Hadrokkosaurus bradyi Welles & Estes, 1969

So, Calvin & Mann, Arjan, 2024, A large brachyopoid from the Middle Triassic of northern Arizona and the diversity of brachyopoid temnospondyls from the Moenkopi Formation, Fossil Record 27 (1), pp. 233-245 : 233-245

publication ID	https://doi.org/ 10.3897/fr.27.117611
publication LSID	lsid:zoobank.org:pub:C084A289-5E44-4FD0-8FBB-AEEB61A83DD1
persistent identifier	https://treatment.plazi.org/id/7FB097FB-C8FB-5E45-A530-795B5737FD59
treatment provided by	by Pensoft
scientific name	Hadrokkosaurus bradyi Welles & Estes, 1969
status

Treatment

Holotype.

UCMP 31699 View Materials (right mandible lacking only the articular).

Horizon and locality.

Uppermost channel sandstone of Holbrook Member, Moenkopi Formation; early Anisian, lowermost Middle Triassic. V 3922, Geronimo (Holbrook) fossil vertebrate quarry near Holbrook, Coconino County, north-eastern Arizona.

Referred material.

UCMP 36200, anterior right dentary; UCMP 36201, partial right dentary; UCMP 36203, partial left dentary; UCMP 36205, partial left pre-articular; UCMP 36836, left pre-articular; UCMP 36837, left pre-articular; UCMP 36838, right surangular.

Revised diagnosis

(modified from Ruta and Bolt (2008)). A brachyopid temnospondyl with the following unique combination of features: total length of angular ventrolateral margin greater than or equal to half of total lateral mandible length; angular posteriormost margin straight in lateral aspect; greatest depth of angular lateral surface less than or equal to greatest depth of dentary lateral surface; ventral margin of posterior Meckelian fenestra formed only by angular; anterior Meckelian fenestra in middle third of postsplenial mesial lamina.

Description.

The holotype right mandible possesses several features that maintain its status as a brachyopoid, such as the long postglenoid area and the curvature of the mandible that can be extrapolated to fit a broad and short-snouted temnospondyl (Fig. 1 View Figure 1 ). It maintains the full plesiomorphic complement of ten bones found with most temnospondyls. The postglenoid area is substantially long, as present in other brachyopoid mandibles ( Warren and Marsicano 2000). Several partial and fragmentary dentaries besides the holotype have been referred to Hadrokkosaurus .

The dentary of Hadrokkosaurus is thin (Fig. 1 View Figure 1 ). The dorsal surface of the dentary (i. e. the dental shelf) is narrow, contributing to the gracile and narrow appearance of the mandible, but towards the symphysis, the shelf broadens to accommodate the larger symphyseal tusk. The dentition of the dentary is represented by small, lingually recurving teeth, with wider than long bases. There is a count of 32 tooth positions. Towards the tips, the dentary teeth are labiolingually compressed and carinated. UCMP 36200, UCMP 36201, UCMP 36203, UCMP 75434 and UCMP 152391 (Suppl. material 1: fig. S 1) are partially preserved dentaries; they are likely also Hadrokkosaurus , based on the narrowness of the dental shelf and tooth sockets that would be implanted with small teeth.

The dentary forms a wide parabolic shape that curves towards the symphysis (Fig. 1 View Figure 1 ). The straight linear measurement from the anteriormost tip of the symphysis to the posteriormost extent of the dentary measures approximately 21 cm. From the anteriormost tip of the mandibular symphysis to the anteriormost sutural contact between the first and second coronoid, it measures 9.5 cm. At the mandibular symphysis, the dentary is vertically short, but, as it continues posteriorly, it deepens considerably. The dentary sutures to the presplenial and postsplenial ventrally. The symphysis is formed entirely by the dentary. Lingually, the large postsymphyseal foramen is bounded by the dentary dorsally, the first coronoid posteriorly and the presplenial ventrally. The foramen exits into an open Meckelian canal that opens on the dentary beginning on the lingual surface and ends on the ventral surface as it continues towards the symphysis (Fig. 1 B, D, E, G View Figure 1 ). The size of the postsymphyseal foramen is large and comparable to the condition in Bathignathus poikilops ( Damiani and Jeannot 2002) . The symphyseal tusk is angled posterodorsally on the symphyseal plate (Fig. 1 View Figure 1 ). The dental row is framed lingually and labially by ridges. In the transverse aspect of the dentary, the ridges are formed by the dental shelf on the lingual side, while the labial ridge is formed by a thin lamina running along the length of the dentary. The dentary teeth are implanted within individual sockets.

While the dental shelf is narrow, the teeth are even narrower, resulting in the dorsal exposure of the dental shelf along the length of the dentary (Fig. 1 F, H View Figure 1 ). The labial lamina of the dentary is shortest at the symphysis, but becomes taller towards the posterior. The lingual side of the dental shelf possesses a lamina that projects ventrally, contributing to the lingual surface of the mandible. The lingual lamina primarily projects ventrally for most of the dentary, but, towards the posterior extent of the element, the lamina possesses a 90 - degree torsion before suturing to the third coronoid. It overlies and sutures to the first and second coronoids dorsally. The lamina ends where the dentary sutures to the third coronoid posteriorly, leaving the dental shelf occupied by the last few teeth on the dentary without a lingual lamina. The surface of the dentary lingual lamina is smooth until the portion just before the suture to the third coronoid, where the texture of the surface changes drastically. It is marked with anteroposteriorly orientated striations. This roughness may have been the attachment site for musculature. The labial side of the dentary is deep and forms the majority of the anterior labial surface of the mandible (Fig. 1 A, C View Figure 1 ).

A deep trench runs along the labial side of the dentary (Fig. 1 A, C View Figure 1 ). Ruta and Bolt (2008) discuss this trench; they note that this groove is present and homologous in the dentaries from V 3922 and in other brachyopoids ( Damiani and Kitching 2003), but they do not consider it to be the oral sulcus. They suggest that an external mandibular artery may have been set within the lateral groove (Morales and Shishkin 2002) and question its identity as an oral sulcus. Lydekkerinids have been described to have an oral sulcus that extends from the posterior mandible on to the dentary and towards the mandibular symphysis ( Jupp and Warren 1986; Jeannot et al. 2006), which is the case in Hadrokkosaurus . The mandible of Brachyops allos (Warren 1981) possesses a “ groove ” in the same topological position, but it is identified as an oral sulcus. Similar grooves can be observed across other trematosaurians (e. g. Sulej (2007); Schoch (2019)) and capitosaurians (e. g. Morales and Shishkin (2002); Eltink et al. (2016 )), suggesting the feature to be broadly distributed across stereospondyls. The groove on the labial surface of the dentary has been widely discussed amongst other descriptions of stereospondyls. Given the aquatic nature of stereospondyls, it is likely that the groove is a lateral line sulcus.

Three coronoid bones are present as in other temnospondyls (Fig. 1 B, D, F, H View Figure 1 ). The first coronoid is a long, splint-like element on the lingual surface of the mandible, wedged between the dentary dorsally and the presplenial ventrally. Posteriorly, the first coronoid is sutured to the second coronoid. The first coronoid frames the posteriormost tip of the Meckelian canal forming the postsymphyseal foramen. The second coronoid forms an interdigitating suture with the first coronoid anteriorly. It is foreshortened as it compensates for a lengthened first coronoid. Ventrally, the second coronoid is sutured to the postsplenial. Posteriorly, it is sutured to the third coronoid.

The third coronoid is tooth-bearing (Fig. 1 B, D, F, H View Figure 1 ). It is positioned more dorsally compared to the other coronoids, almost reaching the tips of the crowns of the marginal dentition. The third coronoid sutures to the second coronoid anteriorly. Ventrally, it is sutured to the pre-articular. The body of the third coronoid is lingually expanded to form the anterior margin of the adductor chamber (Fig. 1 F, H View Figure 1 ). A process of the third coronoid extends posteriorly, lingual to the posterior process of the dentary to form the anterior half of the labial margin of the adductor chamber. The posterior process is well exposed in labial view and forms an interdigitating suture with the surangular. The third coronoid also possesses a lamina that descends from its body and contributes to the lingual surface of the mandible. The third coronoid teeth are smaller than the marginal teeth, but they are similar in shape. There are eight tooth positions forming a row on the third coronoid. The coronoid process is formed by the third coronoid without contribution by the dentary.

The presplenial is short and trough-shaped, positioned near the symphysis on the ventral surface of the mandible (Fig. 1 B, D, E, G View Figure 1 ). The presplenial forms the ventral margin of the canal into which the postsymphyseal foramen exits. It sutures to the dentary dorsally within the canal and to the postsplenial posteriorly. The suture between the presplenial and postsplenial is interdigitating. The presplenial also forms a suture with the first coronoid lingually towards its posterior. The suture between the presplenial and the postsplenial is interdigitating and visible on the lingual surface of the mandible. The suture continues around the ventral mandibular mandible, where it is obscured by plaster. In labial view, the presplenial is barely visible as a narrow splint, where it also sutures to the labial component of the dentary. It does not participate in the mandibular symphysis.

The postsplenial is longer than the presplenial (Fig. 1 B, D, E, G View Figure 1 ). Anteriorly, it is similarly trough-shaped, but towards the posterior, it twists and becomes flat and primarily exposed lingually. It forms interdigitating sutures with the presplenial anteriorly, with the angular posteroventrally and with the pre-articular posteriorly. On the lingual surface, the postsplenial sutures to the first, second and third coronoid dorsally. On the labial surface, the postsplenial sutures to the dentary dorsally.

The angular is poorly ornamented and forms the majority of the floor of the adductor chamber (Fig. 1 View Figure 1 ). It is trough-shaped, contributing to the ventral labial and lingual surfaces of the mandible. The angular has a low exposure on the labial surface of the mandible, reaching only the mid-point of the height of the dentary (Fig. 1 A, C View Figure 1 ). As the angular curves lingually around the ventral mandible to form the adductor chamber floor, it contributes to a narrow ventral portion of the lingual surface. The angular extends posteriorly to contribute to the ventral surface of the postglenoid area, along the length of which it sutures to the surangular dorsally on the labial surface. The anterior angular on the labial surface sutures to the dentary dorsally. It forms a straight suture with the pre-articular on the lingual surface. Anteriorly, the angular forms an interdigitating suture to the postsplenial.

The surangular of Hadrokkosaurus is a large element on the labial surface of the mandible (Fig. 1 A, C View Figure 1 ). It forms the posterior labial margin of the adductor chamber (Fig. 1 F, H View Figure 1 ), where it forms an interdigitating suture with the third coronoid anteriorly and stepped suture to the angular ventrally on the labial side. The surangular forms a straight simple suture with the angular on the ventral postglenoid area. The surangular would underlie the articular, which is not preserved. It forms the labial half of the postglenoid area, where it forms a simple straight suture with the retro-articular process of the pre-articular on the dorsal surface of the postglenoid area. The surangular forms a low preglenoid process, only slightly taller than the prearticular wall of the adductor chamber (Fig. 1 A, C View Figure 1 ).

The pre-articular is tall and forms the majority of the posterior lingual surface of the mandible (Fig. 1 B, D View Figure 1 ). It forms the lingual wall of the adductor chamber (Fig. 1 F, H View Figure 1 ). UCMP 36836, UCMP 36837 and UCMP 36838 are referred to as partial pre-articulars that share with the holotype a dorsal process that curls lingually (Suppl. material 1: fig. S 1 E, F). UCMP 36839 could be the postglenoid process of the pre-articular or the surangular, but there is not enough information preserved to discern its identity (Suppl. material 1: fig. S 1 H). The pre-articular forms a simple suture with the third coronoid anterodorsally and to the angular ventrally. It shares with the angular an interdigitating suture with the postsplenial. The pre-articular forms the lingual half of the postglenoid area, where it forms a simple suture with the surangular labially on the dorsal postglenoid area.

The articular is not preserved in the mandible. It may have been poorly ossified or it could have been disarticulated during the preservation of the mandible. However, the surangular and pre-articular preserve the facet upon which the articular would sit.

Temnospondyli Zittel, 1888

Stereospondyli Zittel, 1888

Brachyopoidea Lydekker, 1885

Brachyopoidea indet.

Horizon and locality. Uppermost channel sandstone of Holbrook Member, Moenkopi Formation; early Anisian, lowermost Middle Triassic. V 3922, Geronimo (Holbrook) fossil vertebrate quarry near Holbrook, Coconino County, north-eastern Arizona.

Referred material. UCMP 36202, partial posterior left dentary; UCMP 36833, partial anterior right mandible; UCMP 36834, near complete right dentary; UCMP 36385, partial right dentary; UCMP 152390, right dentary fragment.

Description. UCMP 36833 is a well-preserved anterior right mandible that demonstrates different morphology from Hadrokkosaurus (Figs 2 View Figure 2 , 3 View Figure 3 ). Welles (1947) previously noted that some of the dentaries referred to Hadrokkosaurus had different tooth morphology than the type. These additional specimens share the same features as UCMP 36833. While these specimens provide significant morphological detail, they exhibit the same preservation quality as Hadrokkosaurus . UCMP 36202 is a disarticulated posterior left dentary, with partial dentition preserved (Suppl. material 1: fig. S 2). It is noticeably laterally compressed in preservation. The teeth are large and robust, as in UCMP 36833 and unlike in Hadrokkosaurus . UCMP 36834 is a well-preserved right dentary, retaining most if not all of the morphology (Fig. 3 View Figure 3 ). It preserves the same tooth morphology as in UCMP 36833. UCMP 152390 is a mid-section fragment of a right dentary; it also exhibits the same tooth morphology as in UCMP 36833.

UCMP 36833 preserves the dentary, the first coronoid, the presplenial, the anterior second coronoid and the anterior postsplenial (Fig. 2 View Figure 2 ). The mandibular symphysis of UCMP 36833 is partially reconstructed in plaster. Based on UCMP 36834, consisting of a nearly complete dentary, the curvature of the mandible suggests it would have accompanied a very wide and parabolic skull (Figs 2 View Figure 2 , 3 View Figure 3 ). The ornamentation that is typically present on the temnospondyl mandible is significantly eroded, though there are hints of its distribution present. It appears the ornamentation may have been more polygonal on UCMP 36833 in the symphyseal area and more represented by elongate grooves and ridges towards the posterior. Otherwise, the other specimens belonging to this new brachyopoid taxon do not preserve ornamentation. The dentition consists of tooth bases that are anteroposteriorly compressed ovals in the cross section of the base. The teeth are slightly lingually recurved. They also possess a slight labiolingual narrowing at the crown, but are far less labiolingually compressed than the teeth in Hadrokkosaurus . Although slightly eroded, the consistent shape across all teeth in all specimens of the unidentified brachyopoid shows that the lack of carinae is not a result of taphonomic processes. Generally, the tooth morphology can be broadly extrapolated to be larger at the base, rounder overall and fewer in number to accommodate the limited space of the dentary. Amongst brachyopoids, this tooth morphology is found only in Koolasuchus cleelandi from the Early Cretaceous of Australia ( Warren et al. 1997) and an incomplete mandible from the Late Triassic of Argentina (Marsicano 2005). The straight linear measurement from the anteriormost tip of the mandibular symphysis to the anteriormost sutural contact between the first and second coronoid of UCMP 36833 measures 10.7 cm. Measured from the anteriormost tip of the symphysis to the posteriormost extent of the mandible, UCMP 36834 measures approximately 14 cm. In total, there are approximately 21 tooth positions present on UCMP 36834, far fewer than the count of 32 on the Hadrokkosaurus mandible and the count of 40 on the Koolasuchus mandible.

The dentary is more robust compared to the dentary of Hadrokkosaurus (Figs 2 View Figure 2 , 3 View Figure 3 ). The dental shelf is notably wider. The dentary forms most of the labial surface of the anterior mandible, similar to the condition of the anterior mandible of Koolasuchus ( Warren et al. 1997) . It is low anteriorly towards the symphysis and deepens to become a tall and robust element towards the posterior. The width of the dentition is wide enough to span the width of dorsal facing surface of the dentary on which the dentition sits. This differs from Hadrokkosaurus , in which the teeth are smaller, leaving a partially exposed dorsal-facing surface of the dentary. The entire width of the dental shelf is occupied entirely by the width of the dentition, resulting in a dorsally unexposed dental shelf unlike in Hadrokkosaurus . The dentary is markedly exposed on the lingual surface of the mandible, extending ventrally from the dentary shelf. The lingual lamina of the dentary curves horizontally and then posteriorly from the shelf. It forms straight sutures ventrally with the first and second coronoid. It sutures to the presplenial and postsplenial labially and ventrally. On the labial surface, there is a shallow groove that is likely homologous with the “ horizontal groove ” noted by Welles (1947) in Hadrokkosaurus and by Damiani and Kitching (2003) in Vanastega (Fig. 2 F, H View Figure 2 ). As mentioned before, this feature is found broadly across stereospondyls.

The mandibular symphysis is poorly preserved in UCMP 36833 and reconstructed in plaster; however, based on the anterior extent of the presplenial, the symphysis is formed by the dentary alone (Fig. 2 B, D, E, G View Figure 2 ). Although the symphysis is not preserved in its entirety in UCMP 36833, it is well-preserved in UCMP 36834, where the reduction in the height of the dentary can be seen at the mandibular symphysis. The symphysis widens posteriorly and accommodates a pair of symphyseal fangs in both UCMP 36833 and 36834. The first coronoid is a long element that begins at the posterior extent of the symphyseal shelf. The first coronoid composes the lingual wall and margin of the postsymphyseal foramen. It extends posteriorly, where it forms a double scarf suture with the second coronoid. The first coronoid forms a straight suture to the dentary dorsally and to the presplenial ventrally. It is a relatively shorter element compared to the first coronoid in Hadrokkosaurus .

A postsymphyseal foramen is present in UCMP 36833 (Fig. 2 A, C, E, G View Figure 2 ). The foramen exits into a distally widening Meckelian canal positioned on the ventral aspect of the mandible. The foramen and canal are entirely exposed on the mandible ventrally, compared to UCMP 36199, in which the postsymphyseal foramen and canal initially appear on the lingual surface of the mandible before the canal curves ventrally. When viewed at the symphyseal surface, the canal forms a ventrally opening concavity. The presplenial contributes to the ventral margin and the first coronoid forms the lingual margin. Other temnospondyl taxa possess a postsymphyseal groove that lies lingually or sometimes ventrally on the mandible and participate in the mandibular symphysis ( Damiani 2001; Jeannot et al. 2006).

Temnospondyli Jaekel, 1909

Temnospondyli indet.

Referred material. UCMP 36210, partial ventral angular.

Description. UCMP 36210 is a partial right angular that would have floored the adductor chamber of the right mandible (Fig. 4 View Figure 4 ). It was noted by Ruta and Bolt (2008) to belong to a different temnospondyl than Hadrokkosaurus due to a “ boss-like process ” upon the floor that is absent in Hadrokkosaurus (Fig. 1 F, H View Figure 1 ; Fig. 4 View Figure 4 ). The pit-and-ridge ornamentation is slightly worn, but noticeable on the ventral side, unlike in Hadrokkosaurus . In light of the presence of another temnospondyl taxon in this locality, the process may have belonged to a temnospondyl with a strong adductor muscle inserted to produce a stronger bite or hold the prey of the animal. A similar process has been reported in the contemporaries Plagiosternum and Gerrothorax (Schoch and Witzmann 2011) , but noted by Ruta and Bolt (2008) to also be present in Aphaneramma ( Nilsson 1943) , Dvinosaurus ( Shishkin 1973) , Archegosaurus ( Gubin 1997) , Acroplous and Trimerorhachis .