Part

Saurischia: predators and giants

Thecodonts

First coined by Cambridge University's H. G. Seeley in 1887, Saurischia originally consisted of Sauropodomorpha (Chapter 11) and its sister-taxon Theropoda (Chapters 12,13, and 14). Seeley imagined saurischians as having a different primitive archosaurian ("thecodont") ancestor from that of ornithischians (Chapter 5), and thought of dinosaurs as a heterogeneous group of advanced archosaurs. A modern view includes Sauropodomorpha and Theropoda as well as a few primitive taxa that appear to be neither sauropodomorphs nor theropods within Saurischia. Saurischians include both the smallest of dinosaurs and the super-giants, as well as the most agile of predatory dinosaurs and the most ponderous plant-eaters.

Despite such disparate membership, Saurischia is monophyletic, defined as all Dinosauria closer to Allosaurus than to Stegosaurus (Figure III.l). The clade is diagnosed by more than a dozen derived features (Figure III.2), including modifications of the external nares, elongation of the rearward neck vertebrae (contributing to a relatively long neck), the development of accessory articulations between the dorsal vertebrae (the so-called hyposphene-hypantrum articulations), expanded transverse processes of the sacral vertebrae, twisted and enlarged thumb, and modifications of the pelvis. All in all, this is a well-supported dinosaurian clade.

Until recently, saurischians neatly divided into theropods and sauropodomorphs a la Seeley. However, recent discoveries of new taxa and better-preserved material of earlier-known taxa have provided a new

Saurischia

Saurischia

Dinosauria

Figure III.l. Cladogram of Dinosauria, emphasizing the monophyly of Saurischia. Derived characters include: at I fossa expanded into the forward corner of the external naris, the development of a subnarial foramen, a concave facet on the axial intercentrum for the atlas, elongation of the centra of forward 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, and restriction of the medioventral lamina of the ischium to the proximal third of the bone.

Saurischia

Dinosauria

Figure III.l. Cladogram of Dinosauria, emphasizing the monophyly of Saurischia. Derived characters include: at I fossa expanded into the forward corner of the external naris, the development of a subnarial foramen, a concave facet on the axial intercentrum for the atlas, elongation of the centra of forward 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, and restriction of the medioventral lamina of the ischium to the proximal third of the bone.

I Ocm

Axial Vertebral Dinosaurs

10 cm

I Ocm

10 cm

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

window on the complexity of saurischian origins. These include Herrerasaurus, Eoraptor, Staurikosaurus, Saturnalia, and Guaibasaurus.

When J. A. Gauthier first assessed the phylogenetic relationships of theropods, it was necessary to establish who among remaining dinosaurs were the closest relatives to Theropoda. He identified Sauropodomorpha as the sister-group of Theropoda. No problem there; this union is the familiar Saurischia. And as we all know, Ornithischia is the sister-group to Saurischia. However, two relatively poorly known forms, Staurikosaurus and Herrerasaurus (Figure III.3a and b), were problematic. Because they were originally thought to be theropods and therefore dinosaurs, Gauthier's Dinosauria included not only Saurischia and Ornithischia, but also these two lesser well-known forms as basal members of the clade (Figure III.4a).

With new material of Herrerasaurus and the discovery of Eoraptor (Figure III.3c), P. C. Sereno and collaborators in the 1990s suggested that these two forms, plus Staurikosaurus were actually basal members of Theropoda (Figure III.4b). As a result, Dinosauria reverted solely to Saurischia and Ornithischia. With the subsequent discovery of Guaibasaurus and Saturnalia (Figure III.5), and new analyses of all these forms conducted by Max Langer, the situation again became more intricate and controversial. Dinosauria stayed the same, but Saurischia revealed more complexity than before. While theropods share a more recent common ancestor with sauropodomorphs, this relationship does not include Herrerasaurus and Staurikosaurus (together forming Herrerasauridae), Eoraptor, and Guaibasaurus. Instead, Saurischia is divided into two main groups: Herrerasauridae and a so-far unnamed

Guaibasaurus
(b)
Dibujos Para Colorear Jurassic Park Staurikosaurus Skeletal

Figure III.3. (a) Staurikosaurus, (b) Herrerasaurus, (c) £oraptor.

Figure 111.4. (a) Gauthier's interpretation of basal saurischian relationships; (b) Sereno's interpretation of basal saurischian relationships.
Saturnalia FiguresGuaibasaurus

Figure III.5. (a) Saturnalia, (b) Guaibasaurus. Solid black areas are where lack of data (missing bones) have been supplemented by similar features in related dinosaurs.

Figure 111.6. Cladogram of Saurischia.

clade comprising Eoraptor, Guaibasaurus, Saturnalia, Theropoda, and Sauropodomorpha (Figure III.6). The features establishing these relationships are hard to come by because of the incomplete nature of the material of several of these basal saurischians, so this chunk of dinosaurian evolutionary history could well be unstable in the future.

Important readings Gauthier, J. A. 1986. Saurischian monophyly and the origin of birds.

Memoirs of the California Academy of Sciences, 8,1-55. Langer, M. 2004. Basal Saurischia. In Weishampel, D. B„ Dodson, P. and Osmölska, H. (eds.) The Dinosauria, 2nd edn. University of California Press, Berkeley, pp. 25-46. Langer, M. A., Abdala, F., Richter, M. and Benton, M. J. 1999. A sauropodomorph dinosaur from the Upper Triassic (Carnian) of southern Brazil. Comptes Rendus de l'Académie des Sciences, Paris, Sciences de la Terre et des Planètes, 329, 511-517. Sereno, P. C. 1999. The evolution of dinosaurs. Science, 284, 2137-2147. Sereno, P. C., Forster, C. A., Rogers, R. R. and Monetta, A. M. 1993. Primitive dinosaur skeleton from Argentina and the early evolution of Dinosauria. Nature, 361, 64-66. Sereno, P. C. and Novas, F. E. 1992. The complete skull and skeleton of an early dinosaur. Science, 258,1137-1140.

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    What are the main features of a cladogram?
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