsistently vascularized pri- Ophiacodontids exhibited a curious degree of histovari- mary lamellar bone, with sparse longitudinal canals dorsally, ability compared to contemporary groups (Enlow and Brown oblique canals ventrally (associated with the adductor ridge) 1957, Enlow 1969, Ricqlès 1974b, Laurin and Buffrénil 2016). and was nearly avascular posteriorly. Overall, the predomi- The overall structure and compactness profile of Ophiacodon nance of lamellar bone with sparse, but highly ordered vas- limb bones appear similar to other early tetrapods; an irregular cular canals, resembles that of contemporary diadectomorphs, perimedullary margin, thick bone wall and often, an occluded and it may therefore reflect the primitive state of the amniote medullary cavity suggested the possibility of a primitively stem. Shelton (2015) and Lambertz et al. (2016) showed simi- aquatic lifestyle (though this has been questioned by numer- lar tissue textures in limb bones of other European and North ous authors; e.g., Felice and Angielczyk 2014). However, mate- American caseids, demonstrating the paucity of nonlamel- rial referred to the Carboniferous ophiacodontid Clepsydrops lar bone and sparse vascularity, even in purported juveniles. collettii shows a remarkably thinner bone wall with an abrupt Lambertz et al. (2016) further suggested, controversially, the transition between medulla and cortex that is more like that of possible existence of a diaphragm on the basis of Sharpey’s Varanops (Laurin and Buffrénil

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