Sauropodomorpha
Sauropodomorpha is an extinct clade of saurischian dinosaurs that includes the long-necked, herbivorous sauropods and their ancestral relatives. Early, more basal sauropodomorphs were bipedal, and the earliest show evidence of omnivorous or carnivorous diets. Over time, sauropodomorph evolution resulted in a shift to herbivorous diets, larger body sizes, and quadrupedal locomotion. The sauropods themselves generally grew to very large sizes, had long necks and tails, and became the largest animals to ever walk the Earth. The sauropods were the dominant terrestrial herbivores throughout much of the Mesozoic Era, from their origins in the Late Triassic until their decline and extinction at the end of the Cretaceous.
History of study
Early study
Gigantic bones of sauropods have been known for thousands of years and become part of legends and cultures but the beginning of their scientific study was in the 1830s. Most of the early taxonomy was based on incomplete and disarticulated material as relatively complete skulls or skeletons were not discovered until closer to the end of the 19th century. The French anatomist Georges Cuvier studied large fossils from the Jurassic of Stonesfield Slate in England, believing them to have belonged to ancient relatives of whales up to his death in 1832. However, British paleontologist Richard Owen instead classified these large bones, which he gave the name Cetiosaurus in 1841, as marine whale-like crocodilians. This was the second genus of sauropod to be named, following his earlier description of a single isolated tooth as the taxon Cardiodon. At the same time, the first early sauropodomorphs were being described, with Thecodontosaurus named in 1836 for material from England and Wales, and Plateosaurus named in 1837 for material from Germany. Owen would even name the early sauropodomorph Massospondylus in 1854 for partial specimens from southern Africa, though all these early taxa were considered carnivorous theropods rather than relatives of sauropods. Owen's interpretation of Cetiosaurus as a marine crocodilian, which he specifically excluded from Dinosauria when he named it in 1842, began to be disputed with additional discoveries of limb bones of both Cetiosaurus and the new sauropod Pelorosaurus, with the classification of "cetiosaurs" as dinosaurs firmly established by 1874 by which time many the other genera including Aepisaurus and Astrodon had been named.The limited samples of material of sauropods and sauropodomorphs were followed by abundant discoveries in the late 19th and early 20th centuries from North America and east Africa. In the United States, American paleontologists Othniel Charles Marsh and Edward Drinker Cope began substantial excavations and competition that would be termed the Bone Wars. From this, Marsh and Cope would name and describe exemplars of the first complete sauropod skulls and skeletons, as well as the mostly complete skull and skeleton of the sauropodomorph Anchisaurus. Many of the most iconic sauropods-Apatosaurus, Brontosaurus, Camarasaurus, and Diplodocus-were all named during this time. In 1859 Owen had named the group Opisthocoelia to unite the "cetiosaurs" Cetiosaurus and Streptospondylus as crocodilians, followed by the naming of Ceteosauria by Harry Govier Seeley in 1874 for "cetiosaurs" as dinosaurs. Marsh ignored both of these older names to create the group Sauropoda in 1878, which became the accepted name following his work on their classification throughout the end of the 19th century and is now the term used. The early forms of sauropodomorphs were united within Prosauropoda by German paleontologist Friedrich von Huene in 1920 as a primitive stock to give rise to both theropods and sauropods, with Sauropodomorpha being named in 1932 by Huene to unite prosauropods and sauropods as the sister to Carnosauria.
Anatomy
Body size
The earliest and most primitive sauropodomorphs—animals like Buriolestes and Pampadromaeus—were small bipedal animals that were in the range of in length and weighed around. These earliest forms were either fully carnivorous or omnivorous, but herbivory quickly became the dominant feeding strategy of sauropodomorphs. Over the course of the Triassic Period, they increased in size, leading to the evolution of animals like Plateosaurus and Gresslyosaurus, which could reach long and weighed around 2-2.5 tons. During this period, all sauropodomorphs were obligate bipeds, which was the ancestral condition for dinosaurs. The largest bipedal sauropodomorph known from substantial remains was Lishulong. Only the head and neck of Lishulong are preserved, but this was enough to estimate a total length of about, which is about twice the size of the related Yunnanosaurus, which is known from more complete remains and weighed about 3 tons. However, there is a large but relatively incomplete sauropodomorph discovered in South Africa that has not yet been fully described. Andrew Yates and Matthew Wedel have suggested that the morphology of its arm bones meant it was probably an obligate biped. Scaling based on the sympatric genus Aardonyx and femur allometry, this bipedal sauropodomorph would have weighed between 10-15 tons, making it comparable in size to Diplodocus and possibly one of the largest bipedal animals ever.The evolution of obligatory quadrupedality enabled the true sauropods and their closest relatives to achieve very large sizes. The oldest confidently quadrupedal sauropodomorph, Melanorosaurus is not known from very complete remains, and Paul Barrett and Jonah Choiniere declined to suggest a mass estimate in their osteology of Melanorosaurus published in 2024. However, Gregory S. Paul estimated Melanorosaurus to have been about long and weighed around a ton, which is comparable to many bipedal sauropodomorphs. By the end of the Triassic, the 7 ton Lessemsaurus had evolved, marking the origin of the oldest true sauropods. The early Jurassic saw the evolution of the even larger Ledumahadi, which weighed around 12 tons.
The final anatomical bottleneck on the size of the true sauropods was the anatomy of their limbs. Columnar limbs evolved at some point in the early Jurassic. The oldest sauropod known to have had columnar limbs was Vulcanodon, which lived in what is now Zimbabwe around 199-188 million years ago. Earlier sauropods may have had columnar limbs, but their remains are too incomplete to determine if this is the case. Vulcanodon has been estimated to have weighed up to 10 tons, making it one of the largest terrestrial animals of its time. Most sauropods during this period—such as Barapasaurus, Rhoetosaurus, and Patagosaurus—weighed between 7-10 tons. True gigantism emerged at the beginning of the Late Jurassic; genera like Turiasaurus and Mamenchisaurus were up to long and may have weighed up to 30 tons. During the Late Jurassic and throughout the Cretaceous Period, true gigantism evolved independently several times in distantly related sauropod groups. Giants like Brachiosaurus, Dreadnoughtus, and Ruyangosaurus are believed to have exceeded in length and weighed in excess of 50 tons, making them the largest land animals of all time. Higher mass estimates have been made for very poorly-known taxa such as Maraapunisaurus and Bruhathkayosaurus, but these remain controversial.
Sauropods reached a variety of different body proportions, so the "largest" individual species will vary based on the measurement concerned. The longest known sauropod was probably Supersaurus, which may have exceeded in length. The tallest sauropodomorph was probably Sauroposeidon, which had a relatively erect posture and may have been able to reach a height of between. The longest neck of any sauropod known from complete remains is that of Xinjiangtitan, which had a neck meters long. Very large isolated cervical vertebrae from taxa like Hudiesaurus and Mamenchisaurus have been found, but the incompleteness of these remains makes it difficult to assess the total length of their necks. The most massive sauropodomorph known from relatively complete remains is generally accepted to be Argentinosaurus, which may have exceeded 70 tons in weight.
The smallest sauropodomorphs were the oldest and earliest-diverging taxa. Genera like Pampadromaeus and Eoraptor may have been long or shorter and weighed as little as. Sauropods were generally much larger, but several sauropods are believed to have been examples of insular dwarfism. Magyarosaurus, Europasaurus, and Petrustitan are the smallest sauropods known from adult remains; they were between long and weighed less than a ton. Other small sauropods existed throughout the Mesozoic including Haplocanthosaurus, Bonatitan, and Ohmdenosaurus, which were each between 1-2 tons.
Skull and tooth morphology
The skulls of sauropodomorphs are generally small in relation to body size and generally had large bony nares. However they vary considerably in most other aspects of their morphology. Prosauropods had very narrow skulls, and the skull morphology of prosauropods was generally conservative. Some prosauropods like Ngwevu and Yizhousaurus evolved relatively wide and robust skulls, but these were the exception. It was not until the diversification of the true sauropods that a wider variety of skull dimensions evolved. Wide and robust skulls evolved on multiple occasions within Sauropoda. The rebbachisaurid Nigersaurus had a relatively wide skull, and similarly wide and robust skulls evolved in Camarasauridae, Brachiosauridae, Euhelopodidae, and other somphospondylans. More basal sauropods, like Shunosaurus, Mamenchisaurus, and members of Diplodocidae, retained relatively narrow and lightly built skulls. However, the study of these trends is complicated by the relative rarity of sauropod skulls in the fossil record.Study of sauropodomorph tooth morphometrics has been used to divide sauropodomorphs into several evolutionary grades. The first of these, basal sauropodomorphs, show a relatively wide disparity in tooth morphology. This is used to infer a wider variety of dietary preferences. Some of these taxa are inferred to have been omnivorous or possibly even carnivorous, and the transition to herbivory took place within this grade. The next grade, the "core prosauropods", includes the large bipedal herbivorous taxa that became abundant during the Late Triassic. The core prosauropods, with only a few exceptions, have very similar tooth morphology; their teeth are generally non-recurved and are closely spaced to form a relatively continuous cutting edge along the whole length of the jaw. These traits, alongside the generally high mechanical advantage of their jaws, are generally correlated with herbivory, which has been used to infer a broadly similar dietary ecology for the core prosauropods. They were likely very generalized herbivores which may have been facultatively omnivorous. The exception to this general trend is Riojasaurus, which had much more specialized teeth. This suggests that Riojasaurus was a relatively specialized herbivore in comparison to some of its close relatives.
The evolution of basal sauropodiformes saw a diversification in tooth and jaw functionality. Taxa like Jingshanosaurus possessed relatively uniform and recurved teeth, which may reflect a renewed importance of meat in their diet. Conversely, Yunnanosaurus evolved very narrow teeth which did not occlude with each other; this is believed to be an adaptation for a very specialized herbivorous diet. More derived near-sauropods like Anchisaurus and Mussaurus retained the generalist anatomy of the core prosauropods, and likely had similarly generalist diets. The evolution of the true sauropods saw the emergence of the "broad-crowned" sauropod teeth, which are associated with more robust skulls and higher bite forces. This grade includes the bulk of early sauropod diversity, and until the evolution of diplodocoids and titanosaurs, would remain relatively conservative in morphology.
The ability for sauropodomorphs to increase in size so rapidly is attributed to the evolution of "bulk-browsing". This feeding method is typified by the presence of three skull characteristics. Firstly, the tooth-bearing bones of the skull developed plates along their margins to brace these bones against the stresses that occur while feeding. Secondly, the skull itself broadened to allow for greater volumes of food to be eaten at any given time. Thirdly, the soft tissues around the mouth reduced to allow for a wider gape. The last of these three can only be observed indirectly, since the soft cheek tissues are not preserved in the fossils. The lack of cheeks is inferred by the lack of neurovascular foramina along the areas of the skull from which the cheeks would grow. These three traits would become more elaborate and specialized with the evolution of true sauropods, which developed wider skulls and large gaping capabilities.
Over the course of their evolution, sauropodomorph skulls tended to develop more robust muscle attachment sites, which have been used to infer a trend towards higher bite forces in both absolute and relative terms. This trend coincided with the evolution of more precise dental occlusion, which likely made it easier to take bites of large masses of vegetation. This increase in the robustness of the jaw musculature co-evolved with a similar increase in the size and robustness of the tooth-bearing bones of the skull and a reduction in sizes of the and the. The evolution of the true sauropods also saw the migration of the s towards the anterior portion of the jaw, which would have allowed for the development of larger jaw adduction muscles, whichalso increased bite forces. Some researchers have suggested that these evolutionary trends formed a feedback-loop, with each adaptation reinforcing and enhancing the feeding utility of the others.
The absolute increase in size of the true sauropods in comparison to prosauropods naturally led to the evolution of higher bite forces. However, these adaptations also led to a significant relative advantage in the functionality of their feeding apparatus. The skulls of Plateosaurus and Camarasaurus, while very similar in size, differed considerably in their ability to withstand the forces that feeding would have exerted on the skull. A finite element analysis conducted in 2016 by David Button and colleagues found that, especially in the posterior portion of the jaw, the skull of Camarasaurus could withstand forces an order of magnitude higher than that of Plateosaurus while feeding. This was likely due to the robustness of the skull bones themselves, but was also impacted by the general dimensions of the skull. Plateosaurus and prosauropods generally had longer and narrower skulls than derived sauropods. Their analysis also found that the forces able to be exerted by Plateosaurus—and presumably other prosauropods—varied considerably along the length of their jaw, which suggests that different portions of the jaw served different purposes during feeding. This is in contrast to the bulk-feeding methods of the true sauropods, which prioritized volume and efficiency in feeding, leading to the teeth gradually shifting towards being concentrated in the front of the mouth, with few or no teeth towards the posterior portions of the jaw. This line of evidence was also used to suggest that prosauropods like Plateosaurus may have retained some of the carnivorous dietary habits of their more primitive ancestors.
Some uncertainties remain regarding the evolution of soft tissue anatomy in sauropodomorphs. Some prosauropods skulls have been suggested to preserve osteological correlates associated with cheeks, but true sauropods are generally believed to have lacked cheeks. When this transition occurred remains an unanswered question. Uniquely, some of the features on the maxillae of Riojasaurus suggest that it may have had a rhamphotheca.