| Gnathovorax | |
|---|---|
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| Photograph, 3D scans, and illustration of the skull of the specimen | |
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| Restoration | |
Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Clade: | Dinosauria |
| Clade: | Saurischia |
| Family: | †Herrerasauridae |
| Genus: | †Gnathovorax Pachecoet al.,2019 |
| Type species | |
| †Gnathovorax cabreirai Pachecoet al., 2019 | |
Gnathovorax is a genus ofherrerasauridsaurischiandinosaur from theSanta Maria Formation inRio Grande do Sul,Brazil. Thetype and only species isGnathovorax cabreirai, described by Pachecoet 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 withrhynchosaur andcynodont remains. The discovery of this superb specimen has shed light onto poorly understood aspects of herrerasaurid anatomy such as endocranial soft tissues.[1]
Theholotype specimen, CAPPA/UFSM 0009, is an almost complete and partially articulated skeleton, lacking only portions of the leftshoulder girdle and leftforelimb. It was found in 2014 at the Marchezan site, in the municipality ofSão João do Polêsine, Rio Grande do Sul, Brazil. This locality preserved rocks fromSanta Maria Formation in the Candelária Sequence of theParaná Basin. The skeleton was fossilized within amudstone layer, along with small articulated skeletons ofprozostrodontcynodonts. This mudstone layer was then covered by a layer ofsandstone containing disarticulatedrhynchosaur 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 theCarnian stage of theLate Triassic, circa 233.23 ± 0.73 million years ago.[1]
Themonospecific genus was named and described by Cristian Pacheco, Rodrigo Temp Müller, Max Langer, Flávio Augusto Pretto, Leonardo Kerber and Sérgio Dias da Silva in an article published in 2019. The generic name is derived from the Greekgnathos, jaw, and the Latinvorax, "voracious", fromvorō (“devour”) + −āx (“inclined to”). Thespecific epithet was given in honour of Dr. Sérgio Furtado Cabreira, the palaeontologist who found the specimen.[1]
There were three teeth in thepremaxilla, in contrast toHerrerasaurus (which possessed four). The contact between the premaxilla andmaxilla encompassed two fenestrae (holes): a subnarial fenestra which was situated low on the snout and an additional smaller fenestra positioned above it. While a subnarial fenestra is common in herrerasaurids and theropods, the additional fenestra is unique toGnathovorax. Like otherarchosaurs, a small basin known as the antorbital fossa is present in front of theantorbital fenestra. Many early saurischians have a hole called the promaxillary fenestra situated on the antorbital fossa, but this hole is absent inGnathovorax. On the other hand,Gnathovorax does possess a characteristic thin ridge edging an additional emargination within the antorbital fossa; a small depression is present between this ridge and the ridge forming the border of the antorbital fossa. The lower branch of thelacrimal is slender and extends to the middle of the lower edge of theorbit (eye socket), similar toDaemonosaurus but different fromHerrerasaurus. Most other aspects of the skull, such as the thicksquamosal, expanded front branch of thejugal, the shape of theinfratemporal fenestra, and a lack ofpalatal teeth, are most similar to herrerasaurids among early dinosaurs.[1]
Thesupraoccipital bone at the top of thebraincase is trapezoidal when seen from behind (unlikeHerrerasaurus's triangular supraoccipital) and has a blade-like crest on top. Theoccipital condyle is also thicker, thebasioccipital has a V-shaped (rather than U-shaped) contact with theparabasisphenoid, and theparoccipital processes are larger inGnathovorax compared toHerrerasaurus. Otherwise the braincase is similar between the two taxa. Thedentary lackssauropodomorph 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]
The neck is short, with keels along the lower edge of its vertebrae (like other herrerasaurids) and long transverse processes (likeSanjuansaurus 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 thesacrum. However,Gnathovorax also has its last dorsal vertebra encompassed by theilium (upper hip plate), although its transverse processes are not long enough to contact the ilium; therefore, it does not qualify as a sacral vertebra. The tail vertebrae are most similar toHerrerasaurus, with low and elongatedzygapophyses, upwards-pointing neural spines, and transverse processes which are semicircular in cross section.[1]
Gnathovorax is the only herrerasaurid combining features of thescapula such as posterior curvature, an expanded tip, and no fusion with thecoracoid. The rest of the forearm is similar to that ofHerrerasaurus, 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 ofStaurikosaurus. Unlike other herrerasaurids (but likelagerpetids), theischium has a hole known as an obturator foramen at its base. Thefemoral 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 thefemur possesses all the muscle attachment sites typical of basal dinosaurs. Thetibia is 90% the length of the femur, unlikeStaurikosaurus which has a longer tibia. The rest of the lower leg is similar toHerrerasaurus, but the foot has threephalanges (toe bone) in the fifth toe, whileHerrerasaurus 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 lateCarnian includingGnathovorax with sauropodomorphsPampadromaeus andBuriolestes, which showed that the invasive air sac system was absent and that their bones were not pneumatised. These results indicate that pneumatisation in archosaur groups (pterosaurs,theropods andsauropodomorphs) are not homologous, but are traits that independently evolved at least 3 times.[2]
Gnathovorax was a member of theHerrerasauridae, a group of small to medium-sized carnivorous dinosaurs. The paper describingGnathovorax contained acladistic analysis which argued that the Herrerasauridae are basalSaurischia, outside of theTheropoda andSauropodomorpha. Within Herrerasauridae,Gnathovorax was found in a polytomy withHerrerasaurus andSanjuansaurus.[1][3]
The holotype skeleton ofGnathovorax is so well preserved that its describers were able to reconstruct a digitalendocast of the brain. It possessed a largefloccular fossa lobe (FFL) of thecerebellum, 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 latersauropodomorphs but not in mosttheropods. This soft tissue data provided byGnathovorax 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 theCarnian 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 theNorian, theropods became the dominant predatory dinosaurs at multiple size tiers, encouraging sauropodomorphs to acquire larger, more herbivory-focused ecologies.[1]
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