Gnathovorax

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Gnathovorax
Temporal range:
Ma
Photograph, 3D scans, and illustration of the skull of the specimen
Restoration
Scientific classification Edit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Clade: Dinosauria (?)
Clade: Saurischia (?)
Family: Herrerasauridae
Genus: Gnathovorax
Pacheco et al., 2019
Type species
Gnathovorax cabreirai
Pacheco et al., 2019

Gnathovorax is a genus of

saurischian dinosaur from the Santa Maria Formation in Rio Grande do Sul, Brazil. The type and only species is Gnathovorax cabreirai, described by Pacheco et al. in 2019.[1]

The type specimen comprises a mostly complete and well-preserved articulated skeleton (considered one of the best herrerasaurid skeletons ever discovered), which fossilized in close association with rhynchosaur and cynodont remains. The discovery of this superb specimen has shed light onto poorly understood aspects of herrerasaurid anatomy such as endocranial soft tissues.[1]

Discovery

Location and fauna of the Marchezan site

The holotype specimen, CAPPA/UFSM 0009, is an almost complete and partially articulated skeleton, lacking only portions of the left shoulder girdle and left forelimb. It was found in 2014 at the Marchezan site, in the municipality of São João do Polêsine, Rio Grande do Sul, Brazil. This locality preserved rocks from Santa Maria Formation in the Candelária Sequence of the Paraná Basin. The skeleton was fossilized within a mudstone layer, along with small articulated skeletons of prozostrodont cynodonts. This mudstone layer was then covered by a layer of sandstone containing disarticulated rhynchosaur remains. These fossil remains are indicative of a diverse community of animals at Marchezan. Stratigraphically correlated beds from a nearby site were dated as the middle of the Carnian stage of the Late Triassic, circa 233.23 ± 0.73 million years ago.[1]

The

specific epithet was given in honour of Dr. Sérgio Furtado Cabreira, the palaeontologist who found the specimen.[1]

Description

Skull

Skull diagram with preserved bones in white

There were three teeth in the

infratemporal fenestra, and a lack of palatal teeth, are most similar to herrerasaurids among early dinosaurs.[1]

The

supraoccipital bone at the top of the braincase is trapezoidal when seen from behind (unlike Herrerasaurus's triangular supraoccipital) and has a blade-like crest on top. The occipital condyle is also thicker, the basioccipital has a V-shaped (rather than U-shaped) contact with the parabasisphenoid, and the paroccipital processes are larger in Gnathovorax compared to Herrerasaurus. Otherwise the braincase is similar between the two taxa. The dentary lacks sauropodomorph qualities and instead likely possessed a sliding joint at the chin similar to other herrerasaurids, although preservation is not good enough to fully confirm this. The 3 premaxillary, 19 maxillary, and 14 dentary teeth are all thin, curved, and blade-like. There are serrations on the distal (rear) edge of all the teeth, and the maxillary teeth also have serrations on the mesial (front) edge.[1]

Postcrania

Pelvis and hindlimbs

The neck is short, with keels along the lower edge of its vertebrae (like other herrerasaurids) and long transverse processes (like Sanjuansaurus specifically). The dorsal vertebrae are similar to those of other herrerasaurids, though the tips of the neural spines are only slightly expanded. As with herrerasaurids and most non-dinosaur reptiles, only two vertebrae form the

zygapophyses, upwards-pointing neural spines, and transverse processes which are semicircular in cross section.[1]

Gnathovorax is the only herrerasaurid combining features of the scapula such as posterior curvature, an expanded tip, and no fusion with the coracoid. The rest of the forearm is similar to that of Herrerasaurus, with a strongly developed elbow area and a long hand. The ilium is short, although it has a pointed front tip in contrast with the rounded front edge of other herrerasaurids. Gnathovorax is the only herrerasaurid in which the pubis is both sinuous in front view, and projects straight down in side view. The pubis also possesses a large pubic boot with a smooth lower edge, similar to other herrerasaurids, although it lacks the beveled pubic boot of Staurikosaurus. Unlike other herrerasaurids (but like lagerpetids), the ischium has a hole known as an obturator foramen at its base. The femoral head is more simple than that of other herrerasaurids, with a small craniomedial tuber (front inner bump) and no caudomedial tuber (rear inner bump). Otherwise the femur possesses all the muscle attachment sites typical of basal dinosaurs. The tibia is 90% the length of the femur, unlike Staurikosaurus which has a longer tibia. The rest of the lower leg is similar to Herrerasaurus, but the foot has three phalanges (toe bone) in the fifth toe, while Herrerasaurus only has one.[1]

In 2022, Aureliano and colleagues performed a mirco-computed tomography scan on the postcranial skeletons of some of the earliest saurischian dinosaurs that lived during the late

sauropodomorphs) are not homologous, but are traits that independently evolved at least 3 times.[2]

Classification

Gnathovorax was a member of the

cladistic analysis which argued that the Herrerasauridae are basal Saurischia, outside of the Theropoda and Sauropodomorpha. Within Herrerasauridae, Gnathovorax was found in a polytomy with Herrerasaurus and Sanjuansaurus.[1][3]

Dinosauria

Paleobiology

Braincase and virtual endocast

The holotype skeleton of Gnathovorax is so well preserved that its describers were able to reconstruct a digital

floccular fossa lobe (FFL) of the cerebellum, a portion of the brain generally (but not universally) considered useful for motor control of the eye, head, and neck. These features mean that most paleontologists consider a large FFL to be indicative of an active predatory lifestyle, explaining why it is reduced in later sauropodomorphs but not in most theropods. This soft tissue data provided by Gnathovorax is congruent with other evidence for carnivory in herrerasaurids, such as the structure of their teeth and claws. The tooth proportions of herrerasaurids and basal carnivorous sauropodomorphs each overlap slightly with theropods according to a morphological variation analysis, but herrerasaurids and sauropodomorphs do not overlap with each other. It is inferred that during the Carnian stage of the Triassic, herrerasaurids occupied large predatory niches while small basal sauropodomorphs occupied small carnivorous and omnivorous niches. After the extinction of herrerasaurids and carnivorous sauropodomorphs in the Norian, theropods became the dominant predatory dinosaurs at multiple size tiers, encouraging sauropodomorphs to acquire larger, more herbivory-focused ecologies.[1]

Gallery

  • Maxilla and dentary
    Maxilla and dentary
  • Neck and tail vertebrae
    Neck and tail vertebrae
  • Shoulder girdle and forelimbs
    Shoulder girdle and forelimbs

References