The current consensus is that there are three herrerasaurids in the Ischigualasto Formation of Argentina- Herrerasaurus (including Ischisaurus and Frenguellisaurus), Sanjuansaurus, and the undescribed partial forelimb MACN-PV 18.649a (Ezcurra and Novas, 2007). But I'm suspicious.
First is the possibility MACN-PV 18.649a belongs to Sanjuansaurus. The only overlapping elements are the distal ulna, manual ungual and possibly vertebrae. The unnamed taxon is supposed to have apomorphic Y-shaped side grooves on its manual unguals, which may be visible in Sanjuansaurus (or may be an illusion caused by damage). It's also supposed to differ from Herrerasaurus in having "subequal ulna-ulnar articular contacts", which I admit to not understanding.
More intriguing is the idea Herrerasaurus has been overlumped. Novas (1989, 1993) sunk Ischisaurus and Frenguellisaurus into Herrerasaurus, but now that we have additional herrerasaurid taxa from the same formation, this seems less clear. Most of the differences between Herrerasaurus and Ischisaurus proposed by Reig when he described them were said by Novas to be due to parts of Herrerasaurus paratype PVL 2558 not belonging to the genus. PVL 2558 includes jaw material with a low number of teeth (3 premaxillary, 8 maxillary, 12 dentary), round alveoli, and a dorsally expanded dentary symphysis. This differs from the Ischisaurus holotype and paratype, which have four premaxillary teeth, a slender dentary with about 15 teeth, and teeth that are more laterally compressed and recurved. The latter agrees with specimen PVSJ 407, which Sereno and Novas (1993) described. Maybe the PVL 2558 cranial material belongs to a crurotarsan or something, but Novas does not give any reasoning for his calling it Archosauria indet. except that it doesn't match with other material he refers to Herrerasaurus. Even ignoring that though, Reig states the calcaneum of Ischisaurus is larger than Herrerasaurus, and the proximal tibia is "laterally shortened." These are not addressed by Novas. Novas (1993) states the Herrerasaurus holotype has shorter posterior dorsal centra than the Ischisaurus holotype, PVSJ 373, 407 and 461, which makes one question his comment earlier on the same page that the Herrerasaurus and Ischisaurus holotypes "exhibit the same autapomorphies and do not exhibit any differences." Of course those autapomorphies could just be herrerasaurid synapomorphies, as Sanjuansaurus has most them it can be coded for- e.g. narrow, U-shaped antiorbital fossa; spine tables on posterior dorsal and first sacral vertebrae; prominent acromion on scapula; acromial process extends distally with respect to glenoid lip, forms nearly right angle with scapular blade; unexpanded distal scapula; anteroproximal keel on femur; anterolateral subcircular muscle scar on distal femur. Are Ischisaurus and/or Frenguellisaurus Sanjuansaurus instead? Also interesting is that Reig states Ischisaurus lacks ventral keels on its cervical centra, while Sereno and Novas (based mostly on PVSJ 407) say Herrerasaurus has them. Based on this, at least Ischisaurus wouldn't be Sanjuansaurus. I haven't even looked at Frenguellisaurus yet to see where it fits into this mess.
Even if Ischisaurus and Frenguellisaurus are synonymous with Herrerasaurus, there seems to be the distinct possibility some of the referred Herrerasaurus specimens are actually Sanjuansaurus and/or the unnamed genus. After all, Sanjuansaurus has almost every supposed Herrerasaurus autapomorphy it can be examined for. Or would Novas and Sereno sink Sanjuansaurus into Herrerasaurus as well?
Unfortunately, I doubt I'll be able to get much resolved using the literature. Reig didn't describe the specimens much at all and only illustrated a few elements (pelvis and hindlimb of the Herrerasaurus holotype; dentary of PVL 2558; femur and humerus of the Ischisaurus holotype), while Novas and/or Sereno only illustrate one specimen for each element (except the neck and pelvis, which are composites of several specimens). Almost all of their descriptions are also composites using several specimens. So there's no way to check out each specimen and see which characters it has, how it differs from other spcimens, etc..
References- Reig, 1963. La presencia de dinosaurios saurisquios en los "Estratos de Ischigualasto" (Mesotriasico Superior) de las provincias de San Juan y La Rioja (República Argentina) [The presence of saurischian dinosaurs in the "Ischigualasto beds" (upper Middle Triassic) of San Juan and La Rioja Provinces (Argentine Republic)]. Ameghiniana. 3, 3-20.
Novas, 1989. The tibia and tarsus in Herrerasauridae (Dinosauria, incertae sedis) and the originn and evolution of the dinosaurian tarsus. Journal of Paleontology. 63, 677-690.
Novas, 1993. New information on the systematics and postcranial skeleton of Herrerasaurus ischigualastensis (Theropoda: Herrerasauridae) from the Ischigualasto Formation (Upper Triassic) of Argentina. Journal of Vertebrate Paleontology. 13, 400-423.
Sereno and Novas, 1993. The skull and neck of the basal theropod Herrerasaurus ischigualastensis. Journal of Vertebrate Paleontology. 13, 451-476.
Ezcurra and Novas, 2007. New dinosaur remains (Saurischia: Herrerasauridae) from the Ischigualasto Formation (Carnian) of NW Argentina. Ameghiniana. 44, 17R.
Bonaparte seems to consider both Frenguelli and Ischisaus valid. Here are the entries for these genera in a book he published in 2007:
ReplyDeleteFrenguellisaurus ischigualastensis Novas, 1986
Este primitivo género de dinosaurios saurisquios procede del sector inferior de la formación Ischigualasto. Está representado por un craneo muy distorsionado, la mandíbula inferior y algunas vertebras dorsales y caudales.
La mandíbula inferior está completa y se observa un rasgo significativo que lo distingue de Herrerasaurus: la longitud del dentario (el hueso que contiene a los dientes), medida entre el punto más anterior de la sínfisis y el borde anterior de la abertura mandibular, muestra que es proporcionalmente más corto, representando sólo el 43% del largo total de la mandíbula. En Herrerasaurus, el dentario es más largo en proporción y corresponde al 47% del total de la mandíbula inferior.
Esta diferencia sugiere que Frenguellisaurus tenía una adaptación especial en la mandíbula para atrapar y sostener la presa, cualidad en la que posiblemente estaban involucrados los grandes dientes caniniformes y un sistema muscular más efectivo, o simplemente diferente del de Herrerasaurus.
Novas (1992) también consideró a Frenguellisaurus como sinónimo de Herrerasaurus, un procedimiento sin demostración fehaciente, como en el caso de Ischisaurus.
Ischisaurus cattoi Reig, 1963
Este pequeño dinosaurio de la formación Ischigualasto está basado en dos esqueletos incompletos que son aproximadamente un 30 % más pequeños que el ejemplar tipo de Herrerasaurus. Entre el material original existen algunos fragmentos de cráneo y vértebras dorsales, húmero, ulna, fémur y otras piezas fragmentarias de las extremidades. Reig (1963) caracterizó a Ischisaurus como un dinosaurio saurisquio primitivo, anatómicamente diferente de Herrerasaurus en las proporciones relativas de las extremidades: la anterior proporcionalmente más pequeña en Ischisaurus, el húmero con una cresta deltopectoral prominente y la ulna con un proceso olécranon muy desarrollado. Las vértebras dorsales de Ischisaurus indican que es diferente de Herrerasaurus, por cuanto los centros vertebrales de este último género son cortos y altos, y las espinas neurales, más bien cuadrangulares, que señalan rasgos de especialización en la columna vertebral que están presentes en Ischisaurus.
No obstante, a pesar de estas diferencias, Ischisaurus fue considerado sinónimo de Herrerasaurus por Novas (1992), como así por Sereno y Novas (1993), con unos pocos argumentos que se contradicen con la anatomía y el grado evolutivo presente en uno y otro género. Ischisaurus cattoi es considerado un género y especie válidos, hasta tanto se demuestre, si es posible, la sinonimia con Herrerasaurus.
source: José Bonaparte / "Dinosaurios y Pterosaurios de América del Sur" / Editorial Albatros, 2007 / pp. 62-63