Theropoda: Basal Theropods: Herrerasauridae

1. Herrerasauridae X

Herrerasaurus feeding on a small cynodont Artwork by Dmitry Bogdanov, Wikipedia

Introduction

These medium-sized hunters are among the earliest known and most primitive dinosaurian lineage. They died out before the Triassic period ended. There is some disagreement whether Herrerasaurs should be considered primitive theropod (Sereno 1999, Nesbitt et al 2009, Nesbitt 2011, Sues et al 2011), basal saurischian (Langer & Benton 2006; Ezcurra 2010, Novas et al 2011) or pre-Saurischian dinosaur ancestors. If herrerasaurs are theropods, that indicates that the three main dinosaurian groups diverged very early on, and that all three lineages independently evolved several dinosaurian features, such as a more advanced ankle joint or, say, an open acetabulum (where the hind limb attaches to the pelvis). Indeed, the discovery of the extremely primitive prosauropod Saturnalia seems to confirm this. (adapted from MAK, revised MAK120207)

Phylogeny

The first two of the following dendrograms are adapted from Wikipedia The first follows one proposed analysis by Novas et al (2011). In this review, Herrerasaurus is a primitive saurischian, but not a theropod. The second is based on an analysis by Sues et al 2011, according to which Herrerasaurus is a basal theropod.

The implication of these very different results may be that Herrerasaurs in the broad sense constitute a paraphyletic assemblage of ancestral or stem group theropods, saurischians, or even dinosaurs, at first prior to, and then evolving alongside more specialised and typical dinosaur groups, in a similar manner to the relation of various dinosauromorph lineage sto dinosaurs as a whole. During the late Triassic, early dinosaur and proto-dinosaur evolution constituted a rich tapestry of diverse lineages. Of these many early lineages, only the three crown groups - the neotheropods, sauropodomorphs, and ornithischians, survived the end Triassic extinction, and dominated mnedium and large herbivore, ominover and carnivore niches for the next 150 million years. MAK1201204

Descriptions

Herrerasauridae: Herrerasaurus, Staurikosaurus.

Range: Late Triassic of South America & North America.

Phylogeny: Either basal/ancestral dinosauria, basal saurischia or basal theropoda, if the latter then Theropoda : (Eoraptor + (Daemonosaurus + (Tawa + (Podokesauridae + (Ceratosauria + Tetanurae))))) + * MAK020206

Characters: 2-5m bipedal predators; large teeth; intramandibular joint; vertebrae: 15+9+2+~50 (note primitive 2 sacrals); dorsal vertebrae and processes axially compressed; accessory dorsal intervertebral articulations present; axial skeleton pneumatized; distal caudal prezygapophyses overlap next vertebra; scapula narrow & unexpanded; well developed acromial process of scapula (deltoid muscle); elongated manus; manus IV&V greatly reduced, I, II, and II elongate w large claws; but acetabulum bony and only slightly perforate; brevis fossa insignificant; great distal enlargement of pubis; leg primitive; pes with 5 digits; pes I-IV with uncurved claws. ATW031204.

Links: Introduction to Herrerasaurus; Herrerasaurus « Dave Hone's Archosaur Musings; Paleo DataBase - Herrerasauridae; Herrerasaurus - Wikipedia and Herrerasauridae - Wikipedia (follow links for other taxa); Herrerasaurus - Jurassic Times; Herrerasaurus dinosaur fact net; Dino Family Spotlight: Herrerasauridae - Super Raptor Paleoblog; Re: Herrerasauridae - Dinosaur Mailing list archives 2002; DINOBASE, John Sibbick's drawing; van Dijk's Dinosaurus Sted - Herrerasaurus. Referencer; Herrerasaurus Fact Sheet - EnchantedLearning.com; Herrerasaurus and Sanjuansaurus - David Peters   MAK120206