1.3.1. FMNH UC 699

This complete left ramus in the Field Museum of Natural History comes from the Admiral Formation and is the stratigraphically oldest of all examined specimens (Figs 1a, b, 8a). Its accompanying label does not permit unambiguous assignment to either the lower-middle or the upper part of the Admiral Formation which, in current stratigraphic terms, are assigned to the Nocona and Petrolia Formations, respectively (Milner & Schoch Reference Milner and Schoch2013). FMNH UC 699 occurs within the known stratigraphic range of Trimerorhachis insignis but has a much more gracile appearance than any of the Belle Plains rami that we have had an opportunity to inspect. Its most peculiar feature is a ‘pleated’ surface in the anteriormost one-third of the precoronoid fossa (an excavation situated posterior or posterolingual to the dentary fangs; see relevant descriptive section below), consisting of a series of slightly oblique and irregular ridges (Fig. 8a). No other specimens show this pleated surface, but the lack of additional material of a similar age makes it premature to attribute a taxonomic value to it. In its general proportions, FMNH UC 699 broadly resembles FMNH UR 73 and UR 136 (Figs 27, 28), two rami of T. rogersi from the Choza Formation (late portion of Middle Clear Fork Group; middle-late Kungurian). However, both rami are heavily worn, such that detailed comparisons with FMNH UC 699 are not possible (see Discussion).

1.3.2. CMNH 11322

This almost complete and nearly undistorted right ramus in the Cleveland Museum of Natural History comes from the Early Permian of southern Ohio and shows exquisitely preserved details of the external bone surface (Figs 3d–g, 12a–c, 17a–c, 20b, 22c, d, 25). It was figured by Hansen (Reference Hansen, Feldmann and Hackathorn1996) and has been prepared in the round from the block of limestone in which it was originally partially exposed in lateral view. The symphyseal region, including most of its articulation surface, as well as the anteriormost part of the dentary and associated marginal teeth, are missing. A plaster-filled crack occurs at about the mid-length of coronoid 1, separating two portions of the ramus that match each other closely. A second crack occurs around a small, incompletely preserved portion of the symphysis. CMNH 11322 resembles several mid- to large-sized lower jaw rami from Belle Plains, and we are confident of its assignment to T. insignis, based on the following characters: (1) shape of precoronoid fossa (Fig. 12c), including the spatial relationships and relative contributions of the bones that participate in its formation (dentary; splenial, coronoid 1); (2) details of splenial and postsplenial morphology, such as the extent and location of the ventromedial flange of the splenial (Fig. 12a), the course of the dentary-splenial suture (Fig. 12a), the occurrence of a sulcus in the anteriormost portion of the medial lamina of the splenial that is wrapped around the bulbous symphyseal region (Fig. 12a), the presence of an elongate, subelliptical foramen straddling the splenial-coronoid 1 suture and situated approximately at the level of the mid-length of the intercoronoid fossa (Fig. 12a), and the shape and distribution of foramina along the medial lamina of the postsplenial (Fig. 12b); (3) construction of posterior extremity of the ramus (Figs 20b, 22c, d, 25), including the relative size and orientation of the pre- and postcondyloid processes (Figs 20b, 22c, d), the position of foramina around the chorda tympani foramen (Fig. 22c) and those surrounding the adductor process inside the adductor fossa (Fig. 25c), and the profile of the glenoid (Figs 22d, 25b).

CMNH 11322 was collected from the Dunkard Group, but there are no precise locality data. Its accompanying label indicates that it originates from Belpre, in the upper Washington Formation, Washington County, Ohio. However, hand-written on one side of the originally enclosing block of limestone, the following can be read: ‘CLARKS [sic] HILL, MONROE CO., OHIO’. The Clark [not Clarks] Hill locality occurs in the Greene Formation of Monroe County, Ohio (Berman Reference Berman1978), which directly overlies the Washington Formation. Berman (Reference Berman1978, p. 496) supplied the following information on this locality: ‘Clark Hill on County Route 43, 1.1 miles west of junction with State Route 7, sec. 16, Salem Township, Monroe County, Ohio.’ The same conflicting information is in the catalogue entry for this specimen. Overall, we are inclined to accept Clark Hill as the most probable locality for CMNH 11322. Neither Hlavin (Reference Hlavin1968) nor Olson (Reference Olson1970) reported Trimerorhachis in their fossil faunal lists from Belpre, although Berman (Reference Berman1978, and personal communication, 2005) noted that Clark Hill has yielded several undescribed Trimerorhachis specimens from the Nineveh Limestone (?Sakmarian) of the Greene Formation. Thus, Clark Hill may be the only locality in the Dunkard Group that has produced Trimerorhachis material, as far as we can tell.

1.3.3. FMNH UR 178

Originating from the Clyde Formation (currently placed in the lower-middle part of the Waggoner Ranch Formation), this specimen in the Field Museum of Natural History represents a nearly complete left ramus in which only a small portion of the symphyseal region and the rearmost extremity are missing. It is particularly interesting, both because of its age and because of its unusual features, including its robust aspect and poorly pronounced curvature in dorsal or ventral aspect (Figs 2a–c, 7b, 24e). It may well represent the youngest record (lower-middle Kungurian) of a reasonably complete and well-preserved Trimerorhachis jaw that we are aware of. It appears to belong to the lower end of the stratigraphic range of T. mesops and is slightly older than the two jaw rami of T. rogersi mentioned previously (Figs 27, 28). Its most peculiar feature is a system of small, transverse bony struts in the lateral part of the precoronoid fossa (see descriptive section below) (Fig. 7b), delimiting a series of small, contiguous recesses. The taxonomic attribution of FMNH UR 178 is difficult, but its gentle curvature in dorsal aspect resembles that of the lateral margins of a large T. mesops skull in the American Museum of Natural History (AMNH 4591) figured by Milner & Schoch (Reference Milner and Schoch2013, fig. 11b).

4.6.1. Dentary

The morphology of the dentary conforms for the most part to Williston's (Reference Williston1914) and Case's (Reference Case1935) descriptions. In lateral view and anterior to the triple sutural joint with the angular and postsplenial, the depth of the dentary is approximately constant. Posterior to this joint, its depth decreases rapidly. Its elongate postdental process ends in an acuminate extremity and extends posteriorly for two-thirds to four-fifths of the length of the adductor fossa (Figs 1a, c, 2d, 3d, 5a, 18b, c). Anterior the postdental process, the dorsal margin of the dentary is smoothly concave. Slightly posterior to the level of the posterior dentary fang, the anteriormost portion of the dorsal margin slopes gently but distinctly dorsalward, conferring a slightly ‘hooked’ aspect to the anterior extremity of the ramus in lateral aspect, particularly evident in large specimens (Figs 1b, f, 3d). The ventral margin of the dentary is irregular immediately posterior to the point where it meets the angular and postsplenial. The course of the anterior one-third of the dentary-angular suture exhibits an abrupt change marked by a small but distinct ‘step’. A second ‘step’ is located posterodorsal to the point where the dentary, angular, and surangular meet. In dorsal view, a conspicuous shelf extends along most of the length of the dentary (Figs 5b, 7, 8, 9a–c, 10a, b, d, 11a–c), medial to the marginal teeth. In its anterior part (symphyseal shelf) and just posteromedial to the dentary fangs, the shelf expands rapidly in a lingual direction, bordering the anterior part of the precoronoid fossa. At this level, the dentary forms a W-shaped or subtriangular acuminate process strongly interlocking with the splenial (Figs 5b, 7b, c, 8c, d, 9a, 10b, d, 11a, b). Immediately posterior to the fangs, the shelf is distinctly wider than the bases of the largest, adjacent marginal teeth (Figs 5b, 7, 8, 9a–c, 10a, b, d, 11a–c). Further posteriorly, the shelf slopes increasingly ventrolingually towards the posterior extremity of the jaw and, in some specimens, it is inclined at about 45 degrees to the horizontal at the level of the most posterior marginal teeth (Figs 5b, 7, 8, 9a, c, 10b, d, 11a, c, 14, 15b, 16a, b). The shelf abuts against coronoid 1 at a level situated immediately posterior to the precoronoid fossa (Figs 5b, 7, 8, 9c, d, 11a–c, 14a, d, 15a, b, 16a, b). Beginning at the level of the anterior extremity of coronoid 2, the shelf is flanked by a subvertical bony surface formed by the lateral portions of coronoids 2 and 3 (see below). As a result, a broad and deep sulcus is formed between the shelf, laterally, and coronoids 2 and 3, medially. The coronoid-dentary suture can be traced along the deepest part of this sulcus (Figs 2b, e, 3g, 5b, 14a–c, 16c, 17d). The width of the shelf is greatly reduced by the enlarged marginal teeth situated between the levels of the posterior one-third of coronoid 1 and the mid-length or anterior one-third of coronoid 2 (Fig. 5b) and disappears gradually along the length of the postdental process.

4.6.2. Splenial

The complex morphology of the splenial results from its contribution to the precoronoid fossa and its relationships with immediately surrounding bones (Figs 1b, e, f, 2c, f, 3b, e, 5a, c, 6–12, 14–16). The splenial is shaped like a slender and elongate trough with a gently arcuate cross-section. In labial view, its external surface is extensively ornamented (Figs 2a, 4b, 6c, d, 19a) and meets the dentary along an interdigitated suture which runs approximately anteroposteriorly and subparallel to the ventral margin of the ramus (Figs 2a, 5a). More posteriorly, the similarly interdigitated suture between the splenial and the postsplenial slopes distinctly posteroventrally (Figs 4b, 5a). The ventral edge of the splenial is occupied by the anteriormost portion of the mandibular lateral line, which marks the separation between the ornamented labial surface and the smooth medial lamina of the bone (Figs 1b, e, f, 4b, 7–12). The anteriormost portion of the medial lamina is occupied by a depressed area of varying depth and proportions (Fig. 12a) which merges anteriorly into the sulcus surrounding the posterior and ventral portions of the brassicate symphyseal surface (see above). This area corresponds in position and proportions to a similar area – termed the free ventral flange of the splenial – observed in several Upper Devonian and Lower Carboniferous stem tetrapods (Ahlberg & Clack Reference Ahlberg and Clack1998). The area is delimited ventrally by a thin, sharp ridge and dorsally by a stronger ridge with a thick, convex medial surface (Figs 2f, 7, 8c, d, 9a, b, 10a, b, d, 11, 12a). The dorsal ridge continues smoothly into the dorsally facing narrow region of the splenial that continues into the symphyseal shelf of the dentary (Figs 1b, f, 2f, 7, 8, 9b, c, 10a, e, f, 11, 12a, c, d). At this level, the dentary-splenial suture shows an elaborate course. After sweeping posterodorsally along the sulcus that surrounds the raised brassicate surface, the suture continues posteriorly for a short tract on the symphyseal shelf before turning abruptly anteriorly and then lateroventrally into the precoronoid fossa. Here, the course of the suture can be traced along the anterior wall of the fossa, before it sweeps rearward to cross the lateralmost one-third of the floor of the fossa (that is, the bottom of the reniform recess described above). From this point, the suture runs posteriorly, describing a slightly indented course before disappearing in the rearmost part of the reniform recess, underneath and immediately in front of the spatulate anterior extremity of coronoid 1 (Figs 10, 11, 12c). A substantial portion of the dorsal surface of the splenial lies inside the precoronoid fossa, just medial to the dentary-splenial suture. More posteriorly, the portion of the dorsal surface of the splenial that contributes to the precoronoid fossa narrows abruptly along its sutural contact with coronoid 1 (Figs 7, 8b–d, 9c, 11, 12c). The splenial-coronoid 1 suture sweeps posteromedially and climbs along the posterior wall of the precoronoid fossa, crossing the sharp and narrow dorsomedial edge of the splenial and continuing posteriorly in an almost straight direction (Figs 5c, 6a, 14a, c, d, 15, 16a, b).

4.6.3. Postsplenial

The postsplenial is slightly longer than the splenial (Figs 1b, c, e, f, 2c, f, 3a, b, d–f, 4, 5a, c, 6a, b, 9d, e, 12b, 14c, 15, 16c). As in the latter, its labial surface shows an extensive dermal sculpture of pits and ridges and terminates ventrally against the lateral rim of the mandibular sulcus. The spatial relationships of the postsplenial with the splenial are best observed in ventral view (Figs 4a, 6a, b, d). The slightly interdigitating splenial-postsplenial suture runs anterolabially to posterolingually, straddling the mandibular sulcus. In lateral view, this suture slopes posteroventrally away from the triple junction between the dentary, splenial, and postsplenial (Fig. 5a). In medial view, it runs posterodorsally in its anterior tract, delimiting the boundary between the medial laminae of the splenial and postsplenial. The dorsal margin of the medial lamina of the postsplenial contacts coronoid 2 along a short tract and continues posteriorly forming an almost straight, subhorizontal suture with the anterior extremity of the prearticular (Figs 5c, 9d, e, 12b, 15, 16c). In its posteriormost tract, the dorsal margin of the lamina is irregularly sinuous and slopes posteroventrally. It then reaches the anteriormost one-third of the dorsal margin of the posterior Meckelian fenestra. Ventral to the fenestra, the medial lamina of the postsplenial contacts the medial lamina of the angular (Figs 5c, 9e, 15, 16c, 17e, 20a). In its ventral part, the strongly indented postsplenial-angular suture is orientated nearly transversely across the mandibular sulcus for a short section but is distinctly oblique on both sides of the sulcus (Figs 4a, 5c).

4.6.4. Angular

The shape and proportions of the angular vary little (Figs 1a, c, 2d, f, 3a, d, 5a, c, 17e, 18b, c, 19–21, 22c, 23a, d, e, 24a). The angular contributes to approximately two-thirds of the lateral surface of the posterior half of the ramus. In lateral view, it is approximately semicrescentic with an irregularly convex dorsal margin. From its dorsalmost point, this margin slopes gently anteroventrally, terminating at the triple junction between the angular, postsplenial, and dentary. A vertical line drawn from the dorsalmost point of the dorsal margin divides the external surface of the angular into a longer, rapidly tapering, and wedge-shaped anterior portion and a shorter, subtriangular posterior portion. The angular wraps around the posteroventral margin of the ramus, conferring a smoothly convex profile to it in lateral aspect. On the posterior surface of the ramus, some distance below the retroarticular process, the angular forms a narrow and elongate triangular process ending in an acuminate extremity that is tightly pressed against, or interlocking with, the articular (Figs 4a, 5a, c, 18b, c, 20b, c, 21a–c, 22c). This process usually extends as far back as, or slightly posterior to, the highest point of the postcondyloid process (Figs 5a, 20b, c, 21a, b, 22c). The medial lamina of the angular is less deep than in Case's (Reference Case1935) and later authors' reconstructions. As in other dvinosaurs, this lamina forms a narrow, smooth bony sliver. It has an irregularly indented dorsal margin and its depth is about 30 % or less of the depth of the adjacent portion of the prearticular (Shishkin Reference Shishkin1973; Coldiron Reference Coldiron1978) (Figs 8e, 9e, 20, 21a, d). The anteriormost part of the lamina forms a small contribution to the posteroventral margin of the posterior Meckelian fenestra (Fig. 17e).

In dorsal aspect, a conspicuous protuberance is visible on the floor of the adductor fossa. It is topologically and anatomically associated with the angular (Figs 2b, e, 5b, 24, 25c, 26, 27d) and is presumably homologous with the processus adductorius (adductor process) described by Shishkin (Reference Shishkin1973) in Dvinosaurus egregius and D. primus, and considered by him to represent an ossification within Meckel's cartilage (see also Shishkin & Sulej Reference Shishkin and Sulej2009). We agree with Shishkin (Reference Shishkin1973) that the occurrence of this protuberance is likely to reflect some features of adductor muscle organisation (e.g., insertion for the major component of the pterygoideus posterior muscle). The adductor process of Trimerorhachis is situated either subcentrally on the floor of the adductor fossa (Figs 24a, 25c, 26), slightly lateral to its deepest part (Fig. 24c, f), or even distinctly medial to it (Fig. 24b, e). Its shape varies from dome-like to subconical, with a subcircular or anteroposteriorly elongate subelliptical profile around its base, where it merges indistinctly into the adjacent surface of the angular. The dorsal surface of the process may appear either smooth or weakly corrugated, and its highest point is situated either centrally or slightly anterior to its mid-length.

At first, we interpreted the adductor process as being structurally part of the angular, forming a prominence arising from the floor of the adductor fossa. However, its nature remains unsettled. In FMNH UC 1118a, a natural break visible across the adductor fossa permits direct, albeit limited, observations of the contact between the adductor process and the floor of the fossa in cross-section (Fig. 26). In this specimen, the process is approximately dome-like in cross-section, situated immediately lateral to the angular-prearticular suture (visible as a zig-zag line running along the cross-sectional area), and aligned vertically with the mandibular sulcus. The portion of the angular immediately below the process is markedly arcuate and forms a band of solid cortical bone that merges indistinctly into the process, thus revealing only faint traces of a boundary. The surface of the process has a slightly granular texture, in contrast to the generally smooth surface of the surrounding fossa. However, we cannot ascertain whether this boundary represents a suture or a preservation artefact. Data from other jaws (Fig. 24a–c, e, f) are equally inconclusive in this respect, and it is hoped that future palaeohistological analyses will shed light on this issue.

The adductor process is sporadically documented in a handful of temnospondyl species and its occurrence in many others is dubious, certainly as a result of the dearth of thoroughly prepared and fully exposed adductor fossae. We were able to retrieve only a few examples from a quick literature survey where this process is reported and/or described, and we feel certain that other examples exist. During our investigation, the adductor fossa of an isolated jaw ramus of Eryops in the collections of the Field Museum of Natural History (FMNH UC 708, not illustrated here) was partially freed from matrix (except in its anterior one-fourth) but did not show any evidence of a process. In contrast, mechanical preparation of specimen FMNH UR 73 of Trimerorhachis rogersi (Fig. 27d) revealed a conspicuous, albeit poorly preserved adductor process of vaguely subellipsoid shape. In relation to the size of the adductor fossa, this process is comparatively more pronounced than in T. insignis. Mechanical preparation also revealed a small but distinct process in the adductor fossa of the eobrachyopid dvinosaur Acroplous vorax (Fig. 29; specimen KUVP 28352; see also Coldiron Reference Coldiron1978, fig. 1). The adductor process of Acroplous is comparatively smaller and more elongate and occupies a more anterior position inside the adductor fossa than in Trimerorhachis. Some distance behind the process, the floor of the adductor fossa of Acroplous shows a narrow and elongate ridge. This ridge is faint immediately behind the process, but it its posterior portion it forms a distinct raised keel (Fig. 29). According to Nilsson (Reference Nilsson1943; quoted in Shishkin Reference Shishkin1973), an adductor process is present in the lower jaw of the trematosaur Aphaneramma, but we are uncertain about the accuracy of this claim, as Nilsson's figures and descriptions do not appear to us to be fully convincing on this point. Similarly, Gubin (Reference Gubin1997) noted the presence of a weakly developed adductor process in the lower jaw of the basal stereospondylomorph Archegosaurus (apparently, A. decheni), although he did not figure such process. However, Witzmann (Reference Witzmann2006) did not report an adductor process in his comprehensive redescription of the cranial anatomy of A. decheni. A boss-like process has also been described in the plagiosaur Gerrothorax pulcherrimus (Hellrung Reference Hellrung2003; Schoch & Witzmann Reference Schoch and Witzmann2012). Schoch & Witzmann (Reference Schoch and Witzmann2012) stated that a similar process is present in the plagiosaur genus Plagiosternum. In another plagiosaur, Plagiosuchus pustuliferus (Damiani et al. Reference Damiani, Schoch, Hellrung, Werneburg and Gastou2009), the anterior half of the adductor fossa of a particularly well-preserved jaw ramus shows a thin, elongate longitudinal ridge. However, it is not known whether an adductor process is present more posteriorly, as the posterior half of the fossa is not preserved. Shishkin & Sulej (Reference Shishkin and Sulej2009, p. 55) reported a ‘tuberculum adductorium’ in the form of a ‘small elongate elevation … preceded and followed by small elongate pits’ in the capitosaurid Parotosuchus speleus. A spur-like adductor process also features in the trematopid dissorophoid Mattauschia laticeps (Milner Reference Milner, Ruta, Ahlberg and Smithson2019, and personal observation by the senior author, 2014), although it was neither formally described nor illustrated in the reconstruction of the jaw ramus (Milner Reference Milner, Ruta, Ahlberg and Smithson2019, fig. 7). It does, however, appear in the camera lucida drawing of the ramus (Milner Reference Milner, Ruta, Ahlberg and Smithson2019, fig. 4). Additional data are necessary to investigate the significance of the distribution of this process, either as a possible shared derived trait within some temnospondyl clades, as a more general lower jaw feature in the group, or as a trait that is more widely distributed among early tetrapods.

4.6.5. Surangular

The surangular is vaguely subrectangular in lateral aspect and reaches its maximum depth at the level of the posterior projection of the precondyloid process (Figs 1a, c, 2d, 3d, 5a, 18, 19). Its lateral surface is extensively sculptured in its ventral two-thirds but only lightly ornamented in its dorsal one-third (Fig. 5a). The dorsal part of the lateral surface is overlapped for about half of its length by the rapidly tapering, posteriorly acuminate postdental process of the dentary (Fig. 19). The rest of the lateral surface of the surangular is deeply incised by the three main branches of the lateral line system (see below; Fig. 18). Immediately dorsal to the accessory sulcus of the lateral line, the surangular forms a conspicuous posterior process that is structurally continuous with the lateral and posterolateral portions of the postcondyloid process of the glenoid (Figs 5a, 18). The process is developed in various degrees and is usually smoothly convex in lateral aspect and slightly flattened mediolaterally. The triple junction between the surangular, angular, and postdental process of the dentary generally occurs slightly anterior to the point where the surangular, the postdental process of the dentary, and the posterodorsal process of coronoid 3 meet. The lingual surface of the surangular forms most of the lateral wall of the adductor fossa and contributes in part to the formation of a surangular crest where it meets the posterior extremity of the posterodorsal process of coronoid 3 (Fig. 20). Immediately anterior to the glenoid, the dorsal margin of the surangular gradually widens anteriorly. Before contacting coronoid 3, the upper surface of this margin becomes flat or slightly shallow. The lateral edge of this surface is slightly taller and sharper than its medial edge, and diminishes in height abruptly anteriorly (Figs 23, 24). Within the adductor fossa, specifically along the angular-surangular suture, the lingual surface of the surangular appears slightly raised and thickened relative to that of the angular (Figs 17a, 20).

4.6.6. Coronoid series

Coronoid 1 contributes to the posteriormost part of the precoronoid fossa (see above) and extends posteriorly to a point situated approximately between the anterior and the middle one-thirds of the ramus length (Figs 1d, 2b, e, 3c, g, 5b, c, 7, 8b–d, 9c, 11, 12c, 14a, c, d, 15, 16a, b, 17a, d). The bone is approximately rectangular in dorsal aspect, with anterolateral to posteromedially orientated anterior and posterior margins sutured with the splenial and coronoid 2, respectively. The anterior one-fourth of coronoid 1 narrows abruptly anteriorly, ending in a spatulate subtriangular extremity with a gentle anteromedioventral slope (Figs 7, 8c, d, 11a, b, 12c, 14d). The surface of this extremity is slightly depressed and covered in small pits and rugosities. Its lateralmost portion forms a subvertical lamina, the dorsal margin of which produces a smooth, nearly straight sutural contact with the dentary shelf (Fig. 11a, c). More anteriorly, the lamina narrows abruptly and ends in a stout process appressed against the posterior one-third of a laterally convex crest situated in the bottom of the precoronoid fossa (see above), which is morphologically part of the splenial. The anterior extremity of coronoid 1 and the portion of the dentary shelf adjacent to it form a narrow canal representing the posterior continuation of the reniform recess in the bottom of the precoronoid fossa (see above; Fig. 11a, c). More medially, the spatulate extremity of coronoid 1 is tightly sutured to the splenial and contributes to the rearmost section of the medial wall of the precoronoid fossa (Figs 7c, 8c, d, 11a, 12c, 14d). A shallow depression visible on the posterior portion of the dorsal surface of coronoid 1 is part of the intercoronoid fossa and extends for a short tract onto coronoid 2 (Figs 8b, 9c, 11a). The surface of the intercoronoid fossa is irregular and covered in closely spaced small pits, foramina, and tuberosities. The deepest part of the intercoronoid fossa forms a narrow and elongate depression. Other variably expressed features of the dorsal surface of coronoid 1 include round protuberances and short, edentulous keels and crests (Figs 11a, 14d). Coronoid 1 forms a slightly indented suture with coronoid 2. From its medialmost point, at the triple junction between coronoid 1, coronoid 2, and splenial, the suture runs posteriorly and slightly laterally for a short distance, then turns abruptly anteriorly and terminates against the dentary shelf following a strongly oblique, indented course (Figs 7b, 8b, 14c, 15, 16a, b, 17 a–d).

Coronoid 2 is about one-third longer than coronoid 1 (Figs 5b, c, 9d, 14a–c, 15, 16, 17b, d). Its anterolateral portion is tightly wedged between coronoid 1 and the dentary shelf (Fig. 17b). In some of the largest specimens, its anterolateral portion features a shallow depression corresponding to the rearmost part of the intercoronoid fossa. More posteriorly, the dorsal surface of coronoid 2 is raised and forms a strong, mediolaterally flattened keel covered in clusters of teeth (Figs 9d, 14a–c, 15, 16b, c, 17b). The keel starts as a low prominence anteriorly but widens rapidly and becomes much taller posteriorly. Its lateral surface is nearly flat and subvertical and occurs close to the dentary shelf, from which it is separated by a straight and narrow sulcus (see above; Fig. 14a–c). The position of the anteriormost portion of the keel is marked by a sudden narrowing of the dentary shelf. The lateral surface of the keel is occupied by large, shallow elliptical depressions and irregular, deeper excavations, a pattern that extends further posteriorly along coronoid 3 (Fig. 14a, b). The medial surface of the keel is either flat or slightly arcuate dorsoventrally and merges smoothly into the medial part of the dorsal surface of coronoid 2 (Figs 15, 16b, c, 17b). This surface shows small pits and sulci and continues medially into a low but distinct longitudinal ridge sutured with the medial laminae of the splenial and postsplenial (Fig. 16b). More posteriorly, the keel expands, forming a flat and slightly raised platform (Figs 15, 16b, c, 17b). The suture between coronoids 2 and 3 runs anterolaterally to posteromedially (Figs 15c, 16c, 17c), and is usually slightly more irregularly indented than that between coronoids 1 and 2. In all specimens in which it is visible, this suture commonly crosses the denticulated area posterior to the keel on coronoid 2. The course of this suture is recognisable because, on both sides of it, teeth appear less densely packed than anywhere else on the dorsal surface of coronoid 3 (Figs 14a–c, 15, 16, 17b).

Coronoid 3 consists of an anterior, broadly rectangular, and tooth-bearing region and a robust, posterodorsally directed process (Figs 5b, c, 9d, 15c, 16c, 17c–e, 18b, 19, 20). The tooth-bearing region is orientated mostly dorsally but slopes gently anteroventrally in its anterior or anteromedial half before it contacts coronoid 2. In several specimens, the medial margin of coronoid 3 is separated from the denticle-bearing area by a narrow, featureless strip of bone orientated gently ventromedially (Figs 1e, f, 2b, c, e, f, 5b, c, 9d, 15c, 16c, 17c, e, 20a). The tooth-bearing region has a low, dorsoventrally sloping profile and is distinctly raised relative to coronoid 2 in medial view (Fig. 5c), as revealed by the slanting course of the prearticular-coronoid 3 suture. The posterodorsal process both overlaps and interdigitates strongly with the surangular (Figs 1d–f, 2b–f, 3c–g, 5, 9d, 14b, 15c, 16c, 17c–e, 18b, 19, 20, 23c, 24b, e, f), and is comparable in length to, or marginally longer than, the tooth-bearing region (Figs. 17c, e, 20a). The process extends posteriorly to a greater degree than in Case's (Reference Case1935) reconstruction, contributes to the anterodorsal part of the internal surface of the adductor fossa, and is tightly pressed against the anterior half of the postdental process of the dentary (Figs 3g, 5a, b, 12b, c, e, 24e). In medial aspect, the upper margin of the process slopes slightly posterodorsally. At its posterior extremity, this margin contributes to the formation of a poorly pronounced crest (‘surangular crest’) together with the adjoining portion of the surangular. In some specimens, the process is slightly visible in lateral view, and appears as a small, subtriangular, narrow shard of bone situated anterior to the surangular and immediately dorsal to the postdental process of the dentary (Figs 1c, 3d, 19). In other specimens, the process is more extensive, and contributes to most of the surangular crest in lateral and medial views (Figs 17e, 18b). The medial surface of the process is slightly twisted anticlockwise along its greater axis in an anterior to posterior direction.

4.6.7. Prearticular

In medial aspect, the prearticular consists of three main regions: a subtriangular anterior region delimited posteriorly by the anterior margin of the adductor fossa; a crescent-like middle region comprised between the anterior margin of the adductor fossa and the precondyloid process of the glenoid; and a spatulate posterior region extending posterodorsally to the level of, or slightly anterior to, the postcondyloid process of the glenoid (Figs 1b, e, f, 2b, c, e, f, 3b, c, e, g, 5b, c, 9d, 15c, 16c, 17d, e, 20, 21, 22a, c, d, 23a, b, d–f, 24b, e, f, 25a, b). The external surface of the combined anterior and middle regions is either flat or gently arcuate, being slightly concave anteroposteriorly and convex dorsoventrally, and appears as a narrow sliver of bone in dorsal view. The generally smooth, anterior region is shorter than the combined middle and posterior regions. Its rapidly tapering anterior extremity is deeply wedged between coronoid 2 and coronoid 3, dorsally, and the medial lamina of the postsplenial, ventrally (Figs 1f, 2b, c, f, 3e, 5c, 9d, 16c, 17e). In some specimens, this extremity appears bifid, being divided into a small, anteriorly convex dorsal lobe and a narrowly triangular, slender ventral process (Figs 1f, 5c). In other specimens, it forms two triangular lobes of subequal size (Fig. 15c). The smooth to finely striated middle region delimits the adductor fossa lingually. Its dorsal margin (medial margin of the adductor fossa) is divided into two, shallow concave portions – the anterior portion slopes gently posteroventrally; the posterior portion rises steeply posterodorsally. Where the two portions meet, the profile of the margin is more distinctly arcuate (Figs 5c, 17e, 20). In most specimens, the dorsal margin of the middle region is markedly offset ventrally relative to the lateral margin of the adductor fossa (Figs 1f, 3e), whilst in others it is only slightly displaced ventrally (Figs 1b, 2f). In cross-section, the dorsal margin of the middle region is sharp or narrowly convex along most of its length, but widens slightly in its steeply ascending posteriormost part, immediately in front of the precondyloid process of the articular (Figs 5c, 23b, c, 24b, c, 25b). At this level, the dorsal surface of the margin becomes shallowly concave, and is delimited laterally by a mediolaterally oblique and slightly sigmoid ridge (Figs 23b–e, 24b, 25a, b). Lateral to this ridge, the prearticular surface descends steeply into the adductor fossa alongside the adjacent internal surface of the precondyloid process, to which it articulates (Figs 5b, 23b–e, 24a–c, 25a, b). The ventral margin of the middle region broadly follows the curvature of the ventral margin of the jaw ramus and its shape becomes irregularly sinuous where it meets the medial lamina of the postsplenial. The ventral margin terminates anteriorly at a level situated slightly anteroventral to the posteriormost point of the posterior Meckelian fenestra. The subquadrangular, posterior region (Fig. 20) shows distinct anterodorsal and posterodorsal corners and a nearly straight dorsal margin between them. The anterodorsal corner is tightly pressed against the lateral surface of the precondyloid process. The dorsal margin runs close to the lateral margin of the glenoid surface. The posterodorsal corner is situated immediately dorsal to the chorda tympani foramen. From this point, the posterior margin of the posterior region of the prearticular descends anteroventrally along the medial surface of the ramus forming a slightly indented suture with the articular (Figs 5c, 20, 21).

4.6.8. Articular

For ease of exposition, the description of the glenoid surface precedes that of the main corpus of the articular. The coarsely granular texture of the glenoid surface is readily distinguished from the smoother texture of the main corpus, from which it is separated by a narrow peripheral rim. In dorsal aspect, the glenoid surface is vaguely saddle-shaped with a subtrapezoidal to subelliptical outline (Figs 1a, d, 2e, 3g, 5b, 22d, 23b–f, 24a–c, 25b). In common with several other early tetrapods, it is described as ‘twisted’ or ‘corkscrew-shaped’, in that it slopes in different directions at different angles. In medial and lateral views, the glenoid surface is delimited anteroposteriorly by the precondyloid and postcondyloid processes and is divided by a weak oblique ridge into a larger lateral portion and a smaller medial portion (Figs 24a, 25b). Both portions are slightly ‘sunken’ below the level of the glenoid rim. The medial portion is anteroposteriorly elongate, remarkably more so than in other temnospondyls (e.g., Eryops; Sawin Reference Sawin1941), and slopes upward anteromedially to terminate in the conspicuous precondyloid process (Nilsson Reference Nilsson1943) (Figs 1a, b, f, 2e, f, 3a, e, 5a, c, 18b, c, 21a, d, 23b–d, 24b, c, 25a, b). The posterior face of this process carries a distinct, approximately dorsoventrally orientated, and shallow groove of slightly variable length and depth, presumably for ligament or muscle attachment (Figs 22d, 25b). The subrectangular lateral portion of the glenoid surface is bounded posterolaterally by the stout postcondyloid process (Nilsson Reference Nilsson1943). This process is delimited by a sharp dorsal rim that usually curves slightly forward, thus overhanging slightly the glenoid surface. Immediately medial to the postcondyloid process, the posterior surface of the articular carries a shallow depressed area visible slightly ventral to the glenoid rim (Figs 20, 21c, d, 22, 25b). Opposite the mid-point of this depressed area, the glenoid margin shows two irregular notches, clearly visible in dorsal aspect, between which is a subconical tubercle with a rugose surface (Fig. 22b). As far as we can discern, this tubercle appears as a constant, yet previously undescribed feature of the Trimerorhachis jaw. The depression and accompanying tubercle occupy approximately the same area as the ‘retractor pit’ described by Bolt & Lombard (Reference Bolt and Lombard2001) in the colosteid Greererpeton burkemorani.

4.6.9. Meckelian ossification

Among early tetrapods, the morphology and extent of the Meckelian ossifications, as well as their relationships with surrounding dermal bones, have been documented in sufficient detail in a handful of species (Ahlberg & Clack Reference Ahlberg and Clack1998). Detailed mechanical preparation of the adductor fossae of several Trimerorhachis jaw rami has provided new information on these structures. Conspicuous Meckelian ossifications are visible immediately in front of the anterior margin of the glenoid surface. In this region, these ossifications form two extensive bony sheets differing slightly in size and descending along the posterior, posterolateral, posteromedial, and posteroventral surfaces of the adductor fossa (Figs 5b, 23, 24a–d, 25a). These sheets show a smooth or lightly striated texture and irregularly distributed small foramina (Fig. 5b). They project anteriorly from the strongly vaulted posterior surface of the adductor fossa, partly covering the posterior portion of the internal surfaces of the prearticular, surangular, and angular. The sheets abut against each other, are sometimes slightly offset dorsoventrally relative to one another, and terminate anteriorly in unfinished irregular margins (Figs 23a, b, e, 24a, 25a).

4.7.1. Dentary fangs

In the anterior part of the dentary, and anterolateral to the symphyseal shelf, most rami carry a single fang and a replacement pit. In some specimens, both fangs are present (Figs 2b, d–f, 8d). Both fangs are approximately conical, with a subcircular to slightly elliptical cross-section near their crown bases. They are slightly but distinctly compressed proximodistally in their upper portions, and curve slightly posteromedially (Figs 2b, d–f, 3a–e, 5b, c, 7, 8, 9a, b, 10a, b, d–f, 11a). The fangs are larger and more robust than the largest marginal teeth (height:basal diameter ratio ~1.4) and this difference is approximately maintained in jaws of different sizes (Figs 2, 3). The single preserved fang of CMNH 11322 is comparatively more acuminate and more elongate than the fangs of other specimens and shows a markedly higher height:basal diameter ratio (~2.4) (Fig. 3d, e).

4.7.2. Marginal dentary teeth

The marginal dentary teeth vary considerably in size along the dentary arcade but exhibit negligible changes in shape. Tooth size increases slowly anteroposteriorly from the symphysis to a point situated approximately at the boundary between coronoids 1 and 2 (Fig. 5). At this level, three or four teeth may be larger than the remaining teeth. More posteriorly, tooth size decreases uniformly (Figs 1c–f, 2b–f, 3, 7, 8b–d, 9d, 14–16, 17d, e). The marginal teeth are invariably acuminate (where the tips are preserved intact) and markedly compressed proximodistally near their bases. Their cross-section is subcircular near the crown tips, which curve slightly lingually. In all rami, a short tract of the dentary tooth row projects slightly medially in dorsal aspect (Milner & Schoch Reference Milner and Schoch2013) (Figs 2b, e, 3c, g, 16a). Anterior and posterior to this curvature, the tooth positions mostly follow the general curvature of the jaw ramus.

4.7.3. Coronoid dentition

The coronoid dentition is highly variable in terms of the number and distribution of denticles (Figs 1d–f, 2b, c, e, f, 3b, c, e, g, 9d, e, 14a–c, 15b, c, 16c, 17b–e, 20a, 24e). The morphology and size of the denticles change little across jaws of different sizes, certainly less so than in the case of the marginal dentition. The denticles generally vary in shape from dome-like to subconical in labial or lingual aspect, and feature a round basal cross-section and a pointed or smoothly convex tip (Figs 15c, 16c, 17d). Occasionally, some coronoid teeth resemble scaled-down versions of the larger dentary teeth (Figs 14b, 17c, e). Coronoid 1 is normally edentulous, but in a single specimen (Fig. 7b) the anterior half of coronoid 1 bears an isolated broken denticle sitting atop an irregular crest and recognisable from the cross-section of its crown. The anterior portion of the mediolaterally compressed keel on coronoid 2 usually carries a single row of teeth, and additional teeth may be present along the upper part of the medial surface of the keel. The dentition pattern on the posterior portion of this keel is variable and shows either multiple tooth rows, an irregular tooth cluster, or few enlarged teeth (Figs 9d, 14a–c, 15, 16, 17b). Coronoid 3 usually shows a patch of small robust teeth, sometimes arranged in longitudinal rows, especially in the posterior part of the bone. Such teeth sit on a slightly raised bony surface that deepens slightly in a lateral direction. In its lateralmost portion, this bony surface terminates in a longitudinal ridge that represents a continuation of a similar ridge present on coronoid 2 (Figs 9d, 14a, b, 15c, 16c, 17c–e, 20a, 24e).