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Abstract We describe the lower jaw of the parareptile Delorhynchus from the Early Permian of Oklahoma, on the basis of a complete, isolated right ramus, and histological thin sections. The lower jaw of Palaeozoic amniotes is generally less well known than other parts of the cranium, largely because they are often preserved in tight occlusion with the skull. Thus, complete information about the dentition and other features of the lower jaw are rarely available. This specimen allows us to recognize for the first time the presence of two coronoid ossifications in a Palaeozoic reptile. Both coronoids bear numerous small teeth, a feature that is commonly found in anamniotes, but rare in amniotes. The distribution of these two features among amniotes reveals that they represent a reversal, and a synapomorphy of the parareptile clade Lanthanosuchoidea, possibly associated with specialized feeding behaviour among these small predators. In addition, Delorhynchus has a coronoid process that is anatomically distinct from those in other parareptiles, indicating multiple, independent origins of this feature within Parareptilia. Furthermore, the complete mandible of Delorhynchus allows us to recognize that the genus Bolterpeton is a junior synonym of the genus Delorhynchus, whereas the species Bolterpeton carrolli is a nomen dubium. mandible, Palaeozoic, Reptilia, teeth INTRODUCTION The immense taxonomic diversity and exceptional preservation found at the Richards Spur locality, Oklahoma, have greatly increased our understanding of upland terrestrial vertebrate communities of the Early Permian. In addition to yielding one of the richest and most diverse faunal assemblages of the Palaeozoic, this locality has preserved many taxa that are currently considered to be endemic (MacDougall & Reisz, 2012). Although this endemism is likely related to the unprecedented taxonomic diversity found at this locality, the fossils from Richards Spur provide unique insights into the initial stages of amniote diversification. Here we provide a description of the lower jaw of the parareptile Delorhynchus, which not only shows critical information on early parareptilian anatomy, but also offers a basis for comparisons with that of the coeval eureptile Captorhinus, which has the only other well-known reptilian lower jaw of this period. The Richards Spur locality has recently yielded many articulated and pristinely preserved specimens, allowing us to revaluate the taxonomic identity and relationships of previously described fragments. These recent discoveries have shown that several parareptile species are present at this locality: Bolosaurus grandis Reisz et al., 2002, Feeserpeton oklahomensis MacDougall & Reisz, 2012, Microleter mckinzieorum Tsuji, Müller, & Reisz, 2010, Abyssomedon williamsi MacDougall & Reisz, 2014, Colobomycter pholeter Vaughn, 1958 (Modesto, 1999), Colobomycter vaughni MacDougall et al., 2016, Delorhynchus cifellii Reisz, MacDougall, & Modesto, 2014, and Delorhynchus priscus Fox, 1962 (Reisz et al., 2014). The genus Delorhynchus was previously known only through fragmentary remains (D. priscus, KU 11117), which were initially described as pelycosaur material (Fox, 1962) before being reassigned to Parareptilia (Modesto, 1999). The holotype of D. cifellii (OMNH 73515) and other specimens of this species show features that are comparable with those of Bolterpeton carrolli, a taxon that was previously described on the basis of fragmentary remains. This is of particular interest as the lower jaw of Delorhynchus, described here in detail, this lower jaw cannot be diagnosed at the species level as there is no lower jaw for D. priscus; however, it is evident that it is indistinguishable from the holotype of B. carrolli Anderson & Reisz, 2003. Herein we describe the anatomy of the lower jaw and associated dentition of Delorhynchus in detail, which has only been previously documented with this level of histological detail in Captorhinus (Heaton, 1979). This was facilitated through a series of coronal sections of the lower jaw, allowing us to determine the exact contribution of each element to the morphology of the mandible. The preservation of this jaw has also allowed for the identification of two coronoid ossifications and their associated dentition, which were referred to previously as denticles (Anderson & Reisz, 2003). The coronoid and its associated dentition were also examined histologically, revealing that the coronoid dentition is indistinguishable from that of the marginal dentition on a microanatomical basis; therefore, we refer to the coronoid dentition as true teeth rather than the problematic term denticles (Gee et al., 2017). Institutional abbreviations—OMNH, Sam Noble Oklahoma Museum of Natural History, Norman, Oklahoma, USA; KU and KUMNH, University of Kansas, Museum of Natural History, Lawrence, Kansas, USA; ROM, Royal Ontario Museum, Toronto, Ontario, Canada. MATERIALS AND METHODS The illustrations found in Figures 1 and 2 were made using Adobe Photoshop CS6 to create outlines, and detailed shape was added to the illustrations using charcoal pencils and coquille paper. The diagrams for Figures 3 and 4 were made using Adobe Illustrator CS6. To access normally inaccessible anatomy of the lower jaw and its dentition, a near-complete right mandible was serially sectioned along a coronal plane, both thin and thick sections were made. The coronoid region was thin sectioned and used in Figure 3, while the remaining thick sections were photographed and then outlined to make the schematic diagram found in Figure 4. All specimens in this study have been photographed using a Canon EOS40D, prior to sectioning. All sectioning was done following the ROM histology protocol. Specimens were embedded in AP Castolite acrylic resin, vacuumed and left to cure for a minimum of 24 h. All resin-embedded specimens were cut using an Buhler isomet 1000 wafer saw at a low speed of 275 rpm. The specimens were then mounted on plexiglass slides using Scotch-Weld SF-100 cyanoacrylate. The slides were then mounted on the thin section grinding machine (Hillquist) and ground down using the grinding cup until optical clarity was achieved, subsequently the specimen was then ground by hand using progressively finer grit, beginning with a 600 silicon carbide powder and working down to a 1 μm aluminium oxide powder. All slides produced were imaged using Nikon DS-Fi1 camera mounted to a Nikon AZ 100 microscope fitted with crossed-polarizing and lambda filters, and an oblique illumination slider, image processing was done through NIS-Elements registered to R. R. Reisz of the University of Toronto Mississauga. The photographs in Figures 6 and 7 were obtained using a Jeol Neoscope JCM-5000 scanning electron microscope (SEM). Figure 1. View largeDownload slide Delorhynchus mandible, OMNH 72363. A, photograph of lingual view. B, photograph of labial view. C, illustration of lingual view. D, illustration of labial view. Scale bar equals 1 cm. Abbreviations: an, angular; ar, articular; cr1, anterior coronoid; cr2; posterior coronoid; d, dentary; f int me, foramen intermandibularis medius; f int ca, foramen intermandibularis caudalis; pr, prearticular; sp, splenial; sur, surangular. Figure 1. View largeDownload slide Delorhynchus mandible, OMNH 72363. A, photograph of lingual view. B, photograph of labial view. C, illustration of lingual view. D, illustration of labial view. Scale bar equals 1 cm. Abbreviations: an, angular; ar, articular; cr1, anterior coronoid; cr2; posterior coronoid; d, dentary; f int me, foramen intermandibularis medius; f int ca, foramen intermandibularis caudalis; pr, prearticular; sp, splenial; sur, surangular. Figure 2. View largeDownload slide Delorhynchus mandible, OMNH 72363. A, photograph of ventral view. B, photograph of dorsal view. C, illustration of ventral view. D, illustration of dorsal view. Scale bar equals 1 cm. Abbreviations: an, angular; ar, articular; cr1, anterior coronoid; cr2; posterior coronoid; d, dentary; pr, prearticular; sp, splenial; sur, surangular. Figure 2. View largeDownload slide Delorhynchus mandible, OMNH 72363. A, photograph of ventral view. B, photograph of dorsal view. C, illustration of ventral view. D, illustration of dorsal view. Scale bar equals 1 cm. Abbreviations: an, angular; ar, articular; cr1, anterior coronoid; cr2; posterior coronoid; d, dentary; pr, prearticular; sp, splenial; sur, surangular. Figure 3. View largeDownload slide Interior view of Delorhynchus mandible, ROM 76624. A, anterior portion of the mandible. B, anterior portion of the first coronoid. C, posterior portion of the first coronoid. D, anterior portion of the second coronoid. E, maximum height of the coronoid process. F, posterior adductor foramen. Scale bar equals 1 cm. Abbreviations: add for, adductor foramen; an, angular; ar, articular; cr1, anterior coronoid; cr2; posterior coronoid; d, dentary; f int or, foramen intermandibularis medius; f int ca, foramen intermandibularis caudalis; pr, prearticular; sp, splenial; sur, surangular. Figure 3. View largeDownload slide Interior view of Delorhynchus mandible, ROM 76624. A, anterior portion of the mandible. B, anterior portion of the first coronoid. C, posterior portion of the first coronoid. D, anterior portion of the second coronoid. E, maximum height of the coronoid process. F, posterior adductor foramen. Scale bar equals 1 cm. Abbreviations: add for, adductor foramen; an, angular; ar, articular; cr1, anterior coronoid; cr2; posterior coronoid; d, dentary; f int or, foramen intermandibularis medius; f int ca, foramen intermandibularis caudalis; pr, prearticular; sp, splenial; sur, surangular. Figure 4. View largeDownload slide Histology of coronoid dentition in Delorhynchus, ROM 76624. A, lingual view of Delorhynchus mandible, ROM 76624 with thin cut section indicated. B, thin section showing both the marginal and coronoid dentition. C, a close up of the coronoid dentition. D, a close up of the coronoid dentition under cross-polarized light. Scale bars equal 500 μm. Abbreviations: ab, alveolar bone; cr, coronoid; d, dentary; de, dentine; lb, lamellar bone; pc, pulp cavity; rl, reversal line; so, secondary osteon. Figure 4. View largeDownload slide Histology of coronoid dentition in Delorhynchus, ROM 76624. A, lingual view of Delorhynchus mandible, ROM 76624 with thin cut section indicated. B, thin section showing both the marginal and coronoid dentition. C, a close up of the coronoid dentition. D, a close up of the coronoid dentition under cross-polarized light. Scale bars equal 500 μm. Abbreviations: ab, alveolar bone; cr, coronoid; d, dentary; de, dentine; lb, lamellar bone; pc, pulp cavity; rl, reversal line; so, secondary osteon. Figure 5. View largeDownload slide Strict consensus tree obtained from the six optimal trees produced by the phylogenetic analysis. Solid black triangles indicate the presence of coronoid dentition, whereas empty black triangles indicate the absence of coronoid dentition. Solid red squares indicate the presence of two coronoids, whereas empty red squares indicate the presence of a single coronoid. Nodes of clades of interest are labelled: A, Amniota; B, Reptilia; C, Eureptilia; D, Parareptilia; E, Lanthanosuchoidea. Figure 5. View largeDownload slide Strict consensus tree obtained from the six optimal trees produced by the phylogenetic analysis. Solid black triangles indicate the presence of coronoid dentition, whereas empty black triangles indicate the absence of coronoid dentition. Solid red squares indicate the presence of two coronoids, whereas empty red squares indicate the presence of a single coronoid. Nodes of clades of interest are labelled: A, Amniota; B, Reptilia; C, Eureptilia; D, Parareptilia; E, Lanthanosuchoidea. Figure 6. View largeDownload slide Scanning electron microscope dentition comparison between the holotype of Bolterpeton carrolli, OMNH 71111, and Delorhynchus mandible, OMNH 72363. A, scanning electron microscope (SEM) photograph of the B. carrolli holotype showing coronoids and marginal dentition. B, SEM photograph of the Delorhynchus mandible showing coronoids and marginal dentition. C, close up SEM photograph of the marginal and coronoid dentition of Bolterpeton. D, close up SEM photograph of the marginal and coronoid dentition of Delorhynchus. Figure 6. View largeDownload slide Scanning electron microscope dentition comparison between the holotype of Bolterpeton carrolli, OMNH 71111, and Delorhynchus mandible, OMNH 72363. A, scanning electron microscope (SEM) photograph of the B. carrolli holotype showing coronoids and marginal dentition. B, SEM photograph of the Delorhynchus mandible showing coronoids and marginal dentition. C, close up SEM photograph of the marginal and coronoid dentition of Bolterpeton. D, close up SEM photograph of the marginal and coronoid dentition of Delorhynchus. Figure 7. View largeDownload slide Replacement in coronoid dentition of Delorhynchus. A, photograph of the denticulate region of the coronoid of Delorhynchus OMNH 72363. B, scanning electron microscope (SEM) photograph of replacement tooth in matrix and surrounding reabsorption. Figure 7. View largeDownload slide Replacement in coronoid dentition of Delorhynchus. A, photograph of the denticulate region of the coronoid of Delorhynchus OMNH 72363. B, scanning electron microscope (SEM) photograph of replacement tooth in matrix and surrounding reabsorption. The phylogenetic analysis was performed in PAUP 4.0a152 (Swofford, 2017) using the methodology of MacDougall & Reisz (2014) [tree length = 571, consistency index (CI) = 0.338, rescaled CI = 0.215, retention index = 0.635]. The data matrix used was the MacDougall et al. (2017) matrix, which contained 169 characters and 39 taxa; no modifications were made to this matrix. Referred Specimens—ROM 76624, partial right mandibular ramus; OMNH 73363, complete right mandibular ramus; OMNH 52364, holotype of B. carrolli, partial right mandibular ramus; OMNH 71111, B. carrolli, partial right dentary; KU 11117, holotype of D. priscus, a partial right maxilla. SYSTEMATIC PALAEONTOLOGY Parareptilia Olson, 1947 Procolophonomorpha Romer, 1964 Ankyramorpha Debraga and Reisz, 1996 Lanthanosuchoidea Efremov, 1946 Delorhynchus Fox, 1962 (Figs 1–5) Diagnosis Parareptile characterized by the following apomorphies: presence of robust, slightly recurved homodont dentition; presence of well-developed dorsal lamina in the anterior portion of the maxilla, forming the tall posterior edge of the external naris with a well-developed anteromedial shelf; and dermal sculpturing consisting of low tuberosities in adults that largely replaces a pattern of diffuse, circular dimples present in subadults. Remarks In spite of the lack of mandibular characters in the original diagnosis, the denticulate coronoids ascribed to D. cifellii (Reisz et al., 2014) in combination with the morphology of the marginal dentition, and overall relative proportions of the mandible allow us to refer Bolterpeton to the genus Delorhynchus. It should also be noted that the specimens of ‘Species X’ as described by Bolt (1980) have not been examined in this study, it is possible that they are ontogenetic stages of Delorhynchus or entirely distinct species. DESCRIPTION These isolated mandibles of Delorhynchus present a relatively rare opportunity to study in detail the anatomy of the lower jaw of a Palaeozoic amniote. The most common mode of preservation of early amniote jaws is either as fragmentary remains, or in close association with skulls. In the latter case, the lower jaw tends to be tightly appressed against the palate, obscuring critical aspects of its anatomy, as has been the case in other lanthanosuchoids, such as F. oklahomensis (MacDougall & Reisz, 2012) and Acleistorhinus pteroticus (deBraga & Reisz, 1996). We are therefore fortunate in finding isolated, completely preserved mandibular rami, which allows us to study and illustrate in detail most aspects of their anatomy. The mandible of D. cifellii (OMNH 73363) is indistinguishable from the specimens described here (OMNH 71111, OMNH 52364 and ROM 76624); OMNH 73363 was previously mentioned and described briefly by Reisz et al. (2014). Mandible The mandible of Delorhynchus is an elongate structure, subdivided into three distinct regions, a slender anterior tooth bearing region, a tall coronoid region immediately posterior to the tooth row and a broad posterior region that includes the articular cotyles for the jaw articulation. The mandible is most robust in the region of the articular bone, which forms not only the articular surfaces with the skull, but also maintains well-developed medial and posterior processes. As noted in Reisz et al. (2014), the mandible is characterized by the presence of a well-developed coronoid process; however, what is more striking is the occurrence of two denticulate coronoid ossifications which can been observed on the medial surface (Fig. 1). The dentary is a long element, forming most of the lateral surface of the mandible in the region of the tooth row, and extends from the mandibular symphysis to slightly beyond the coronoid process. The lateral surface of the dentary lacks the type of ornamentation found on the skull roof of Delorhynchus, but the anteriormost portion of the bone exhibits a series of mental foramina. This pattern of clustering of anterolateral foramina near the symphysis is present in all Richards Spur lanthanosuchoids, and is prominent in all D. cifellii specimens (Reisz et al., 2014). Ventrally, most of the dentary is overlain by the splenial, forming a linear suture best observed in lateral and ventral views (Figs 1D, 2C). Posterodorsally, the dentary overlies the surangular bone, a slender process of the dentary extends between the lateral exposure of that bone and the coronoid process. The dentary contacts the angular posteroventrally, forming a highly interdigitated suture that continues dorsally to form a similar suture with the surangular. Lingually, the dentary forms the lateral wall of the Meckelian fossa, as well as contacting the dorsal edge of the medial portion of the splenial, and the anterior coronoid. The splenial is a large, smooth superficial element that forms much of the lingual and ventral surfaces of the lower jaw, spanning more than half the length of the mandible (Fig. 2). Only a small portion of the splenial, directly under the dentary and angular bones, is visible in labial view. The splenial has an uneven posterior edge that contributes to the anterior margin of the large intermandibular caudalis foramen, as is the case in other parareptiles, such as Macroleter poezicus (Tsuji, 2006). The posterior flange of the splenial continues to contact and overlap the labial exposure of the angular. The posterior dorsal edge of the splenial contacts the first coronoid process along its ventral vedge. The foramen intermandibularis medius, as described in the eureptile Captorhinus laticeps (Heaton, 1979), is not present. Instead, there is a large uncovered region of the Meckelian canal, as the splenial does not continue anteriorly to contact the mandibular symphysis, and therefore does not create a foramen proper as in C. laticeps (Heaton, 1979). Most parareptiles have a mandibular symphysis that is formed entirely by the dentary bone, and a similar pattern where the most anterior part of the dentary is uncovered by the splenial is also observed in the procolophonid Scoloparia glyphanodon (Sues & Baird, 1998). Anteriorly, the splenial has two tongue-like processes that attach to the dorsal and ventral edges of the dentary. The angular is a large, dorsally curved element that forms the posterior portion of the labial and ventral surfaces of the mandible. This element is bordered anteroventrally by the splenial, anterodorsally by the dentary, dorsally by the surangular, and extends posteriorly to meet the articular. In lingual view, the angular extends dorsally from the ventral ridge to form much of the medial wall and floor of the adductor fossa, and underlies the splenial anteriorly, forming an elongate anteriorly slanted suture. Dorsally, the angular contacts the prearticular bone, and together with the splenial and the prearticular, contributes to the large intermandibular caudalis foramen. Thin sections have allowed us to determine that a process of the angular underlies the splenial anteriorly (Fig. 3B); this internal anterior process of the bone extends to the level of the suture between the two coronoids. The angular bone extends even farther anteriorly as a relatively thin sheet of bone that appears to brace the internal surfaces of the dentary and splenial, at least to the level of the 12th tooth position. Comparison with Captorhinus indicates that the anterior, internally located process of the angular is much shorter in this eureptile (Heaton, 1979) than in Delorhynchus, and it does not appear to extend anteriorly beyond the level of the foramen intermandibularis caudalis. The angular shows the presence of a distinct pattern of rugosities posteroventrally that can be best viewed labially (Figs 1D, 2C). The rugosity on the labial surface of the mandible likely represents dermal sculpturing, although it differs from the pattern of round pits found on the skull roof, or the tuberosities present along the edges of the orbit (Reisz et al., 2014). It is unlikely that this pattern of rugosities is related to muscle attachments, like the pterygoideus muscles, because a sharp ventral ridge separates the labial and lingual surfaces of the bone (Fig. 2C). This ventral ridge has an uneven edge, and slight rugosities are present on either side. It is possible that this ventral ridge may have been the origin of muscles that extended posteriorly, possibly to the hyoid apparatus (equivalent to mandibulohyoideus mm. in squamates). The surangular is a complex element that contributes to several features of the lower jaw. Anterodorsally, the surangular contributes to the prominent coronoid process, forming a rugose eminence that meets the coronoid bone to form the labial component of the coronoid process proper. Extending posteriorly from this eminence, the dorsal edge of the surangular has a wide, flattened surface that forms a shelf, as in numerous other parareptiles (Reisz & Scott, 2002; Tsuji et al., 2010). This is in strong contrast to the condition present in eureptiles and synapsids primitively, where the dorsal edge of the surangular is rounded (Heaton, 1979; Reisz & Berman, 1986). The presence of this enlarged dorsal shelf is likely related to the modified pattern of adductor muscle attachment that may have evolved in parareptiles (Reisz & Scott, 2002). Posteriorly, the surangular is broadened labiolingually, and abuts against the body of the articular bone; a thin posteriorly oriented sheet of bone extends from the main body of surangular and wraps around the articular bone. The posterior sheet of bone of the surangular covers much of the articular cotyle in labial view. Lastly, the surangular, along with the angular, forms the internal wall of the adductor fossa, which can be observed lingually (Figs 1, 2). Thin sections of the mandible indicate that the surangular has a long internal anterior process, on that underlies the dentary bone, well anterior to the coronoid process, almost to the level of the anterior edge of the posterior coronoid bone (Fig. 3D), which is in contrast to the condition that is observed in the eureptile Captorhinus. The prearticular is another complex element of the lower jaw; it is exposed in lingual, ventral and dorsal views. It is a relatively slender, elongate bone that is wedged anteriorly between the splenial and the posterior coronoid, and reaches far anteriorly beneath the splenial as a thin sliver of bone, extending to the midpoint of the anterior coronoid (Fig. 3C). Ventrally, it borders the splenial, the large intermandibularis foramen caudalis, and the angular, and underlies the articular posteriorly. The dorsal edge of the prearticular forms the lingual margin of the adductor fossa. Furthermore, along this dorsal edge there is a sharp ridge that runs the length of the element, moving posteriorly this ridge twists ventrally and forms a lingually slanted shelf. The articular is a large-bodied and robust element that is composed of two cotyles and a retroarticular process that makes up the posterior portion of the mandible. Labially, the articular contacts the angular and surangular, whereas it contacts the prearticular lingually. The mandibular fossa of the articular is large and located dorsally, and is characterized by the porous unfinished bone that would have likely been capped with cartilage to aid in articulation with the condyle of the quadrate. The articular surface of the fossa is surrounded by rugose areas of thicker bone that give the bone its robustness, and includes a particularly rugose medioventrally projecting pterygoideus process. The retroarticular process projects posteriorly beneath the main body of the articular, and its rounded circumference is marked by rugosities that were probably areas of origin for jaw-opening musculature (depressor mandibularis). Perhaps the most interesting aspect of the mandible of Delorhynchus is the distinctive coronoid complex. It is composed of two separate bones, a simple, sheet-like anterior coronoid bone, and a more complex posterior coronoid. Both coronoid elements bear numerous small teeth all of which are on elevated pads of bone that are distinct from the coronoid element itself. Histological thin sections of this region have shown that the dentigerous pads are composed of several generations of alveolar bone similar to that observed in LeBlanc & Reisz (2013), this is best illustrated in Figure 4. These pads are more distinct and occupy less of the total coronoid area in the smaller and presumably younger specimen (ROM 76624), whereas in the larger specimen (OMNH 72363) the pads occupy the majority of the surface area of the individual coronoid elements. The larger mandibular ramus (OMNH 72363) has dentigerous pads essentially forming a continuous field of teeth on the dorsolingual surface of the lower jaw, which extends over most of the anterior coronoid and covers the anterior half of the posterior coronoid. The posterior coronoid differs from the anterior coronoid in having an edentulous posterior region that meets dorsally with the surangular; together they form the coronoid process. The coronoid process is rather rugose in appearance with a number of tuberosities on the lingual surface. This is complimented by the rugose surface of the underlying surangular, together making up a robust muscle attachment site at the apex of the coronoid process. This is in contrast to some other parareptiles like Belebey vegrandis where the coronoid process has a tripartite composition of dentary, coronoid and surangular (Reisz et al., 2007), or the condition in Owenetta, where it is composed of the dentary alone (Reisz & Scott, 2002). Dentition Marginal The morphology of the marginal dentition associated with the mandibular ramus described by Reisz et al. (2014) is indistinguishable to the maxillary dentition of D. priscus, D. cifellii and B. carrolli. The general morphology of the mandibular dentition can be described as moderately recurved and distinctly monocuspid with an uneven apex that leads to a cutting blade running labiolingually down the body of the tooth. Delorhynchus marginal dentition also consistently shows fluting crownward, this is a feature that is also found among eureptiles (Reisz et al., 2015), other parareptiles (Modesto, 1999) and anamniotes (Fröbisch & Reisz, 2012), indicating that this feature is likely primitive for tetrapods. In the specimens attributed to Delorhynchus, the teeth have a characteristic, slightly recurved outline, and rootwards there are external plications, which are caused by the infolding of dentine (MacDougall, LeBlanc & Reisz, 2014). The shape and the presence of plicidentine near the root of the teeth are a feature found not only within the genus Delorhynchus, but also other closely related parareptiles, such as Colobomycter, Feeserpeton and Microleter (MacDougall et al., 2014). Coronoid Interestingly, all lanthanosuchoid taxa that have exposed coronoids exhibit dentition on these elements. Re-examination of computed tomography (CT) scans of the holotype and only know specimen of F. oklahomensis (MacDougall & Reisz, 2012) revealed the presence of coronoid teeth, their disposition and the size of the dental pads could only be determined by CT scans because that part of the mandible was not visible externally. Coronoid teeth have also been found to be present in C. pholeter (MacDougall et al., 2017). It should be noted that the coronoid dentition mirrors that of the marginal teeth in external and internal morphology. Externally there is a colour variation on the crown portion of the coronoid dentition, this is also found in the marginal dentition, and likely indicates the portion of the crown that is covered by enamel. Additionally, there is the same fluting of the enamel on the coronoid teeth as there is in the marginal dentition. Internally we see other similarities between coronoid and marginal dentition, for instance the amount of enamel, dentine and alveolar bone are proportionately the same between the coronoid and marginal dentition. There seems to be disparity in the size range of the individual teeth on the dentulous pads, with the more labial dentition being larger. This is more evident on the larger specimen (OMNH 72363) than on the smaller ramus (ROM 76624). The size of these dental pads and therefore the number and size of coronoid teeth appears to vary between individual specimens of Delorhynchus, and this variation may be related to ontogeny with the larger specimens having larger coronoids, hence more teeth. Histology The mandibular ramus ROM 76624 was serially sectioned to better understand the internal morphology of the element; particular interest was paid to the coronoids and their associated dentition. Two thin sections were made of (ROM 76624); these thin sections were located in the centre of the anterior and posterior coronoid, respectively. The thin sections allowed for histological analysis of the marginal and coronoid dentition, as well as the underlying bone. The marginal dentition is implanted in a pleuroacrodont fashion with a bias towards acrodonty; we assign the term pleuroacrodonty to this specimen due to the unequal geometry of the dentary underlying the marginal dentition. It should also be noted that although there is a bias towards a higher labial wall, the labial wall is mostly composed of alveolar bone rather than true jawbone. The marginal dentition of Delorhynchus has been previously sectioned and interpreted (MacDougall et al., 2014), for this reason the histological description of the marginal dentition will remain brief as the focus of this study is the coronoid dentition. The coronoid dentition is similar in attachment to that of the marginal dentition of multiple tooth-rowed captorhinids (LeBlanc & Reisz, 2015), where alveolar bone ankylosis the dentition to the underlying bone; furthermore, there seems to be secondary attachment via alveolar bone to the neighbouring teeth as well as resorption of the neighbouring teeth which is also seen in captorhinids (Fig. 4). The enamel found on the coronoid dentition is uneven favouring the lingual side; this is likely due to wear or taphonomic damage. The enamel-covered crown makes up about one-third of the total height of the tooth (Fig. 4). The dentine underlying the enamel is characterized by radiating dentine tubules that terminate at the pulp cavity. In the dentine Lines of Von Ebner can be observed, these are much more prominent towards the crown, they are unevenly spaced, eventually becoming indistinct towards the root portion of the tooth. Rootwards there does not seem to be any plicidentine infolding which is clearly visible in the marginal dentition, this is likely a character of scale, supporting the idea that smaller dentition may need less infolding to properly attach to the underlying tissues (MacDougall et al., 2014). The dentition is attached to the coronoid by a fibrous mineralized tissue that contains trabeculae that we here identify as alveolar bone (LeBlanc & Reisz, 2013, 2015). This attachment tissue is present at the base of the dentition as well as between where the teeth come in contact with one another. The alveolar bone is parallel fibered in appearance under cross-polarized light, which is distinguishable from the lamellar bone making up the coronoid and the dentary. There are also two reversal lines separating the alveolar bone and the underlying layers, the layers are composed of alveolar bone super imposed on one another, this is likely a remnant of tooth replacement at this location. The reversal lines not only show separation between the various generations of alveolar bone, but also mark the boundary between the fibrous alveolar bone and the underlying lamellar coronoid bone (Fig. 4). In thin section the coronoid is made up of lamellar bone that is highly organized with few vascular spaces. There are incremental growth lines that are evenly distributed along the coronoid, indicating even growth and slow bone deposition through ontogeny. Unlike the captorhinid condition in which fast growing jaw bone is highly vascularized and continuously remodelled (LeBlanc & Reisz, 2015), here the coronoid and dentary bone are made of evenly deposited lamellar bone, where the only resorption and remodelling are done at the dentulous surfaces (Fig. 4). DISCUSSION Multiple coronoids and coronoid dentition among Palaeozoic reptiles Multiple coronoids have been used often as a character in analyses of microsaurs (Carroll & Currie, 1975; Anderson & Reisz, 2003) and other anamniotes (Gregory, 1948; Carroll & Currie, 1975), and the presence of two coronoids has been reported in numerous synapsid amniotes, such as Haptodus garnettensis (Laurin, 1993) and Edaphosaurus boanerges (Modesto, 1995). The presence of multiple coronoids has also been documented in Seymoria baylorensis (Laurin, 1996), Tseajaia campi (Kissel, 2010) and Limnoscelis paludis (Berman, Reisz & Scott, 2010). However, the presence of multiple coronoids in Reptilia has not been previously noted. A single coronoid element is present in many well-known eureptiles: the captorhinids Captorhinus aguti (Fox & Bowman, 1966; Heaton, 1979), C. laticeps (Heaton & Reisz, 1980) and Labidosaurus hamatus (Modesto et al., 2007); the basal captorhinomorphs Paleothyris acadiana (Carroll, 1969) and Protorothyris (Clark & Carroll, 1973); as well as in the basal diapsids Petrolacosaurus (Reisz, 1977) and Araeoscelis (Reisz, 1977; Reisz, Berman & Scott, 1984). It was therefore assumed that Reptilia lost the anterior coronoid, or that the two elements fused. This latter hypothesis was supported by the presence of an elongate anterior process of the coronoid in captorhinids (Heaton, 1979). Although we seem to have evidence of multiple cases of loss of the anterior coronoid as a separate ossification in all amniote lineages (Synapsida, Eureptilia and Parareptilia), the absence of information about the condition of the coronoids in such critical reptiles as Hylonomus (Carroll, 1964), Cephalerpeton (Gregory, 1948; Carroll & Baird, 1972) and Macroleter (Tsuji, 2006) makes a comprehensive evaluation of this evolutionary event difficult. Nevertheless, when the presence of one or two coronoids is mapped onto a cladogram of Palaeozoic amniotes (Fig. 5), the overall pattern supports the hypothesis that the presence of a single coronoid is a synapomorphy of Reptilia. Thus, the fusion of the coronoids into a single element, or the loss of the anterior coronoid, appears to have occurred at the base of Reptilia. The occurrence of two coronoids in the lanthanosuchoids Feeserpeton and Delorhynchus appears to represent a reversal to the primitive tetrapod condition, or an independent acquisition of this condition. The association between the presence of two coronoids and the presence of coronoid dentition may suggest that these two character states are associated in lanthanosuchoids. Coronoid dentition is rare among amniotes (Laurin, 1993), but has been found to be widely distributed among anamniotes (Carroll & Currie, 1975; Fröbisch & Reisz, 2012); furthermore, it is often referred to as denticles in literature. Here we show that this character now extends into reptiles, and that coronoid denticles are in fact true teeth (sensuHuysseune, Sire & Witten, 2009). The two coronoid elements in Delorhynchus exhibit a large number of teeth, a feature that appears to characterize lanthanosuchoids. Two coronoids and coronoid dentition is occasionally present in other amniotes, like the synapsid H. garnettensis (Laurin, 1993). The presence of two coronoids with dentition in Delorhynchus and Feeserpeton, as well as at least one dentulous coronoid in Colobomycter (MacDougall et al., 2017), is in strong contrast to the condition observed in other parareptiles. This can be most parsimoniously interpreted as support for the hypothesis that the primitive condition for this clade is the presence of a single, edentulous coronoid. Mesosaurs, millerettids and bolosaurids have only one edentulous coronoid, as do Microleter, Nyctiphruretus, procolophonids and pareiasaurs (Fig. 5). As previously noted, the coronoid dentition mirrors that of the marginal dentition in external and internal morphology; therefore, it is reasonable to assume that the replacement mode for these teeth would follow that of the pleurodont marginal dentition. This however would be difficult to prove without further histological sampling, the material examined for this study shows that it is likely that multiple generations of teeth occupied the coronoids on the basis of several generations of alveolar bone being present, and super imposed (Fig. 4). Each layer of alveolar bone demarcates a tooth attachment site, and prominent reversal lines show that these layers were deposited sequentially with minimal resorption between generations. External evidence of replacement is visible in the larger specimen (OMNH 72363), there are several empty sockets that show no outward damage due to attritional processes; furthermore, a small compete enamel cap is present in the matrix on the posterior coronoid amongst ankylosed dentition (Fig. 7). This enamel cap (Fig. 7) is positioned in an area that largely lacks dentition, additionally there seems to be a depression in neighbouring tooth as well as the surrounding bone, suggesting resorption of the adjacent tooth as well as the surrounding bone prior to deposition of new alveolar bone and subsequent ankylosis of this new tooth, making it reasonable to deduce that this is a replacement tooth that would have been in soft tissue. As previously noted, the coronoid teeth vary in size which cannot be attributed to replacement rate, with the largest of them being positioned labial to the smallest, this is best observed in OMNH 72363, this heterodonty likely becomes more pronounced through ontogeny as it is harder to visualize in the smaller specimen (ROM 76624). New evidence regarding the internal anatomy of the mandible in Delorhynchus indicates not only that the dentary and angular are surprisingly robust when viewed in transverse section, but also that the superficial elements are extensively reinforced through the anterior extension of the angular, surangular and prearticular bones (Fig. 3). This is a particularly interesting phenomenon because the anterior extension of these bones provides additional internal reinforcement of the region where the tooth-covered coronoids are found. This raises the possibility that these anatomical features are related to the presence of coronoid dentition in animals that have unusually large number of palatal teeth, likely related to their hypercarnivorous lifestyle. This is in contrast to the condition observed in coeval captorhinid eureptiles, where there is no coronoid dentition, palatal dentition is reduced, and the marginal chewing surface is expanded via multiple rows of teeth or large bulbous dentition (LeBlanc & Reisz, 2015; LeBlanc et al., 2015; Reisz et al., 2015). Furthermore, in captorhinids the angular, surangular and prearticular are not anteriorly extended as in Delorhynchus, likely because further reinforcement to the interior of the mandible is not necessary for captorhinids. Future work will allow us to expand these types of comparisons, by including parareptiles and other eureptiles that lack coronoid dentition. Taxonomic status of Bolterpeton carrolli Anderson & Reisz (2003) erected the taxon B. carrolli on the basis of a few morphological features of a fragmentary lower jaw (OMNH 71111), but mainly on the presence of the two coronoids, as well as some apparently unique features of its dentition. Since the discovery of the fragmentary jaw of Bolterpeton, many new parareptiles from Richards Spur, as well as more complete specimens of previously known taxa, have been described (Modesto, 1999; Tsuji et al., 2010; MacDougall & Reisz, 2012; Reisz et al., 2014; MacDougall et al., 2017). Many of these taxa share dental and mandibular features that were assumed to only belong to anamniotes like Cardiocephalus and Euryodus (Anderson & Reisz, 2003). For instance, the delicate fluting on the enamel that is present on the lingual side of the crown is commonly found in parareptiles, including Colobomycter, Microleter and Delorhynchus (Fig. 6). Similarly, the anterior and posterior cutting edges or ridges, misinterpreted as ‘weakly bicuspid’ (Anderson & Reisz, 2003: p. 503, fig. 3) because they lack a sulcus that would separate the lingual and labial cusps, this term was originally used by (Bolt, 1977) to describe the amphibamids Doleserpeton and Tersomius, are also clearly present in Delorhynchus (Fig. 6). However, upon re-examination of the holotype of Bolterpeton the original observation of ‘weakly bicuspid’ morphology can be attributed to uneven wear of the teeth on the lingual side. In fact, the tooth morphology of B. carrolli is indistinguishable from that observed in D. cifellii (OMNH 73363) (Fig. 6). Finally, the presence of two coronoids that are covered with dentition is clearly present in both the holotype of B. carrolli and D. cifellii. The anatomy of the preserved portion of the posterior coronoid, with its anterior region being covered by teeth, and the posterior edentulous portion, where the base of the coronoid process is preserved, is also identical to that in D. cifellii. Therefore, we have no hesitation in declaring that the genus Bolterpeton is a junior synonym of the genus Delorhynchus. However, there are no known lower jaws belonging to D. priscus, making it impossible to determine if B. carrolli is a junior synonym of D. priscus or D. cifellii. Therefore, we declare that the species B. carrolli is a nomen dubium. CONCLUSIONS The Dolese Brothers Limestone Quarry, near Richards Spur, continues to provide a wealth of information on the anatomy and evolution of Early Permian terrestrial vertebrates, notably parareptiles. New material of the parareptile Delorhynchus reveals the presence of multiple coronoids on the mandibular ramus, as well as dentition on these coronoids. The multiple coronoids in Delorhynchus have lead to a more detailed look at the coronoids of lanthanosuchoids, although there are some critical species on which we have no coronoid data, there is still compelling evidence that lanthanosuchoids either independently acquired denticulate multiple coronoids, or show a reversal to the condition found in anamniotes. This study has also shown how the reinforced internal anatomy of the jaw of Delorhynchus is likely associated with the dentition found on the coronoids, and the purpose of this re-enforcement is to cope with the physiological stresses put on the element during feeding. The discovery of the multiple coronoids and associated dentition has furthered our understanding of the evolution of morphology of the parareptilian jaw and their adaptations to hypercarnivory. The histological data presented have shown unambiguous parallels between the coronoid and marginal dentition, in that they are histologically identical when it comes to the tissues that form these structures, and by that logic we promote the term teeth rather than denticles for the coronoid dentition. Lastly, through the analysis of the lower jaw of Delorhynchus all the anatomical features that were initially assigned to Bolterpeton can now be attributed to the genus Delorhynchus, effectively making the genus Bolterpeton a junior synonym of Delorhynchus. ACKNOWLEDGEMENTS We would like to thank Diane Scott for her valuable input, preparation of specimens and all of the associated photography, as well as Steven Leduc for the illustrations found in Figures 1 and 2. We also thank Richard Cifelli, Jennifer Larsen and Bill May for specimen loans and hosting our visits to the OMNH. We further thank Jason Anderson for helpful discussions regarding Bolterpeton. 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Zoological Journal of the Linnean Society – Oxford University Press
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