Gnathovorax, Herrerasaurus, Staurikosaurus and Sanjuansaurus

Herrerasaurus ischigualastensis (Reig 1963) Ladinian, middle Triassic ~230 mya, 3m in length was derived from a sister toPseudhesperosuchus Herrerasaurus preceded members of the Dinosauria, including Eodromaeus and Buriolestes at the base of the Phytodinosauria and Tawa at the base of the Theropoda.

Distinct from Pseudhesperosuchus, the skull of Herrerasaurus had a smaller antorbital fenestra. The naris was small and placed high on the rostrum. The lateral temporal fenestra was more open as the quadrate and squamosal retreated. The mandible had a larger fenestra straighter tip.

Distinct from Gracilisuchus, the skull was taller and narrower posteriorly. The squamosal did not contact the jugal. Note the similarity of the medially meeting pterygoids.

Distinct from Gracilisuchus the cervicals were smaller and the cervical ribs were longer. The neural spines were taller, especially near the pelvis. The caudals tapered distally.

The coracoid and ventral scapula were enlarged. Manual digits 4 and 5 were vestiges.

The pubis was deeper and angled posteriorly with a large "boot." The ilium was taller. The ischium was more gracile. The hind limb was more robust. The calcaneal heel was absent. The metatarsals were shorter. Digit 5 had at least one phalanx.

The premaxilla as reconstructed above is different than the traditional reconstruction.

Skull of Herrerasaurus (from Digimorph.org) animated GIF shows nasal and two possible postfrontals (wide proximally and not) every 5 seconds.

The traits that distinguish basal dinosaurs, like Herrerasaurus, Eodromaeus and Tawa, from their ancestors, like Junggarsuchus and Lewisuchus, include the following.

skull shorter than cervical series (not Herrerasaurus):
skull width less than 1.2x height;
posterolateral premaxilla narrower than naris (not Herrerasaurus);
quadrate curls posterodorsally (not Herrerasaurus);
dentary tip straight or descends (does not rise);
deep canine maxillary teeth absent (not Herrerasaurus);
nine or more cervicals (not Herrerasaurus);
cervical ribs slender, parallel centra (Herrerasaurus unknown);
3–4 sacral ribs (not Herrerasaurus, which has 2);
mid-caudal centra 3x longer than tall where known (not Herrerasaurus);
clavicles poorly ossified or absent (reappear in later taxa);
interclavicle poorly ossified or absent;
manual unguals long with penultimate phalanges longer than proximal phalanges;
tibia not shorter than femur (not Herrerasaurus);
fourth trochanter sharp (also in the proximal dinosaur outgroup taxon PVL 4597);
tibia not < 2x ilium length (not Herrerasaurus);
advanced and simple metatarsal joint without a calcaneal tuber (also in Lewisuchus, convergent in Silesaurus);
compact metatarsus;
longest metatarsal: #3;
metatarsal #1 < 75% of metatarsal #3 length;
metatarsals 2–4 >= tibia (not Herrerasaurus);
proximal metatarsals 1 and/or 5 reduced in diameter.

In summary: Arising from the morphology of basal bipedal crocodylomorphs, many of these basal dinosaur traits document a longer, leaner bipedal morphology with an increasingly robust fulcrum (pelvis and sacrum), along with longer distal hind limb elements with more reliance on the middle digits for faster locomotion. By contrast the proximal outgroup clade, Junggarsuchus and kin, documents a return to quadrupedalism terminating with Trialestes and its long forelimbs. The increasingly elongate radiale and ulnare of the Junggarsuchus clade turn out to be convergent, rather than homologous, with those of other bipedal crocodylomorphs, like Terrestrisuchus and Hesperosuchus, a clade that also transitioned toward a quadrupedal configuration, as seen in extant crocodylomorphs. We’ll take a closer look at this clade and this transition later this week.

Sanjuansaurus gordilloi (Alcober and Martinez 2010) Ladinian, middle Triassic ~230 mya was a sister to Herrerasaurus, but had a more primitive pubis that was more anteriorly oriented and shorter. The scapula was longer and narrower. The dorsal spines were shorter. The nine cervicals were more robust and longer.

Herrerasaurus and the origin of the Dinosauria are featured in

Herrerasaurus coracoid compared to Sanjuansaurus

The coracoids of Herrerasaurus and Sanjuansaurushave been imagined and repaired as crescentic discs, but remove most of the plaster and you have a more elongate coracoid more similar to that of Pseudhesperosuchus and crocs.

The origin of dinosaurs to scale. Hererasaurus is the basalmost dinosaur here.

The origin of dinosaurs to scale. Barberenasuchus is the basalmost dinosaur here.

Staurikosaurus pricei (Colbert 1970; Late Triassic, 225 mya; 2.25 m in length) is traditionally considered a herrerasaurid and nests here with Herrerasaurus.

Colbert EH 1970. A Saurischian dinosaur from the Triassic of Brazil. Amercian Museum Novitates2405; 1-39.

Novas FE 1994. New information on the systematics and postcranial skeleton of Herrerasaurus ischigualastensis (Theropoda: Herrerasauridae) from the Ischigualasto. Journal of Vertebrate Paleontology 13(4):40–423.

Pacheco et al. 2019. Gnathovorax cabreirai: a new early dinosaur and the origin and initial radiation of predatory dinosaurs. PeerJ. 7: e7963. doi:10.7717/peerj.7963

Reig OA 1963. La presencia de dinosaurios saurisquios en los “Estratos de Ischigualasto" (Mesotriásico Superior) de las provincias de San Juan y La Rioja (República Argentina). Ameghiniana 3: 3-20.

Sereno PC and Novas FE 1993. The skull and neck of the basal theropod Herrerasaurus ischigualastensis. Journal of Vertebrate Paleontology 13: 451-476. doi:10.1080/02724634.1994.10011525.

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wiki/Herrerasaurus
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Herrerasaurus

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