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of evolutionary theory allows for extrapolating a greater range represented by ghost lineages. Nonetheless, dinosaur remains discovered from Early Triassic rocks would be an extremely significant find, and similar body fossils from Permian rocks would be completely unexpected.

Tracks would be considerably less convincing evidence than skeletal remains for the first dinosaurs or their immediate ancestors, despite the valuable information potentially conveyed by such a find (Chapter 14). Even more suspect evidence would be eggs and nests, minus accompanying skeletal material (Chapter 8). Coprolites, gastroliths, and toothmarks attributable to the first dinosaurs would probably warrant the most skepticism because of the current lack of firm identity attached to such trace fossils (Chapter 14). Consequently, the origin of the first dinosaurs can only be postulated on the basis of skeletal evidence and the stratigraphic position of this evidence, although other indicators or supporting evidence of their existence is possible through trace fossils. The problem with a trace fossil approach for finding evidence of dinosaur ancestors is threefold:

1 trace fossils could have been made by tracemakers that had a similar morphology to the first dinosaurs but may have been distantly-related archosaurs;

2 the criteria for what constitutes a dinosaur in the fossil record is currently based on anatomical criteria; and

3 most dinosaur paleontologists have limited their studies to bones and have not looked for trace fossil evidence.

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FIGURE 6.11 Cast of the small dinosauromorph Marasuchus from the Late Triassic of Argentina: Sam Noble Oklahoma Museum of Natural History, Norman, Oklahoma. Marasuchus is not a dinosaur, but is very, very close to being one. Length about 40 cm.

As a result, the body fossil record for dinosaur ancestors is currently considered to be the primary basis for phylogenetic reconstructions of dinosaur lineages.

Based on known lineages of archosaurs before the oldest known dinosaurs found in the geologic record and their anatomical traits, a prediction of the ancestral archosaur, the "mother of all dinosaurs," can be made. This hypothetical ancestor would have had, at a minimum, the following traits distinctive from other diapsids:

■ Bipedal, with long hind limbs relative to the forelimbs.

■ Four or five digits on its manus, with digits IV and V reduced in size.

■ Long metatarsals and phalanges on its pes.

■ Ankle with a hinge developed between the astragalus and calcaneum.

■ A tibia-fibula length greater than the femur.

Of fossil finds so far, those closest to this ancestor are Marasuchus (Fig. 6.11), synonymous with Lagosuchus in some studies, and Lagerpeton, which are small but long-limbed reptiles occurring in the Middle Triassic strata of Argentina. Marasuchus and Lagerpeton were among the first ornithodirans, and their successors could have diverged into either pterosaur or dinosaur lineages. Additionally, small three-toed footprints documented from Early and Middle Triassic strata may be associated with ornithodiran tracemakers that preceded or were contemporaneous with the aforementioned species represented by body fossils.

These possible ancestral forms are succeeded in the geologic record by what are considered by many paleontologists as the earliest known dinosaurs: Eoraptor lunensis and Herrerasaurus ischigualastensis from the Ischigualasto Formation of Argentina, as well as Staurikosaurus pricei from the Santa Maria Formation of Brazil (Fig. 6.12). All three of these specimens are from the earliest part of the Late Triassic (Carnian Age, which was about 221 to 228 Ma); radiometric age dates of 40Ar/39Ar

FIGURE 6.12 Three Late Triassic fossil archosaurs proposed as primitive dinosaurs. (A) Eoraptor lunensis.

(B) Herrerasaurus ischigualasto.

(C) Staurikosaurus pricei. Modified from Paul (1988), Sereno et al. (1993), and Sereno (1994).

FIGURE 6.12 Three Late Triassic fossil archosaurs proposed as primitive dinosaurs. (A) Eoraptor lunensis.

(B) Herrerasaurus ischigualasto.

(C) Staurikosaurus pricei. Modified from Paul (1988), Sereno et al. (1993), and Sereno (1994).

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