| Archeopelta | |
|---|---|
 | |
| A vertebra and adjacent osteoderms from CPEZ-239a | |
Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Reptilia |
| Clade: | Archosauria |
| Clade: | Pseudosuchia |
| Clade: | Suchia |
| Family: | †Erpetosuchidae |
| Genus: | †Archeopelta Desojoet al., 2011 |
| Type species | |
| †Archeopelta arborensis Desojoet al., 2011 | |
Archeopelta is anextinctgenus ofcarnivorousarchosaur from the late Middle or early LateTriassic period (lateLadinian to earlyCarnian stage). It was a 2 m (6 ft) longpredator which lived in what is now southern Brazil.[1] Its exact phylogenetic placement within Archosauriformes is uncertain; it was originally classified as adoswelliid, but subsequently it was argued to be anerpetosuchidarchosaur.[2]
It is only known from theholotype CPEZ-239a, which consists of partialskeleton (includingvertebrae, partial right front and hind limbs, a partial hip, and an undetermined bone which may be part of a tibia) andbraincase. It was found in theSanta Maria 1 Sequence, previously known as theSanta Maria Formation, inChiniquá region,São Pedro do Sul ofRio Grande do Sul State. It was first named by Julia B. Desojo, Martín D. Ezcurra and César L. Schultz in2011 and thetype species isArcheopelta arborensis. Thegeneric name comes fromarchaios, ancient inGreek andpelta, shield, in reference to its thickosteoderms. Thespecific name is derived fromarbor, tree inLatin, in reference toSanga da Árvore where the fossils were found.[3]
CPEZ-239a's braincase is low, without an occipital neck. Theexoccipitals do not meet each other medially. The paraoccipital processes extend outwards and slightly downwards, and the supraoccipital has a ridge. Uniquely, a very deep fossa is present in the corner of the opisthotics. The exit of thehypoglossal nerve is a single opening. The basal tubera are very low and separated by a deep notch. The parabasisphenoid is short and its exits for theinternal carotid arteries are small and pushed to the rear edge of the bone. The basipterygoid processes are close to each other and project anterolaterally. The exit for thefacial nerve is present on thevestibule, and the lamina separating the metotic foramen and thefenestra ovalis is very thin.
The back vertebrae are only slightly elongated and are not constricted from the sides. The diapophyses are thick, subrectangular, and elongated. Theprezygapophyses are short while the neural spines are long and oval-shaped in cross section. The first 'primordial'sacral vertebra (likely the second sacral) is low and wide, with characteristically-shaped sacral ribs. The sacral ribs expand anteroposteriorly at their tips, with the anterior expansions being large and subtriangular. The first primordial's unique prezygapophyses are very large and circular, with their faces pointing upward and inwards. On the other hand, the postzygapophyses are short, downwards-pointing, and connected by a V-shaped hyposphene. Although incomplete, the second 'primordial' sacral (likely the third sacral) is also low, with sacral ribs similarly shaped to those of the first 'primordial' sacral.[3]
Thehumeral head is offset from the humeral shaft, and the righthumerus as a whole is wide and distally tapering (although missing the distal portion). A thin bone has been interpreted to be a rightilium, with an unusual illiac blade which is S-shaped in posterior view. However, this interpretation is uncertain and the bone's shape may be a result of postmortem deformation.[2] The rightischium has a long and deep pubic peduncle but a very short illiac peduncle. The ischial shaft is thin and the lower edge bends towards the midline. The rightfemur is S-shaped from the front, with a transversely very wide distal end and poorly developed condyles and tubercules. The righttibia is anteroposteriorly wide but distally tapering and missing its distal tip. An unusual rod-like bone may be the distal part of a tibia.[3]
Severalosteoderms are preserved with CPEZ-239a. At least two rows of osteoderms attached to each neural spine of the vertebrae, and there is evidence that additional rows of lateral osteoderms were also present. The osteoderms are very thick and quadrangular in shape, with straight posterior and medial borders and rounded anterior and lateral borders. They were rough and covered in deep, circular pits, with each possessing an anterior articular lamina (a smooth area where the preceding osteoderm would have overlapped the front edge of the following osteoderm). Although they had serrated edges, they did not possess a raised keel or peak (a dorsal prominence) on their surface. Small, circular plates attached to the femur may be appendicular osteoderms, although poor preservation makes this uncertain.[2]
Upon the initial description ofArcheopelta, it was placed as a close relative ofDoswellia in the newly created familyDoswellidae. This referral was due to the structure of its osteoderms, which were very similar to those ofDoswellia. In addition,Archeopelta shared several other features withDoswellia which were unknown inTarjadia, which was rather incomplete at the time ofArcheopelta's description. Among these features are the wide first primordial sacral, a long and laterally deflected illiac blade, and anterolaterally-projected basipterygoid processes. This is thecladogram from the study, after Desojo, Ezcurra & Schultz, 2011:[3]
In 2013, Lucas, Spielmann, and Hunt claimed thatArcheopelta was ajunior synonym ofTarjadia due to a lack of distinguishing features between the two.[4] However, Ezcurra (2016) provided several differences between the two genera. For example,Archeopelta has unconstricted dorsal vertebrae and a ridge on the supraoccipital, whileTarjadia has the opposite traits. Nevertheless, Ezcurra observed that both genera had very thick osteoderms, and he considered that both of them were doswellids more closely related to each other than toDoswellia.[5]
New specimens ofTarjadia described in 2017 provided aphylogenetic analysis that argued thatTarjadia andArcheopelta were not doswellids, but ratherpseudosuchianarchosaurs of the familyErpetosuchidae. The strict consensus tree of this analysis is given below. Erpetosuchid osteoderms are similar to Doswellid osteoderms, which explains the earlier classification ofTarjadia andArcheopelta. Even in non-parmonious phylogenetic trees of the analysis which forced them to be recovered as doswellids,Tarjadia andArcheopelta still formed a clade withErpetosuchus. Despite the change inArcheopelta's classification, it still forms a clade withTarjadia as an erpetosuchid. This is due to both of them possessing a very short or absent occipital neck and osteoderms without a dorsal prominence.[2]
