Describer

Reig, 1963

Time

Triassic Late Carnian

Classification

Saurischia Theropoda Herrerasauria Herrerasauridae

Diet

Carnivore

Fossilsite

Ischigualasto Formation, San Juan, Argentina

Info

Genus - Typespecies

2 partical skeletons.

From: Reig, O.A. (1963). La presencia de dinosaurios saurisquios en los \\\"Estrados de Ischigualasto\\\" (Mesotriasico superior) de las provincias de San Juan y La Rioja (Republica Argentina). Ameghiniana 3, 3-20 [The presence of Saurischian Dinosaurs in the \\\"Ischigualasto beds\\\" (upper Middle Triassic) of the provinces of San Juan and La Rioja, Argentina.

_{Translated by Matthew T. Carrano University of Chicago, 2/98}

Ischisaurus, n. gen.

Ethymology: From Ischi, first part of the word Ischigualasto, and the component Saurus, used in the generic names of dinosaurs.Reig dedicated this species to Dr. Noemi V. Cattoi, in recognition of his repeated collaboration in the development of the activities in the Museo Argentino de Ciencias Naturales.

Diagnosis

A saurischian of moderate size, comparable to that of Massospondylus carinatus; premaxilla with four large, laterally compressed, backwards-inclined teeth. Gracile, slender dentary with fifteen teeth of narrow and elongate cross-section, the same as those of the maxilla; low maxilla. Relatively short cervical vertebrae without ventral keel, similar to those of Thecondontosaurus, the same as in the trunk. Gracile femur, lightly curved into an S, with fourth trochanter located on the upper third of the bone; gracile tibia, a little shorter than the femur (T/Fe index: 89), with laterally shortened proximal end. Short humerus, a third of the length of the femur; gracile, with prominent lateral process. Ulna with a typical olecranon. Foot similar to that of Herrerasaurus, the same for the astragalus; however the calcaneum is larger. Short coracoids with rounded borders, similar to those in Plateosauridae.

Typespecies

Ischisaurus cattoi, n. sp.

Holotype

Left femur, both tibiae, elements of both hind feet, both humeri, fragments of forearm and forefoot, fragments of pelvis, two cervical vertebrae, various dorsal and caudal vertebrae, most of both mandibles, right premaxilla and portion of right maxilla; other cranial fragments. This specimen was found by Galileo J. Scaglia on the expedition funded by the Instituto Miguel Lillo, Museo Argentino de Ciencias Naturales, Museo de La Plata and Museo Municipal de Ciencias Naturales de Mar del Plata, carried out in 1960. It was collected about 3700 meters ESE of Aguada de La Peña, in the lower beds of the Ischigualasto Beds. Its preparation was done by Scaglia in the laboratories of the MACN (abbreviation indicates the Museo Argentino de Ciencias Naturales and the collection of its Departamento de Palentología.) It belongs to the collections of this last institution, where it was catalogued with No. 18.060 in the Collección del Departamento de Paleontología.

Hypodigm

Apart from the holotype, specimen MLP 61-VIII-2-3, which includes a complete right femur, various elements of the right hind foot, both humeri, fragments of the forearm, fragments of vertebrae, coracoids, and fragments of maxilla, other cranial elements, and the mandibular rami. Found by Victorino Herrera in the same region as the holotype, about 4000 meters ESE of Aguada de La Peña. Like the holotype, found in the lower beds of the Ischigualasto Beds.

Provenance and age

Ischigualasto Valley, department of Valle Fértil, province of San Juan, Argentina. \\\"Ischigualasto Beds\\\" of the \\\"Ischigualasto-Ischichuca\\\" Series. Upper Middle Triassic.

Discussion

Comparison of Ischisaurus with other Triassic dinosaurs does not allow us to get very clear precision on its relationships for now. In the course of our work we had suggested the hypothesis that the remains we now attribute to Ischisaurus cattoi represent juvenile specimens of Herrerasaurus ischigualastensis, or another smaller species of this same genus. This possibility was established by the great similarity existing in the forms of the femur, astragalus and metatarsals. The marked differences that exist between other skeletal elements in both forms could be attributed, under this hypothesis, to factors of age or deformation, frequent in fossils of great antiquity. However this hypothesis cannot be validated by a closer observation, since we noticed differences that cannot be attributed to the alluded factors, such as the number of teeth and their morphology, the presence of an olecranon on the ulna, the relative height of the maxilla, the relative development of the calcaneum, the form of the proximal end of the tibia, etc.

Another possibility that can be discarded is that the remains which we attribute to Ischisaurus could pertain to Spondylosoma (see Huene, 1936-1942), the genus of saurischian which was found in the approximately coeval fauna of Santa María, Brazil. There exists similarities in size and relative length of the cervical vertebrae with this genus. However, those of Ischisaurus differ from those of the Brazilian genus in the absence of a keel on the inferior border of the centrum, and in the relative positions of the zygapophyses. The humerus attributed by Huene to Spondylosoma, for another thing, differs essentially from that of Ischisaurus.

That Ischisaurus is not a thecodontosaurid can be demonstrated by the shape of the femur, which in the San Juan genus is more gracile, incurved and has a more elevated fourth trochanter than in the known members of the aforementioned family.

At the same time, the relative size of the forelimb, with the humerus alone half the length of the femur, and the particular possession of an ulna with a typical olecranon (exceptional for a saurischian), coincide to exclude Ischisaurus from the Thecodontosauridae.

This last character has been confirmed in the ulnae of the known individuals, because its presence in this form excludes all doubt. Comparison with other families does not allow obtaining more positive results. The proportions between the femur, tibia and metatarsals match those of certain Triassic pachypodosaurs, and not those of coelurosaurs.

Nevertheless, the foot retains certain affinities with that of Ammosaurus (see Huene, 1906; Lull, 1915); however, Ischisaurus differs from this genus in having the tibia shorter than the femur, by the marked character of the olecranon, and in various other traits. The form of the femur, and above all the proportions between the humerus and femur, also suggest looking for its relations among the coelurosaurs. Perhaps we are in the presence of a primitive form which includes primitive characters of pachypodosaurs and coelurosaurs, and which represents a moment in saurischian evolution in which both groups had not entirely differentiated. However any conclusion on the relationships and significance of Ischisaurus is entirely premature; clarification will require new materials and direct comparisons.

_{Source: Polyglot Paleontologist}

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