Sauropodomorpha

Sauropodomorpha

The Sauropodomorpha were a group of long-necked, herbivorous dinosaurs that eventually dropped down on all fours and became the largest animals that ever walked the earth.

Description

Sauropodomorphs were adapted to browsing higher than any other contemporary herbivore, giving them access to high tree foliage. This feeding strategy is supported by many of their defining characteristics, such as: a light, tiny skull on the end of a long neck (with ten or more elongated cervical vertebrae) and a counterbalancing long tail (with one to three extra sacral vertebrae).

Their teeth were weak, and shaped like leaves or spoons (lanceolate or spatulate). Instead of grinding teeth, they had stomach stones (gastroliths), similar to the gizzard stones of modern birds and crocodiles, to help digest tough plant fibers. The front of the upper mouth bends down in what may be a beak.

The earliest known sauropodomorph, Saturnalia, was small and slender (1.5 metres, or 5 feet long), but by the end of the Triassic they were the largest dinosaurs of their time, and in the Jurassic/Cretaceous they kept on growing. Ultimately the largest sauropods like the Supersaurus, Diplodocus hallorum, and Argentinosaurus reached 30–40 metres (100–130 ft) in length, and 60,000–100,000 kilograms (65–110 US short tons) or more in mass.

Initially bipedal, as their size increased they evolved to become graviportal quadrupeds (like elephants). The early sauropodomorphs were most likely omnivores as their shared common ancestor with the other saurischian lineage (the theropods) was a carnivore. Therefore their evolution to herbivory went hand in hand with their increasing size and neck length.

They also had large nostrils (nares), and retained a thumb (pollex) with a big claw which may have been used for defense — though their primary defensive adaptation was their extreme size.

Range

Among the very first dinosaurs to evolve in the late Triassic Period, about 230 million years ago (Mya), they became the dominant herbivores by half way through the late Triassic (during the Norian stage). Their perceived decline in the early Cretaceous is most likely a bias in fossil sampling, as most fossils are known from Europe and North America. Sauropods were still the dominant herbivores in the Gondwana landmasses, however. The spread of flowering plants (angiosperms) and "advanced" ornithischians, another major group of herbivorous dinosaurs (noted for their highly developed chewing mechanisms) are most likely not a major factor in sauropod decline in the northern continents. Like all non-avian dinosaurs, the sauropodomorphs became extinct 65 Mya, during the Cretaceous-Tertiary extinction event.

The most basal sauropodomorph known, Saturnalia, was discovered in 1999, and is dated to the Carnian stage of the late Triassic. However, fragmentary remains from Madagascar may represent an even earlier sauropodomorph from the middle Triassic.

Classification

Sauropodomorpha is one of the two major clades within the order Saurischia. The sauropodomorphs' sister group, the Theropoda, includes bipedal carnivores like Velociraptor and Tyrannosaurus. However, Sauropodomorpha also share a number of characteristics with the Ornithischia, so a small minority of palaeontologists like Bakker place both sets of herbivores within Phytodinosauria (or Ornithischiformes).

In Linnaean taxonomy, Sauropodomorpha (which means "lizard feet forms") is either a suborder or is left unranked. It was originally established by Friedrich von Huene in 1932, who broke it into two groups: the basal forms within Prosauropoda, and their descendants, the giant Sauropoda.

Recent phylogenetic analyses by Adam Yates (2004, 2006) firmly places Sauropoda within a paraphyletic Prosauropoda. Also, finds of late Triassic sauropods demonstrate that there is no gap between the "prosauropod" and sauropod lineages.

Evidence against sauropod ancestry within Prosauropoda comes from the fact that prosauropods had a smaller outer toe on their hind feet than the sauropods. Many maintain that it is easier for digits to be reduced or lost during evolution than the reverse, however there is no evidence for this. The lengthening, or gaining of extra digits is common in marine reptiles, and within the theropods digit lengthening occurred at least once. Therefore using this as evidence against ancestral prosauropods is questionable.

While the sauropodomorphs are still grouped into prosauropods and sauropods for convenience, most modern classification schemes break the prosauropods into a half-dozen groups that evolved separately from one or more common ancestors. While they have a number of shared characteristics, the evolutionary requirements for giraffe-like browsing high in the trees may have caused convergent evolution, where similar traits evolve separately because they faced the same evolutionary pressure, instead of (homologous) traits derived from a shared ancestor.

Since the modern preference is for groups that are composed of all descendants of the same common ancestor (clades), instead of groups that exclude certain descendants of that ancestor (paraphyletic taxa), Prosauropoda is unpopular except as an informal collection of primitive (basal) sauropodomorphs. However, some like Michael Benton, consider the prosauropods and sauropods to be a distinct lineage descended from a common saurischian ancestor. While this is a minority view, supported by weak evidence, there is considerable support for a small, monophyletic Prosauropoda clade containing only smaller percentage of its previous members (taxa).

Saturnalia has the teeth, backbone, pelvis, and legs of traditional prosauropods, while lacking all of the unique sauropod characteristics. This lends some support to the prosauropod paraphyly theory, as it is the most basal sauropodomorph. However, it also lacks some of characteristics traditionally associated with Sauropodomorpha. Although, again being the most basal species this is not too surprising. The suggestion that the lack of some derived sauropodomorph characters in Saturnalia can be taken as evidence that Sauropodomorpha eis polyphyletic (evolved separately from different saurischian ancestors) has not been demonstrated by any cladistic analysis of sauropodomorphs.

Taxonomy

Taxonomy of the Sauropodomorpha after Benton, 2004.

Phylogeny

The following cladogram simplified after those presented in The Dinosauria second edition.

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References

External links

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