Saurischia the big picture

What makes a saurischian a saurischian?

Dinosauria

Figure III.1. Cladogram of Dinosauria, emphasizing the monophyly of Saurischia. Derived characters include: at 1, fossa expanded into the anterior corner of the external naris, the development of a subnarial foramen, a concave facet on the axial intercen-trum for the atlas, elongation of the centra of anterior cervical vertebrae, hyposphene-hypantrum articulation on the dorsal vertebrae, expanded transverse processes of sacral vertebrae, loss of distal carpal V, twisting of the first phalanx of manual digit I, well-developed supracetabular crest, restriction of the medioventral lamina of the ischium to the proximal third of the bone.

For all their differences, Saurischia is mono-phyletic, and is diagnosed by more than a dozen derived features (Figure III.1), two of which are shown in Figure III.2.

Recall from Chapter 4 that, as early as 1887, H. G. Seeley recognized two great clades within Dinosauria: Ornithischia and Saurischia. Having dwelled on Ornithischia for the past three chapters, we now turn to Saurischia for the next four. Seeley's Saurischia originally consisted of Sauropodomorpha (see Chapter 8) and its sister-taxon Theropoda (theros - wild beast; poda - foot; Chapters 9, 10, and 11). A modern view, however, also includes a few dinosaurs that appear to be neither sauropodomorphs nor theropods within Saurischia.

Despite the evident monophyly of Saurischia, some question remains about the relationships of some of its members, in particular the primitive small saurischians

Hyposphene
10 cm

Figure III.2. (a) Dorsal vertebrae of Herrerasaurus indicating the extra hyposphene-hypantrum articulations; (b) hypantrum in medial view; (c) twisted thumb (digit I of the hand).

10 cm

10 cm

(a) Figure III.3. (a) Staurikosaurus;

(b) Herrerasaurus; (c) Eoraptor.

(a) Figure III.3. (a) Staurikosaurus;

(b) Herrerasaurus; (c) Eoraptor.

Eoraptor Skeleton
(b)
Baryonyx Skeleton
(c)
Hyposphene

Staurikosaurus, Herrerasaurus, Eoraptor, Guaibasaurus, and Saturnalia (Figure III.3). J. A. Gauthier's Dinosauria included not only Saurischia and Ornithischia, but also Staurikosaurus and Herrerasaurus outside of Saurischia and Ornithischia (Figure III.4a).

Saurischia Ornithischia Saurischia Ornithischia

Saurischia

Dinosauria

Saurischia

Dinosauria

Theropoda Saurischia

Dinosauria

Figure III.4. (a) Gauthier's interpretation of basal saurischian relationships; (b) Sereno's interpretation of basal saurischian relationships.

New material of Herrerasaurus and the discovery of Eoraptor, however, led P. C. Sereno and collaborators to suggest that these two forms, plus Staurikosaurus were actually thero-pods (Figure III.4b), a return to a more Seeley-like view of dinosaur relationships.

With the subsequent discovery of Guaiba-saurus and Saturnalia, and new analyses of all these forms by paleontologist M. Langer, Saurischia revealed yet more complexity than before. Langer divided Saurischia into two main groups: herrerasaurs (including Staurikosaurus) and a so-far unnamed clade comprising Eoraptor, Guaibasaurus, Saturnalia, Theropoda, and Sauropodomorpha (Figure III.5). Given the rarity and incompleteness of all these basal saurischian fossils, we're betting that we haven't seen the final phylogeny for the group!

Arbre Phylogenetique

Dinosauria

Figure III.5. Cladogram of Saurischia.

Dinosauria

Figure III.5. Cladogram of Saurischia.

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