Erpetosuchidae

Erpetosuchids were lithe but well-armored carnivorous pseudosuchians. Two rows of overlapping armored plates (osteoderms) extended from the neck to the tail, supplemented by an additional row on the back and tail and small oval-shaped osteoderms on the legs and possibly the arms as well. The osteoderms were unusually sculptured by deep pits and ranged in shape and thickness between taxa.[4]

Erpetosuchids also characteristically had upper teeth restricted to the front part of the mouth. While Erpetosuchus and Parringtonia had unserrated and only slightly mediolaterally compressed teeth, Tarjadia had thin and serrated teeth more similar to other carnivorous archosauriforms. Erpetosuchids also had broad maxillae with antorbital fenestrae set approximately in the middle of the deep antorbital fossa, which in turn was completely surrounded by bony ridges. Under the ridge which composes the ventral edge of the antorbital fossa, the maxilla (particularly the rear part) slopes inwards towards the teeth. Although erpetosuchid skulls were not low and flat like those of some archosauriform groups, they were broader than those of other basal pseudosuchians, particularly behind the orbits, which were large and pointed upwards.[2][4]

They also had a deep groove or pit on the top surface of their wide neural spines, a condition quite different from that of other archosaurs. The neural spines were taller on the tail than on the back.[4]

Life restoration of Erpetosuchus

When Erpetosuchidae was first defined to include Erpetosuchus and Parringtonia, three synapomorphies or shared characteristics were identified for the family: teeth restricted to the anterior half of the maxilla, a posterior half of the maxilla that is thicker than it is tall, and no tooth serrations (although Tarjadia was known to have serrated teeth). Other possible synapomorphies of the two taxa were considered tentative due to having not yet been sampled in the cladistic analysis. In Erpetosuchus, the medially inclined lower external surface of the maxilla continues posteriorly onto the jugal, exposing much of the external surface of the jugal in ventral view. This morphology unites the North American and European specimens of Erpetosuchus with Parringtonia gracilis. Other potential synapomorphies include a hypertrophied tuber hypothesized for the attachment of the m. triceps brachii on the posterolateral surface of the proximal scapula blade. Unlike other archosauriforms that have a small tuber in the same location, the size of the tuber in erpetosuchids is exceptionally large in relation to the overall size of the scapula. Furthermore, as Krebs (1976) hypothesized the scapula blade slightly arc anteriorly in both taxa. However, this condition is similar to that of other archosaurs, like Postosuchus kirkpatricki.[2]

While Erpetosuchus granti and Erpetosuchus sp. are nearly indistinguishable, the scapula of Parringtonia differs from E. granti in that it has a small bump or tubercle over its shoulder socket, osteoderms that are nearly square instead of being anteroposteriorly longer than wide, and a foramen (or hole) on the outer surface of the maxilla. Parringtonia has five tooth sockets, Erpetosuchus gracilis only four, and Erpetosuchus sp. six or more (up to nine). Furthermore, Erpetosuchus sp. has a proportionally taller ascending process of the maxilla than Parringtonia.[2]

Dyoplax, a controversial pseudosuchian from Germany, has been proposed to be a close relative of Erpetosuchus. Some similarities between the two taxa include the structure of their osteoderms (including unusual small cervical osteoderms), a recessed antorbital fenestra, ridges completely surrounding the antorbital fossa, large and upward-pointing orbits, and slender limbs. However, the preservation of the only known specimen of Dyoplax, a sandstone mold, makes analysis difficult, and Dyoplax lacks many of the features that characterize other erpetosuchids. Nevertheless, the relation of Dyoplax shares more similarities with Erpetosuchidae than with other groups it has been proposed to be related to, such as Aetosauria and "Sphenosuchia".[3]

New specimens of Tarjadia have doubted its previous classification as a doswellid and instead supports it as a member of Erpetosuchidae. Support for this new assignment is seemingly robust, with Tarjadia possessing many erpetosuchid features. Some of these features include a lack of posterior maxillary teeth, the presence of neural spine pits, an erpetosuchid-style maxilla, and archosaurian traits which contrast with those of the doswellids, which are non-archosaurian archosauriforms. This new classification also places the former doswellid Archeopelta as an erpetosuchid, as it is a very close relative of Tarjadia.[4]

Pagosvenator is a Brazilian genus which has been allied with the ornithosuchids prior to receiving a formal description in 2018. Despite only being known from a skull and a few vertebrae and osteoderms, it shares similarities with several erpetosuchids. Although its description only compared it with Erpetosuchus and Parringtonia, its assignment to this family does have some support. It shares a few traits with these other genera, such as maxillary teeth at only the front of the mouth and a portion of the maxilla under the lacrimal which is higher than long.[5]