Eodromaeus

Chapter • Meet the dinosaur

Overview

Eodromaeus was a small, lightly built predatory dinosaur, only about 1.2 to 1.6 metres long from snout to tail tip, with much of that length taken up by a long, stiffened tail. It weighed roughly five kilograms. The animal walked on two legs with its backbone held horizontally and the tail stretched out behind as a counterweight, exactly as in later theropods. The neck was slender and flexible, the skull low and pointed, and the jaws carried small, backward-curving, serrated teeth. The forelimbs were short but muscular and ended in five-fingered hands, the inner three of which bore substantial claws. No feathers are known from Eodromaeus – no skin or feather impressions have been preserved – and because the animal sits near the very base of the theropod family tree, whether it had simple filamentous body covering or only scaly skin remains genuinely open. Everything we can say about its behaviour comes from skeletal anatomy, since no tracks, nests or eggs have been attributed to the species. The long, slim hind limbs with tightly bunched metatarsals point to a fast-moving runner that chased down prey in open ground. Air-filled cavities in the vertebrae – the same type of hollows seen in modern birds – show that the bird-like respiratory system had already begun to evolve more than 230 million years ago. Whether Eodromaeus lived alone or in groups is unknown; there is no fossil evidence of gregarious behaviour in this species at all. The teeth make the diet clear: they were recurved, flattened side to side and edged with fine serrations front and back, a classic meat-eater's toolkit. Eodromaeus was far too small to tackle the large herbivores of its world and most likely hunted small reptiles, early mammal relatives (cynodonts) and insects. The clawed hands were probably used to grab and pin struggling prey while the jaws did the rest. The holotype skeleton, catalogued as PVSJ 560, was found in 1996 in the Ischigualasto Formation of Valle de la Luna, San Juan Province, north-western Argentina. It was discovered by field volunteer Jim Murphy, whom the species name murphi honours; the genus name means roughly "dawn runner". The animal was formally described in 2011 in the journal Science by Ricardo Martínez, Paul Sereno, Oscar Alcober and colleagues. The paper had wide impact: in the same study the equally ancient Eoraptor was reinterpreted as an early sauropodomorph rather than a theropod, which left Eodromaeus as one of the clearest pictures we have of what a truly early carnivorous dinosaur looked like.

Chapter • Where it lived

Habitat

Eodromaeus lived in the Late Triassic, during the Carnian stage roughly 233–230 million years ago. Every known specimen comes from the Ischigualasto Formation in Ischigualasto Provincial Park (Valle de la Luna), San Juan Province, Argentina. At the time this was a broad river floodplain crossed by meandering channels, under a warm climate with strongly marked wet and dry seasons. Volcanic ash from nearby eruptions blanketed the plain and helped preserve skeletons and plant remains – and the same ash layers allow the fossil horizons to be radiometrically dated with unusual precision. The landscape carried ferns, horsetails and conifer-like woodland along the watercourses. Dinosaurs were still a small minority of the fauna: Eodromaeus shared the plain with Eoraptor, Herrerasaurus, the early sauropodomorph Panphagia, large predatory rauisuchians, and abundant cynodonts and barrel-bodied herbivorous rhynchosaurs.

Chapter • Wild behavior

Behavior & Ecology

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Chapter • Scientific place

Taxonomy