Sauropodomorpha
From Palaeos
| DINOSAURIA (The Dinosaurs) | |
| Taxonomy | Phylogeny (some taxa are of uncertain relationship and hence mentioned twice) |
o Dinosauromorpha `--o DINOSAURIA |--Ornithischia `--o Saurischia |?-Eoraptor |?-Guaibasaurus |?-Herrerasauria `--+--Sauropodomorpha `--o Theropoda |?-Eoraptor `--+?-Herrerasauria `--+?-Guaibasaurus `--Neotheropoda / Avepoda (= Theropoda?) |
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Contents |
Introduction
The Sauropodomorpha constitute one of the two main groups of herbivorous dinosaurs, the other being the Ornithischia. There are two main subgroups, the small to large Triassic Prosauropoda and their giant cousins or descendents the Sauropoda. It is possible to trace a progressive morphic sequence from the small lightly built early thecodontosaurian prosauropods through the large plateosaurs, melanorosaurs and vulcanodonts to the giant Sauropoda of the Jurassic and Cretaceous.
But even the most primitive ancestral prosauropods had the same long neck, tiny head, and weak peg-like teeth which characterized the greatest of the later giants. The sauropodomorphs in fact were "pin-headed" herbivores. Their bodies grew extremely large, but the heads remained tiny. And like modern ostriches and emus, and extinct moas and elephant birds (Aepyornis), these ancient long-necked pinheads swallowed stones (gastroliths) which remained in the stomach to aid in digestion. This bird-like stomach processing of food differs greatly from the ornithischian and mammalian oral processing which relied on grinding teeth.
The first known prosauropod dinosaurs were small, lightly built bipedal forms of the family Thecodontosauridae. Slightly later, larger anchisaurids are found which were probably capable of walking on all fours as well as on their back legs. Early in their history, some of these small and primitive prosauropods developed into increasingly larger forms. These lines continued to evolve parallel to each other after the initial radiation; each independently evolving a gravimorphic form (like the reverse of a Weight Watchers contest), just as the various theropod lines independently took up the gravimorphic tendency. This is indicated by the steady increase in size and bulk of these creatures. The small ancestral types, like thecodontosaurs and anchisaurs, were only 1.5 to 2.5 meters in length, and a svelte 25 kg in weight. From these there evolved the larger types: the lightly-built massospondylids (4 to 5 metres long), the large yunnanosaurs (up to 7.5 metres), and the diverse plateosaurs, the most common of the early prosauropods.
The big plateosaurs, in particular, were much larger and heavier than their close anchisaur cousins, with lengths of 4 to 10 (or even 12) metres, and weights of up to 1500 kg. Their increasing size drove them closer to the earth, making them more comfortable with quadrupedal than with bipedal locomotion, although they may still have been able to walk on their hind legs if the situation required.
The increase in size continues with another line, the melanorosaurs. Melanorosaurs, which may have evolved from something like the early plateosaur Sellosaurus (length 3-6 metres), were large herbivorous animals, 7 to 12 metres long, and one or two tonnes in weight, which may have been closely related to the huge sauropod dinosaurs of the Jurassic and Cretaceous, and were strictly limited to a quadrupedal pose.
A related line, the blikanosaurs, represent an independent and parallel evolution of stocky types that are so far known only from a single medium- sized (length 5 metres) Late Triassic form.
The various prosauropod lines survived well into the Jurassic, and so were contemporary with the earliest giant sauropods. They were probably ousted by the increasing diversity of more efficient herbivorous ornithopod and sauropod dinosaurs in the early Jurassic. They represented, for the most part, early and short-lived evolutionary branches; parallel experiments in gravimorphism.
With the early Jurassic appeared the first true sauropods (the Vulcanodontidae or "volcano tooth", rather inappropriately so called because what were at first thought to be teeth of this type were found between layers of volcanic ash). Unfortunately, the fossil record of Early Jurassic sauropods is still very incomplete, although by the Pliensbachian or Toarcian age at least one large and specialized form, Barapasaurus tagorei (length 15 to 18 metres, weight 15 tonnes), is known. Some 10-20 million years later, by the Middle Jurassic period (Bathonian Age) we find several different families (Brachiosauridae, Shunosauridae, and Cetiosauridae) existing side by side. These early sauropods were 8 to 21 metres long, with weights from 1 to 30 tonnes, depending on the species. The giant, Apatosaurus-sized, 21 metre form was possibly the species Cetiosaurus oxoniensis. Cetiosaurus or "whale saurian" is so called because the bones unearthed in England during the Nineteenth Century were originally thought to be from a prehistoric whale. The unknown ancestors of these different types must have diverged from Vulcanodon-like stock during the middle of the Early Jurassic.
Basically then, early sauropod evolution meant a rapid increase of body size. Beginning with the two-metre long thecodontosaurs and anchisaurs of the Triassic Carnian age, within a few millions of years (Norian and Rhaetian ages) the plateosaurs and melanorosaurs had reached lengths of 8 to 12 metres, and weights of one to two tonnes. By the Early and Middle Jurassic (Pliensbachian to Bathonian ages), the Sauropoda proper appeared and diversified, with several parallel lines of evolution, some species being 15 to 20 metres in length and 12 to 30 tonnes in weight.
By the middle Jurassic at latest, the sauropods were world-wide in distribution, their fossil remains being known from Europe (England), Africa (Morocco and Madagascar), South America (Argentina), China, and Australia (Queensland). As in the Triassic, all the continental land masses were joined together and, although there was now the beginning of a separation into north (Laurasia) and south (Gondwanaland), there were no serious geographical barriers to migration. Hence animals could become global in distribution.
At least as early as the middle Jurassic, the Sauropods had branched into two main clades, which can be called the Macronaria and Diplodocomorpha. Both had long necks for reaching far and/or high up into vegetation, but they differed in body shape, and hence feeding behavior. The macronarians included, as their most advanced representatives, the Brachiosaurs ("arm saurians"), which had long fore-arms (hence the name) a larger, more globular head, stronger teeth, shorter tail, and a sloping giraffe-like back, enabling them to walk and browse from trees. They were huge beasts, yet still quite slightly built relative to their size. They flourished from the Middle Jurassic to the Mid-Cretaceous.
The diplodocomorphs had longer hind limbs, a long tail, more slender bodies, and a smaller, longer head with fewer teeth. Using their tail as a prop, some researchers believe that they could rear up on their large hind legs and feed from the tops of trees, reaching leaves that were beyond the reach of the Brachiosaurs. It was as if they were, in an elephantine way, trying to regain the bipedal posture of their distant ancestors. Meanwhile, the much smaller ornithischian dinosaurs - the ornithopods and stegosaurs - browsed on the undergrowth. Some diplodocomorphs, especially types like Diplodocus, were very lightly built. Others, like the closely related Apatosaurus (or "Brontosaurus"), were much more heftier.
There is still uncertainty regarding the relationships between the different groups of sauropods. But all of the advanced (Late Jurassic to Cretaceous) sauropods had specially hollowed-out vertebrae, which not only lightened the animal but also suggest the presence of avian-style air-sacs (identical hollows being found in the vertebrae of birds) and an advanced metabolism. In contrast, the earlier (Middle Jurassic) and more primitive sauropods had solid vertebrae.
The Late Jurassic was the golden age of sauropods; the age of giants. The great Apatosaurus, the almost unbelievably long-necked Mamenchisaurus, and Brachiosaurus, and the imaginatively named Supersaurus, Ultrasaurus, and Seismosaurus, attained lengths of 20 to 40 metres and weights of 30 to 50 tonnes or more.
While most sauropods disappeared at the end of the Jurassic (possibly the victims of one of the many mass-extinctions that characterize the history of life), some, such as the brachiosaurs, survived and flourished into the Cretaceous period. But by the mid-Cretaceous they, too, declined, perhaps through changing vegetation and (during the late Cretaceous) climatic patterns, or competition from the fast evolving ornithischian dinosaurs.
But the sauropods were far from finished. During the Late Cretaceous, a new family of sauropods, the Titanosauridae ("titanic saurians"), attained world-wide distribution, being known from every continent except most of Asiamerica (where ornithischians were the dominant herbivores) and Australia (where late surviving primitive types, the Austrosaurs, held sway). For the most part, these newcomers had nothing of the of their predecessors, and despite their name they were often small (by sauropod standards only!), around 12 meters long, and rather uniform in appearance. It was the spectacular array of ornithischians that made the Cretaceous such an interesting time -- strangely named beasts like "duckbills" and "boneheads", and various types of armoured and horned dinosaurs.
But there were also a few giants among them as well. Several species of titanosaurids were the size of the Jurassic Apatosaurus ("Brontosaurus"), and the great Antarctosaurus giganteus, known from the latest Cretaceous of South America, is estimated to have been as large as or even larger than the biggest Brachiosaurus - 30 metres long, 50 tonnes or more in weight. All these Late Cretaceous types survived right until the very end of the Mesozoic, when they were exterminated by the same catastrophe that wiped out the rest of the dinosaurs.
| SAUROPODOMORPHA | |
| Taxonomy | Phylogeny |
Class: Reptilia/Sauropsida
|
o Saurischia `--o SAUROPODOMORPHA |--Saturnalia tupiniquim `--+--Thecodontosaurus `--+--Efraasia minor |--o Prosauropoda | `--+--+--Plateosauridae | | `--+--Massospondylidae | | `--Yunnanosaurus | `--Riojasaurus `--o--Anchisaurus `--+--Melanorosauridae `--o Sauropoda (sensu stricto) `==Vulcanodontidae `==Cetiosauridae `--o--Diplodocomorpha `--Macronaria |
Information
Sauropodomorpha: Saturnalia
Range: Late Triassic to Late Cretaceous.
Phylogeny: Saurischia: Theropoda + Sauropodomorpha : Thecodontosaurus + (Efraasia + Prosauropoda + (Anchisaurus + Sauropoda))
note: phylogeny avccording to Adam Yates (Anchisaurus as basal sauropod; other phylogenies differ.
Characters
- $ Small skull: less than 1/2 of femur length.
- Peg-like teeth with ($) spatulate or lanceolate crowns;
- $ Anterior end of premaxilla and dentary deflected down;
- $ External nares enlarged;
- $ 10 or more cervical vertebrae;
- Mid and posterior cervical vertebrae longer than dorsals;
- Trunk long relative to hind limb;
- 3 or more sacral vertebrae (with addition of caudals);
- Humerus with deltopectoral crest extending 50% or more of bone length;
- Ungual of manus 1 much longer than other manual phalanges;
- Acetabulum open (partially closed in Saturnalia);
- $ Anterior process of ileum short and pointed;
- $ Proximal portion of pubes deep; tibia > femur;
- Ascending process of astragalus fits into slot formed by descending process of tibia;
- $ Ascending process of astragalus broad;
- Distal tarsals absent;
- Short feet with large claw on pes 1 (at least as long as other phalanges).
- Gastroliths frequently present;
- All or almost all herbivorous.
NB: Not a strongly supported clade, as most characters may be secondary to high browsing; may be synonymous with Prosauropoda, or conceivably polyphyletic within Saurischia.
Discussion
At the time of this writing (7/00), the unofficial Chief Panjandrum of the Basal Sauropodomorpha is Professor Michael Benton of the University of Bristol. Prof. Benton's particular domain in phylostratigraphic space-time extends out into the sere and barren wilderness of mid-Triassic Dinosauriformes, through the high pass between the Theropoda and Sauropoda, and into the slim, but fertile valley of Upper Triassic and Lower Jurassic Prosauropoda. Recently, forces from Chicago's Field Museum attempted an incursion with prosauropods from the earliest Late Triassic of Madagascar. Flynn et al. (1999). But Flynn's specimens were not securely dated, and Benton immediately countered with Saturnalia, a basal sauropodomorph from the Upper Triassic of Brazil (Langer et al. (1999)) and a redescription of Thecodontosaurus from Triassic Bristol itself. Benton et al. (2000). The latter move was especially dramatic, as the holotype of Thecodontosaurus was destroyed by pin-point Nazi bombing in World War II (undoubtedly the loss of this primitive prosauropod was intended as a stunning blow to British combat paleontology). At the last, Benton was forced to call up his ultimate reserves: casts, paratypes, privately held bits and pieces, and even previously unidentified shards salvaged from the smoking ruins of the Bristol City Museum in 1944.
So, just what does this territory consist of? That's a harder question. In researching this essay, I had set myself the task of trying to determine what Prof. Benton was up to: was there a master hypothesis in back of this recent activity? I am happy to report total failure on that front. I can only state the obvious. Benton and his students have repeatedly made the point that the ancestral sauropodomorph was a remarkably small, gracile creature with a small head and short fore-limbs. Recently, they have suggested that the root of Dinosauria may lie closer to the pterosaurs than previously thought. But the bigger questions remain unresolved.
What is the relationship between the sauropods and prosauropods? Many sauropod workers feel that sauropods are, as originally believed, descended from the prosauropods. That is, traditional Prosauropoda is paraphyletic or, more correctly, that Prosauropoda and Sauropodomorpha are the same. This is consistent with the stratigraphic absence of any sauropod remains until the early Jurassic. However, Benton et al. conclude that this is incorrect and that the prosauropods and sauropods diverged early from a common ancestor. However, they point out that the support for this hypothesis is weak. Furthermore, the analysis in Benton et al. (2000) was performed without the inclusion of the Saturnalia material, which has been named but not yet described in any detail.
Saturnalia might have some strange effects on the results. On the one hand, almost all of the informative characters of Saturnalia are prosauropod. So, for example, the teeth are lanceolate, with coarse serrations oriented at 45 degrees to the vertical, there are three sacral vertebrae, the anterior iliac blade is short and pointed, and the tibia is much shorter than the femur. There is almost nothing distinctive to identify it as related to the Sauropoda. This is not terribly surprising, since by definition, all sauropodomorphs must be referable to one or the other daughter clade.
On the other hand, Saturnalia has some characters so primitive as to raise a few eyebrows. Most notably, the acetabulum (where the hind limb attaches to the pelvis) is not completely open. The open acetabulum is a well-established, in fact possibly the most well-established characteristic of Dinosauria. So this is a little like finding a crocodile with fur. The tibia is strongly S-curved and may be distinctly bent off the mid-line (its hard to tell from the photograph). The ankle joint is said to be "similar to that of Herrerasaurus ... with a broad ascending process of the astragalus." Langer et al. (1999) at 513-14.
The bottom line is that Saturnalia seems to have just about everything it needs to be a prosauropod, but lacks various features apparently shared by all other dinosaurs, or at least by all sauropodomorphs as traditionally understood. If this is all true, one is forced to the tentative conclusion that Sauropodomorpha is polyphyletic. Sauropods and prosauropods may have developed independently from each other. That is, Sauropodomorpha (considered as, say, the last common ancestor of Plateosaurus and Alamosaurus and all of its descendants) may be synonymous with Saurischia or Dinosauria.
Links
- DD: Sauropodomorpha
- Lecture 11 - Triassic
- Sauropodomorpha
- Saurischian Dinosaurs - Enchanted Learning Software
- Sauropodomorpha -- The Dinosauricon
- Basal Sauropodomorpha
- Sauropodomorpha (characters, lecture notes)
- FPDM - Sauropodomorpha (Fukui Museum -- still a good site, but not quite what it was)
- GEOL 104 Lecture 21- Sauropodomorpha- Size matters (Tom Holtz' very clear and useful lecture notes)
- Ficha Sauropodomorpha
- ｿ・｢ﾇ・ﾆｸ・Sauropodomorpha
- Chapter 12 - Clade Sauropodomorpha
- Lec 10 - SAUROPODOMORPHA
References
- Benton, MJ, L Juul, GW Storrs, & PM Galton (2000), Anatomy and systematics of the prosauropod dinosaur Thecodontosaurus antiquus from the Upper Triassic of Southwest England. J. Vert. Paleontol. 20: 77-108.
- Flynn, JJ, JM Parrish, B Rakotosamimanana, WF Simpson, RL Whatley & AR Wyss (1999), A Triassic fauna from Madagascar, including early dinosaurs. Science 286: 763-765.
- Langer, MC, F Abdala, M Richter, & MJ Benton (1999), A sauropodomorph dinosaur from the Upper Triassic (Carnian) of southern Brazil. C. R. Acad. Sci. (Paris): Sci. de la Terre 329: 511-517.
Credits
- Palaeos.com:
- This page MAK061025.
