Ichthyosaurian palaeopathology: evidence of injury and disease in fossil fish lizards

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1 Journal of Zoology. Print ISSN MINI-SERIES Ichthyosaurian palaeopathology: evidence of injury and disease in fossil fish lizards J. M. Pardo-Perez 1,2, B. P. Kear 3,M.Gomez 4, M. Moroni 4 & E. E. Maxwell 1 1 Staatliches Museum f ur Naturkunde Stuttgart, Stuttgart, Germany 2 Vicerrectorıa de Investigacion y Postgrado, Universidad de Magallanes, Punta Arenas, Chile 3 Museum of Evolution, Uppsala University, Uppsala, Sweden 4 Facultad de Ciencias Veterinarias, Universidad Austral de Chile, Campus Isla Teja, Valdivia, Chile Keywords palaeobiology; ichthyosauria; Mesozoic; Reptilia; avascular necrosis; osteotrauma; palaeopathology. Correspondence Judith Pardo-Perez, Staatliches Museum f ur Naturkunde, Rosenstein 1, Stuttgart, Germany. judith.pardo-perez@smns-bw.de Editor: Heike Lutermann Received 1 February 2017; revised 22 May 2017; accepted 13 June 2017 doi: /jzo Abstract The documented record of ichthyosaurian paleopathologies reveals an array of injuryrelated bone modifications and instances of disease evidenced through multiple clades, skeletal regions and body-size classes from the Middle Triassic to middle Cretaceous. Examples include traumatic injuries, as well as a high incidence of articular diseases, including avascular necrosis. Forelimb pathologies are particularly abundant (65% of total reported), and the glenoid region seems to have been especially prone to articular disease. In contrast, pathologies affecting the vertebral column are comparatively underrepresented (6% of reported pathologies). Also notable is the disproportionate commonality of osteopathologies in ichthyosaurian taxa between 2 and 6 m in length (54%), as opposed to demonstrably larger (31%) or smaller bodied (15%) species. Furthermore, osteopathologies are almost exclusively described from skeletally mature individuals, and are best known from taxa of Jurassic age (78%), versus those from the Triassic (15%) or Cretaceous (7%); this likely reflects biases in the ichthyosaurian fossil record through time. Ichthyosaurs evince remarkable similarities in the types of observed skeletal damage relative to other ecologically similar marine amniotes especially cetaceans and mosasaurid squamates, all of which potentially exhibited equivalent palaeoecological and/or behavioural adaptations for life in aqueous environments. Notably, however, the unusually low frequency of vertebral pathologies in ichthyosaurs is peculiar, and requires further investigation to establish significance. Introduction Palaeopathological studies have been used to understand the history of injury and disease in the fossil record (Rothschild & Martin, 2006). They deduce the types of skeletal damage occurring in fossil populations, their underlying cause and by inference, aspects of related palaeoecology and behaviour (Moodie, 1918b; Rothschild & Martin, 2006). Palaeopathologies are usually identified in fossil vertebrates only if they resulted in damage to the skeleton (but see, e.g., Rothschild & Depalma (2013), who reported traumatic skin damage in a hadrosaur) and are typically the result of a traumatic injury (e.g. fractures) involving bone modification and occasional callus development during healing. Infectious diseases can also develop after trauma (e.g. abscesses), or may be age related (e.g. osteoarthritis/osteoarthrosis) (Lingham-Soliar, 2004; Rothschild, 2012). Alternatively, osteological pathologies can result from other factors causing mechanical or physiological stress (Kompanje, 1999; Rothschild & Martin, 2006; Cooper & Dawson, 2009). Examples of putative palaeopathologies have long been observed among fossil vertebrates, and perhaps most famously among Mesozoic amniotes including dinosaurs and various marine reptiles (see Rothschild & Martin, 1993 for summary). Indeed, the latter have proven particularly pertinent for interpreting lifestyle based on evidence of skeletal damage (e.g. bone modification attributed to nitrogen narcosis and diving habits: Rothschild & Martin, 1987; Rothschild, Xiaoting & Martin, 2012b; Rothschild, Schultze & Pellegrini, 2013). The earliest obligate marine amniotes were ichthyosaurs, which constituted a prolific radiation of middle to apex-level pelagic predators in Mesozoic aquatic ecosystems from the Early Triassic to the early Late Cretaceous ( Ma) (McGowan & Motani, 2003; Dick & Maxwell, 2015). They were typified by a fish-like body plan with flipper-like foreand hindlimbs; and relied primarily upon axial propulsion for subaqueous locomotion (McGowan & Motani, 2003). Unlike some other Mesozoic marine amniote clades (e.g. mosasaurid squamates), there is at present little compiled information describing the range and causes of osteopathologies observed in ichthyosaurs, although the frequency of avascular necrosis has been used as a key proxy for deep-diving behaviours (Motani, Rothschild & Wahl, 1999; Rothschild et al., 2012b). Journal of Zoology 304 (2018) ª 2017 The Zoological Society of London 21

2 Ichthyosaurian palaeopathology J. M. Pardo-Perez et al. It is therefore the purpose of this paper to present a survey of the form and distribution of palaeopathological structures in ichthyosaurs based on observations from both the literature and original specimens. We collate the frequency of such traces through time, and supplement with novel histological data, to derive comparisons based on pathologies recognized in other ecologically similar obligate marine amniotes (e.g. mosasaurs, plesiosaurians, cetaceans). Our intention is to construct a framework for future targeted studies of pathological occurrences within discrete fossil assemblages, as a means of understanding the regularity of injury and disease affecting animal populations in deep time. Institutional abbreviations GPIT: Geologisches und Pal aontologisches Museum T ubingen, Germany. PMU: Palaeontological collection, Museum of Evolution, Uppsala University, Sweden. SMNS: Staatliches Museum f ur Naturkunde Stuttgart, Germany. U-MO: Urwelt- Museum Oberfranken, Bayreuth, Germany. Approaches to classifying palaeopathologies To avoid misinterpreting damage caused by taphonomic or diagenetic processes, unambiguous osseous pathologies must show signs of healing (Rothschild & Martin, 2006), for instance through callus development or fibre remodelling (Lingham-Soliar, 2004; Rothschild & Martin, 2006; Jahagirdar & Scammell, 2009). Congenital malformations (e.g. defects in somitogenesis: Witzmann et al. (2014)) and dental pathologies (e.g. Kear, 2001) have also been recorded but are not the focus of our review. Palaeopathologies are usually identified only by macroscopic examination. However, externally visible pathological bone modifications are associated with changes to skeletal microstructure, which can be used to confirm the pathological nature of the specimen and provide additional information on the causes of the pathology and the nature of healing processes. Examination of such microstructural features is accomplished via lct scanning or histological sectioning; however such methods can be difficult to apply for slab-mounted specimens, those preserved in pyrite-rich shales or rare fossils that are not available for destructive sampling. Based on our survey of the published record of ichthyosaur remains, together with some novel observations derived from key museum specimens (Appendix S1), we classified osteological palaeopathologies into three categories for analytical purposes: (1) Simple trauma: a wound produced by sudden physical force and resulting in bone fibre remodelling, possibly with callus or abscess formation. Trauma may also be a factor in the subsequent development of articular disease and joint ankylosis (Figs. 1 3). (2) Articular disease: trauma, congenital disorder or repetitive strain injury resulting in damage to the articular surfaces (Figs. 4 and 5). (3) Ankylosis: trauma or disease causing a postnatal fusion of normally separate skeletal elements (Fig. 1f). To exemplify a histological analysis, we made four sections lm in thickness through the distal shaft of a disarticulated scapula referred to Eurhinosaurus sp. (GPIT NC/20/F/12) bearing obvious macroscopic pathological bone modifications. We also sectioned a humerus referred to Ichthyosaurus sp. (SMNS 96945) bearing a depression on the proximal articular surface consistent with both articular disease (e.g. Rothschild et al., 2012b) and normal morphology (Lomax & Massare, 2017). All sections were prepared using standard palaeohistological techniques (Chinsamy & Raath, 1992). Survey of documented ichthyosaur palaeopathologies Simple trauma Cranial injuries Several instances of traumatic damage to the skull have been described in ichthyosaurs. A specimen of the Late Triassic Early Jurassic (Rhaetian Hettangian) taxon Leptonectes cf. tenuirostris bears an elongate gouge on the lower jaw, extending along the ventrolateral margin of the right angular towards the posterior end (Maisch & Matzke, 2003). A juvenile ichthyosaur from the Early Jurassic (Toarcian) of Germany, referable to Stenopterygius sp., shows marked upward deflection of the premaxilla and dentary (Quenstedt, 1885; reinterpreted as an injury by Abel, 1935). Zammit & Kear (2011) described an osteologically mature individual of Platypterygius australis from the Early Cretaceous (Albian) of Australia, which exhibited marks incorporating visible callus formation, together with lamellar remodelling on the ventrolateral surfaces of the dentaries. In addition to these published accounts, a specimen of Stenopterygius uniter (PMU 24320, also Toarcian in age) mentioned by Wiman (1921) as exhibiting healed ribs was found by this study to also possess healed injures on the lateral surface of posterior section of the lower jaw. These included four deep notches, as well as a concavity on the angular, all of which display evidence of bone remodelling (Fig. 1) likely caused by secondary infection and abscesses following invasive trauma. A protuberance on the ventral edge of the splenial in the holotype of the Early Cretaceous (Barremian Aptian) ophthalmosaur Platypterygius sachicarum Paramo, 1997 reveals fibrous surface remodelling and possible abscesses derived from trauma and infection (EEM, pers. obs.). Injuries to the ribs Mention of presacral ribs with pseudarthroses and prominent callus formation is also relatively frequent in the literature. These have been documented in Shonisaurus popularis from the Late Triassic (Carnian) of Nevada (Camp, 1980), as well as several Jurassic taxa, including Temnodontosaurus 22 Journal of Zoology 304 (2018) ª 2017 The Zoological Society of London

3 J. M. Pardo-Perez et al. Ichthyosaurian palaeopathology (a) (c) (b) (d) (e) (f) (g) (h) Figure 1 Stenopterygius uniter (PMU 24320), with traumatic injury to the mandible and presacral ribs. (a) Right mandible. The black square indicates the damaged area, interpreted as a traumatic injury with evidence of infection and abscesses formation. (b) Magnified view; arrows indicate abscess formation between the angular and surangular. (c, d) Fibre remodelling in two areas of the angular, indicated by black arrows. (e) Location of the injured presacral ribs. (f) Ankylosis observed between the vertebrae 34 and 35. (g) Arrows indicating specific points of damage. (h) Arrows indicating potential abscess. This specimen was counted twice in the analysis of types of pathology since it exhibited both healed traumas and idiopathic ankylosis of the neural arches (not figured). [Colour figure can be viewed at wileyonlinelibrary.com] nuertingensis (SMNS 13488) and T. trigonodon (SMNS 15950) from the Early Jurassic of Germany (Huene, 1930, 1931a,b). Restudy of the latter specimen revealed 19 dorsal rib fractures (von Huene, 1931b only mentioned 11) at vertebral positions 11 24, 30, 35 and 37 39, conspicuously located at different lengths along the rib shafts on the left side of SMNS (Table 1; Fig. 2). At least six fractured ribs are likewise evident on the right side of the axial column. Healed ribs, some with pseudarthroses, have also been reported in Eurhinosaurus longirostris, from the Early Jurassic (Toarcian) of Germany (Huene, 1951; Keller, 2006). Ribs and 47 of GPIT/RE/9412 (Table 1) and ribs of a second specimen (Keller, 2006) are affected. An individual of S. uniter (PMU 24320) possesses calluses on the midsection of ribs Journal of Zoology 304 (2018) ª 2017 The Zoological Society of London 23

4 Ichthyosaurian palaeopathology J. M. Pardo-Perez et al. (a) (b) (c) (d) (e) (f) (g) Figure 2 Fractured ribs in ichthyosaurians. (a f) Temnodontosaurus trigonodon (SMSN 15950). (a) Overview indicating the injured area of the rib cage. (b f) Healed ribs with callus development and pseudarthrosis formation. (g) Stenopterygius sp. (U-MO) rib showing evidence of callus development. [Colour figure can be viewed at wileyonlinelibrary.com] 24 Journal of Zoology 304 (2018) ª 2017 The Zoological Society of London

5 J. M. Pardo-Perez et al. Ichthyosaurian palaeopathology (Wiman, 1921). These traumas are located on the right side of the dorsal series immediately adjacent to the pelvic girdle (Table 1; Fig. 1e), with the healed fractures on rib 32 evidencing extensive osseous overgrowth, with possible abscesses formed in other associated ribs. Finally, our study detected a callus (Fig. 2g) on a rib fragment associated with a specimen referred to Stenopterygius sp. (Rabold & Eggmaier, 2013, pl. 3, fig. 1). Limb and girdle injuries Traumatic injuries to the limbs and girdles are relatively infrequently documented. The holotype of the Middle Jurassic (Callovian) ophthalmosaur Ophthalmosaurus icenicus has a severely deformed and fused left clavicle/scapula (Andrews, 1910; Appleby, 1956), possibly inflicted at an early stage of growth. A broken clavicle with pseudarthrosis and abscess development was also documented in the holotype of Grendelius alekseevi from the Late Jurassic (Tithonian) of Russia (Arkhangelsky, 2001; Stepanov, Arkhangelsky & Ivanov, 2004). We identified an isolated scapula of Eurhinosaurus sp. (GPIT NC/20/F/12) bearing a prominent rugose lesion on its anterolateral margin, along with a small posterolateral notch. The lesion shows extensive surface remodelling and bony overgrowth. Histological thin sections (Fig. 3) indicate that dense cortical bone has been almost completely lost within the lesion, and replaced by woven bone with erosive vacuities and an associated channel possibly linked to abscess formation. Adjacent osteotissues possess a normal structure of internal trabeculae and an external cortical layer (Fig. 3f). However, asymmetrical reactive deposition of primary bone is evident around the exterior notch, which is thus identified as pathological in origin. In addition, a conspicuous crack propagating away from the notch is internally lined with new bone tissue, suggesting that this is a healed fracture rather than a post mortem break (Fig. 3h, i). Based on these data, the damage observed on GPIT NC/20/F/12 can therefore be interpreted as two discrete punctures on the external surface of the scapula, the more posterior of which fractured the bone. Articular disease Avascular necrosis Avascular necrosis is caused by nitrogen embolisms forming within tissues during decompression after diving at depth. Loss of circulation initiates localized tissue death, and subsequent mechanical stress on the articular surfaces of long bones causes fracturing of the underlying necrotic trabeculae, forming subsidence structures (Rothschild et al., 2012b). Previously, structures consistent with avascular necrosis have been reported in the proximal humeri of Early Jurassic ichthyosaurs such as Leptonectes sp. (one individual), Ichthyosaurus (6 isolated specimens) and a femur referred to Temnodontosaurus, 11 humeri and a femur attributed to the Middle Late Jurassic genus Ophthalmosaurus, the humerus of the holotype of the Late Jurassic Undorosaurus trautscholdi and a humerus assigned to Platypterygius from the Early Cretaceous (Motani et al., 1999; Rothschild et al., 2012b; Arkhangelsky & Zverkov, 2014). However, similar depressions on the proximal articular surface of the humerus in Ichthyosaurus spp. have also been interpreted as non-pathological (Lomax & Massare, 2015, 2017). An isolated humerus of Ichthyosaurus sp. from the Early Jurassic of England bears this type of depression (SMNS 96945; Fig. 4). A histological section transecting this feature revealed that the external depression is underlain by a cone of unusually porous bone (Fig. 4c). Signs of resorption on the trabeculae indicate widespread osteoclastic activity in this zone (Fig. 4d). The base of the external depression lacks both cortical bone and calcified cartilage, instead being made of fractured trabecular fragments. A clear ledge separates the subsidence feature from the normal articular surface (Fig. 4e). This combination of histological features suggests that the depression is pathological in nature, and is consistent with osteonecrosis. Other occurrences of articular disease An unusual central prominence and associated groove extending to the periphery of the flattened humeral head in the holotype of Leptonectes solei from the Early Jurassic (Sinemurian) of England (McGowan, 1993) probably evidences articular disease. A gravid female Stenopterygius sp. (Rabold & Eggmaier, 2013, pl. 2, fig. 1) also shows atypical concavities along the posterior margin of the glenoid on the left scapula, as well as the lateral edge of the left coracoid and proximal articular surface of the disproportionately enlarged left humerus (Fig. 5). Finally, Stepanov et al. (2004) described two anterior caudal vertebrae from the Early Cretaceous (Hauterivian) referred to Platypterygius that displayed bone erosion and osteophytic growth on the peripheral articular surfaces, possibly resulting from a trauma. Ankylosis Unambiguous examples of pathological (non-congenital) ankylosis are relatively rare. In L. tenuirostris, co-ossification of the humerus, radius and ulna has been described in four specimens, with co-ossification limited to the radius and ulna in one individual (Delair, 1974; McGowan, 1989). Fr obisch, Sander & Rieppel (2006) reported zygopophyseal fusion, with accompanying bone rugosity and malformation, in cervical vertebrae 6 9 in the holotype of Cymbospondylus nichollsi from the Middle Triassic (Anisian) of Nevada. The S. uniter specimen PMU also displays co-ossification of dorsal neural spines 34 35; however, no obvious callusispresent. Frequency of palaeopathologies in Ichthyopterygia We assessed the anatomical, stratigraphical and body size distribution of pathological specimens in the ichthyopterygian Journal of Zoology 304 (2018) ª 2017 The Zoological Society of London 25

6 Ichthyosaurian palaeopathology J. M. Pardo-Perez et al. (a) (d) (d) (e) (e) (b) (c) (f) (g) (h) (i) (j) (k) 26 Journal of Zoology 304 (2018) ª 2017 The Zoological Society of London

7 J. M. Pardo-Perez et al. Ichthyosaurian palaeopathology Figure 3 Eurhinosaurus sp. right scapula (GPIT NC/20/F/12). (a) External view showing the two planes of sectioning (d, e). (b) Anterolateral view, showing a close up of the notch. (c) Anterodorsal edge of the scapula showing the profile and close up (inset) of the lesion. (d) Distal section spanning the notch and (e) proximal section spanning the lesion. (f) Unmodified bone on the internal surface of the scapula showing the compact cortex and inner spongiosa. (g) External surface at the edge of the notch showing thick deposition of reactive bone tissue. (h) Fractured posterior margin of the scapula with inset (i) showing the development of primary bone within the fracture. (i) Posterior edge of the lesion, showing primary bone, and (j k) anterior edge of the lesion showing absence of cortical bone and large vacuities potentially related to minor infection. [Colour figure can be viewed at wileyonlinelibrary.com] (a) (b) (c) (d) (e) (f) Figure 4 Isolated right humerus referred to Ichthyosaurus sp. (SMNS 96945). (a) Dorsal view. The arrow indicates the depression on the proximal articular surface compatible with avascular necrosis. (b) Proximal view, indicating the osseous depression. (c f) Longitudinal thin section through the dorsoventral plane. (c) Longitudinal section indicating the damaged area. (d) A close up of the affected area. (e) Partially resorbed trabeculae. Signs of osteoclastic activity indicated by arrows. (f) The arrow indicates the edge of the subsidence feature; the undamaged articular surface is to the right. [Colour figure can be viewed at wileyonlinelibrary.com] Journal of Zoology 304 (2018) ª 2017 The Zoological Society of London 27

8 Ichthyosaurian palaeopathology J. M. Pardo-Perez et al. (a) (c) (d) (b) (e) Figure 5 Stenopterygius sp. (U-MO). Articular pathology affecting the glenoid region of the left forelimb. (a, b) Overview of affected elements. (c, d) Arrows indicating the deep depressions observed in the proximal margin of the right humerus and the medial margin of the right coracoid. (e) Large concavities in the glenoid contribution of the left scapula. [Colour figure can be viewed at wileyonlinelibrary.com] fossil record. We used six skeletal subunits to examine anatomical distribution (from Beardmore & Furrer, 2016: skull, anterior vertebral column, posterior vertebral column, ribs and gastralia, pectoral girdle/forelimb and pelvic girdle/hindlimb). Chronostratigraphical correlations were derived from the literature (Paramo, 1997; Arkhangelsky, 2001; McGowan & Motani, 2003), with multiple epochal time bins assigned for wide-ranging genera (e.g. Ophthalmosaurus) and species (e.g. L. tenuirostris). Body-size estimates were based on published accounts (McGowan, 1993; Efimov, 1998; Maisch & Matzke, 2000; McGowan & Motani, 2003; Zammit, Norris & Kear, 2010; Arkhangelsky & Zverkov, 2014) and averaged across ranges where required. Our results revealed that pathological bone modifications most frequently occur in the forelimb module of ichthyosaurs (65%) (Fig. 6a), but this probably reflects the higher rate of reporting on afflictions such as avascular necrosis and its relationship with deep diving. Fractured ribs are also common (15%), and have been attributed to either aggressive interactions with conspecifics (Camp, 1980), collision with or beaching on reefs (Huene, 1931b, 1951) or compression of the rib cage during diving (Keller, 2006). Pseudarthroses might have developed via breathing-related flexion of the rib cage inhibiting healing (Huene, 1931a,b, 1951). Facial injuries (10%) are usually interpreted as healed bite marks (Maisch & Matzke, 2003; Zammit & Kear, 2011), possibly from intraspecific aggression (Zammit & Kear, 2011). Interestingly, the vertebral column (6%) and hindlimb (4%) yielded the least frequent accounts of damage. Examples of articular disease (49%) were more prevalent than traumatic injuries (Table 2), and almost exclusively confined to the pectoral girdle and forelimb. Relative to body size, the most frequently recorded ichthyosaur palaeopathologies (54%) occur in osteologically mature individuals of species approximately 2 6 m in length (e.g. Stenopterygius, L. tenuirostris, Ophthalmosaurus). This accords well with higher skeletal completeness of medium-sized ichthyosaurs relative to large and small taxa (Cleary et al., 2015). Higher rates of fatality or bone remodelling in juveniles could possibly hinder the frequency or detectability of skeletal traumas in younger individuals. Fewest pathologies were documented in taxa >8 m in length (11%; Fig. 6d). We ascribe this phenomenon to the relative scarcity of large ichthyosaurs in Early Jurassic lagerst atten. Nevertheless, larger taxa manifested a higher frequency of healed traumatic injuries, whereas smaller bodied taxa most often exhibited articular diseases. Chronostratigraphical sub-division demonstrated that palaeopathologies are best known from Early Jurassic ichthyosaurs (37%), with decline in reportage from the Late (21%) to Middle (19%) Jurassic and Late Triassic (14%) respectively. No pathological bone modifications have yet been documented from 28 Journal of Zoology 304 (2018) ª 2017 The Zoological Society of London

9 J. M. Pardo-Perez et al. Ichthyosaurian palaeopathology Table 1 Distance (in cm) between the proximal end of the rib and the callus formation in SMNS (Temnodontosaurus trigonodon), PMU (Stenopterygius uniter) and GPIT/RE/9412 (Eurhinosaurus longirostris) SMNS (left) SMNS (right) PMU GPIT/RE/9412 Rib position Distance from vertebral column Rib position Distance from vertebral column Rib position Distance from vertebral column Rib position 11? ? ? ? 47 ~ Distance from vertebral column Early Triassic or Late Cretaceous ichthyosaurs, and only very few instances are identifiable in Middle Triassic (1%) and Early Cretaceous (7%) taxa (Fig. 6c). Such distributional patterns are likely the result of preservational and/or collecting biases because the most complete ichthyosaur skeletons are often derived from Late Triassic Middle Jurassic konservat lagerst atten, and the number of ichthyosaur-bearing localities is particularly high in the Early and Late Jurassic (Cleary et al., 2015). Palaeopathologies in other ecologically comparable marine amniotes Traumatic injuries Cranial trauma is often concentrated along the mandible in mosasaurs, plesiosaurs and cetaceans, all of which constitute viable ecological analogues for ichthyosaurs (Dawson & Gottfried, 2002; Lingham-Soliar, 2004; Everhart, 2008). Lesions interpreted as bite marks have been reported in mosasaurs (Everhart, 2008), and fractured dentaries showing callus formation have been reported in both mosasaurs and elasmosaurid plesiosaurs (Welles, 1949; Lingham-Soliar, 2004). Facial traumas attributed to intraspecific aggression have been documented in extant and fossil odontocetes (Slyper, 1931; Abel, 1935; Dawson & Gottfried, 2002), including tooth breakage that exposes the pulp cavity to propagate infection and abscess in the mandible (de Smet, 1977; Loch et al., 2011). Curiously, such injuries are virtually unknown in marine reptiles (but see Lingham-Soliar, 2004), perhaps because of polyphyodont tooth replacement. As in ichthyosaurs, fractured ribs with callus development have been found in mosasaurs (Slyper, 1931; Abel, 1935) and plesiosaurians (Slyper, 1931; Kear, 2003). Broken ribs, occasionally with pseudarthrosis formation, are also frequently observed in extant and fossil cetaceans and sirenians (Slyper, 1931; de Smet, 1977; Dawson & Gottfried, 2002; Thomas et al., 2008). In stark contrast to ichthyosaurs, vertebral traumas are relatively frequent in mosasaurs (Abel, 1935; Shimada, 1997; Rothschild, Martin & Schulp, 2005), and in marine crocodylomorphs (Auer, 1909; Hua, 1999). Fractured cervical, lumbar and caudal centra, transverse processes and neural spines (spinous processes) have been recorded in extant and fossil cetaceans (Slyper, 1931; Abel, 1935; de Smet, 1977; Thomas et al., 2008). Axial traumas in extant cetaceans have been attributed to anthropogenic activity such as harpoons or ship strikes (Slyper, 1931; Arbelo et al., 2013); however, other mechanisms must also be implicated since broken ribs and vertebral fractures and fusion have been recorded in captive delphinids (de Smet, 1977) and fossil cetaceans (Dawson & Gottfried, 2002; Thomas et al., 2008). A healed femoral fracture has been described from an elasmosaurid plesiosaur (Sato, 2003), but this constitutes a comparative rarity, and such hindlimb/girdle pathologies are otherwise more frequently encountered among marine crocodylomorphs (Auer, 1909; Hua, 1999). Articular disease and ankylosis Sassoon, Noe & Benton (2012) described age-related erosive arthrosis of the mandibular glenoid leading to jaw misalignment Journal of Zoology 304 (2018) ª 2017 The Zoological Society of London 29

10 Ichthyosaurian palaeopathology J. M. Pardo-Perez et al. Figure 6 (a) Distribution of pathologies by anatomical region in ichthyosaurs. (b) Taxonomic distribution of reported pathologies. (c) Changes in the reported frequency of pathologies through geological time. (d) Distribution of pathologies by maximum length. [Colour figure can be viewed at wileyonlinelibrary.com] in a large pliosaurid, but articular disease is otherwise rare in the skull. In the limbs, lesions interpreted as septic arthritis have been reported in the proximal and distal limb elements of mosasaurs (Moodie, 1918a, 1923), and phalangeal fusion occurring in the forelimbs of some delphinids has been attributed to agerelated primary degenerative joint disease (Cooper & Dawson, 2009). Osteochondrosis of the pectoral girdle and proximal limb has likewise been reported in extant and extinct cetaceans (Thomas & Barnes, 2015), together with myositis ossificans traumatica and osteoarthritis (Thomas et al., 2008). As with traumatic injuries, articular damage to the vertebral column is surprisingly less prevalent in ichthyosaurs than in other marine amniotes, despite their reliance on axial propulsion. Idiopathic fusion, arthritis and spondyloarthropathy have been reported in the vertebral column of mosasaurs (Moodie, 1923; Slyper, 1931; Rothschild, Schultze & Pellegrini, 2012a; Rothschild & Everhart, 2015), and spondylitis, spondyloarthritis, zygarthrosis, discarthrosis and osteochondrosis are known from cetaceans (de Smet, 1977; Kompanje, 1999; Thomas & Barnes, 2015). Vertebral exostoses are particularly common in cetaceans, and often result in the fusion of adjacent vertebrae (Slyper, 1931; Kompanje, 1995a,b, 1999; Dawson & Gottfried, 2002). Subchondral cysts have been described in the articular surfaces of the posteriormost cervical vertebrae of an Early Jurassic plesiosaur (Hopley, 2001; reinterpreted by Witzmann et al., 2016). Avascular necrosis has been documented in mosasaur vertebral centra and limb elements (Rothschild & Martin, 1987), as well as in the vertebral centra of fossil cetaceans (Beatty & Rothschild, 2008). In plesiosaurs, as in ichthyosaurs, avascular necrosis is well-documented only in the limb elements (Rothschild & Storrs, 2003). Table 2 Percentage of osteopathologies of ichthyosaurs from the literature and personal observations Pathology Count % Trauma Ankylosis 8 16 Articular disease Total Journal of Zoology 304 (2018) ª 2017 The Zoological Society of London

11 J. M. Pardo-Perez et al. Ichthyosaurian palaeopathology Conclusion Our review of documented ichthyosaur palaeopathologies reveals that a range of skeletal injuries involving osteological trauma, articular diseases, avascular necrosis and idiopathic bone fusions have been evidenced in this clade of obligate marine amniotes over a cumulative history of some 150 million years. However, relative body size and preservational/collecting biases appear to have influenced the frequency at which these abnormalities are reported. The majority of ichthyosaurian osteopathologies affect the forelimb skeleton, as opposed to damage and disease in the vertebral column, which are comparatively rare. Such observations conflict with the documented records of pathologies in other obligate marine amniote clades, especially mosasaurid squamates and cetaceans. Although these constituted ecologically analogous large aquatic predators, they were subject to different phylogenetic and palaeobiological constraints. A possible illustration of this might be the higher propensity for avascular necrosis in the limb articulations of ichthyosaurs and plesiosaurians, whereas in mosasaurids and extinct cetaceans such injuries concentrate in the axial skeleton. Rib fractures and facial traumas are otherwise prevalent in all groups, perhaps suggesting that structural variation in locomotory apparatus imposed disparate functional impacts, while feeding and social behaviours were more influenced by the convergent requirements of large-bodied amniote carnivores successfully adapting to life in aqueous environments. Acknowledgements Thanks to I. Werneburg (GPIT), J.-O. Ebbestad (PMU), J. Rabold and S. Eggmaier (U-MO), and D. Pomar and M. Pardo Jaramillo (DON) for collections access; H. Ketchum (OUMNH) for photographs, C. 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