Sauropodomorpha: Overview

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 tagore (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. (MAK 000117)