Sauropodiform

Sauropodomorpha ( ; from Greek, meaning "lizard-footed forms") is an extinct clade of long-necked, herbivorous, saurischian dinosaurs that includes the sauropods and their ancestral relatives. Sauropods generally grew to very large sizes, had long necks and tails, were quadrupedal, and became the largest animals to ever walk the Earth. The primitive sauropodomorphs which preceded the sauropods, and are informally called prosauropods, were smaller and were often able to walk on two legs. The sauropodomorphs were the dominant terrestrial herbivores throughout much of the Mesozoic Era, from their origins in the Late Triassic (approximately 230 Ma) 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 (from the Ancient Greek words ''sauros'' and ''podus'' as "saurian/lizard feet"), 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.Lu, J., Li, T., Zhong, S., Azuma, Y., Fujita, M., Dong, Z., and Ji, Q. (2007). "New yunnanosaurid dinosaur (Dinosauria, Prosauropoda) from the Middle Jurassic Zhanghe Formation of Yuanmou, Yunnan Province of China." ''Memoir of the Fukui Prefectural Dinosaur Museum'', 6: 1-15. However, there is a large but relatively incomplete sauropodomorph (specimen BP/1/5339) 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. 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 (when viewed from above), 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 (including taxa like ''Eoraptor'', '' Pampadromaeus'', and ''Saturnalia''), 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 (''Plateosaurus'', ''Massospondylus'', ''Thecodontosaurus'', etc). 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 (called "near-sauropods" by some sources) 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 (the premaxillae, maxillae, and dentaries) 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 (including cheeks) 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.

Neck anatomy

Neck elongation was among the first anatomical trends to evolve that differentiated sauropodomorphs from their theropod relatives. Within at least the first 8 million years of sauropodomorph evolution, proportional neck length grew from around one-third of trunk length to almost the same as trunk length. The Late Triassic Santa Maria Formation of Brazil preserves this transition. The morphologically conservative ''Buriolestes'' is known from around 233 million years ago, and by 225 million years ago, the "core prosauropod" bodyplan had emerged with the evolution of the long-necked ''Macrocollum''. Intermediate forms, such as the moderately-elongated neck of '' Bagualosaurus'' indicate that this was a gradual process. This initial phase of neck elongation occurred via the elongation of the cervical vertebrae themselves, rather than an increase in the number of vertebrae (both ''Buriolestes'' and ''Macrocollum'' had 8 cervical vertebrae). This elongation coincided with a shrinking of the skull, probably to reduce the muscular strain on the neck that a large head would create.

The elongation of sauropodomorph necks is suggested to have provided a comparative advantage versus other large herbivores such as aetosaurs and dicynodonts in allowing them to feed on a wider variety of vegetation. This process of neck elongation closely mirrored the evolution of sauropodomorph teeth, which saw a trend towards adaptations for herbivory during this same interval.

The evolution of long necks required a suite of adaptations to sauropodomorph vertebrae. A long neck is necessarily more massive than a short neck, and therefore the evolution of long necks coincided with the development of broad cervical vertebrae to accommodate the expansion of neck muscles. Primitive sauropodomorphs with short necks had weak and small or no . These features of the vertebrae grew broad in sauropodomorphs with longer necks and likely served as attachment sites for large trapezius muscles. These expanded muscles were necessary for sauropodomorphs to hold their necks and heads up, and the presence of large trapezii is also supported by broad muscle attachment sites on the shoulders of these taxa. The evolution of true sauropods saw an increase in the development of broad and robust cervical vertebrae to facilitate the evolution of increasingly long and massive necks.

Skeletal pneumaticity

Sauropodomorphs possessed a system of air sacs throughout their body which were connected to the respiratory system. During development, these air sacs expanded via branching structures called "diverticula" (singular "diverticulum") which invaded and carved holes into parts of the skeleton. These invasive diverticula formed structures called pleurocoels and pneumatic fossae, which are preserved in the fossils and can be used by researchers to infer the presence of these air sacs. These adaptations are also seen in birds, non-avian theropods, and pterosaurs, but are completely absent in ornithischians. This has led to the suggestion that pneumatic skeletons are ancestral to bird-line archosaurs and were secondarily lost in ornithischians. However, an analysis of the skeletons of three dinosaurs including two sauropodomorphs (''Buriolestes'', ''Gnathovorax'', and '' Pampadromaeus'') conducted in 2022 found that none of the taxa studied possessed signs of an invasive air-sac system in their vertebrae. This led the authors of the study to the conclusion that this respiratory apparatus evolved independently in pterosaurs, theropods, and sauropodomorphs.

An important genus in the study of pneumaticity in early sauropodomorphs is ''Macrocollum'', from the Triassic of Brazil. It is one of the earliest sauropodomorphs to achieve sizes larger than , and the skeletal remains of the genus also preserve some of the oldest evidence of pneumatic vertebrae in this group. Earlier taxa are either fully apneumatic (lacking invasive air sacs) or with only very minor pneumaticity, but ''Macrocollum'' possessed signs of an invasive air sac system in both the lower cervical vertebrae and upper dorsal vertebrae. It is notable however that the evolution of vertebral pneumaticity in sauropodomorphs was not a linear process. Taxa that evolved after ''Macrocollum'' including ''Plateosaurus'' possessed invasive pneumaticity in the cervical vertebrae, but seemed to lack this adaptation in the dorsal vertebrae. The Early Jurassic genus ''Aardonyx'' had an extensive air sac system in the lower dorsal vertebrae and sacrum, but had entirely apneumatic cervical vertebrae. It appears that extensive pneumaticity along the entire vertebral column did not become established until the evolution of the true sauropods. The exact locations of pneumatic elements on the bones are highly variable even within taxa, which is a consequence of these elements developing in tandem with the circulatory system, rather than the development of the skeleton itself.

The researchers Matt Taylor and Mike Weddel have written extensively about variation in the invasive air sac systems of sauropodomorphs. Weddel conducted a general review of sauropodomorph skeletal pneumaticity in 2007 which surmised that the ribs and vertebrae of prosauropods were generally much less extensively pneumatized than those of the true sauropods. Some prosauropods, including ''Pantydraco'' (then called '' Thecodontosaurus caudus'') possessed excavations in their cervical vertebrae which were described as "pleurocoel-like", but may or may not have been true pleurocoels created by pneumatic diverticula. Other vertebral structures indicative of pneumaticity in true sauropods (laminae and fossae on the vertebrae) are present in various prosauropods such as ''Plateosaurus'' and ''Pantydraco'', but these are not interpreted as signs of true pneumaticity because the texture of the bone associated with these structures generally does not differ from fully apneumatic bone. Similar laminae and fossae are present in a variety of other archosaurs including pseudosuchians (which do not have any pneumatic elements in the skeleton), which may indicate that these structures are ancestral to archosaurs and evolved independently of an invasive air sac system.

Arms and claws

Like all dinosaurs, sauropodomorphs evolved from bipedal ancestors with five digits on each hand. They did not however possess claws on all of their digits. Early sauropodomorphs only had claws on the first three digits (sometimes called the "thumb"), and their early evolution was marked by an increase in the size of the first digit and its associated claw. As the transition to quadrupedality took place throughout the Triassic and into the Jurassic, the claws on the second and third digits reduced in size and disappeared. However, even into the Late Cretaceous, large sauropods retained a large claw on the first digit of the hand.

The first major radiation of sauropodomorphs (the "core prosauropods"), retained the relatively short forelimbs of their more primitive ancestors. ''Plateosaurus'' and its relatives had forelimbs that were less than half the length of their hind limbs. This meant they were obligate bipeds and used their arms exclusively for non-locomotory purposes. This is further supported by the limited wrist mobility of their hands, which would have prevented them from pronating their hands to plant them on the ground. Functions for their arms, including the large claws on their hands, include for use in feeding and defense from predators, which would have included primitive theropods and large rauisuchians.

A detailed study of the arm musculature of the core prosauropod ''Unaysaurus'' suggested that the adduction-abduction range of motion for the forelimbs was between about 74-130 degrees relative to the body. The humerus likely had a range of motion between 105-153 degrees relative to the coracoid, and the elbow likely had a range of motion between about 70-137 degrees relative to the humerus. Range of motion for the forelimbs changed considerably during the course of sauropodomorph evolution. A more derived prosauropod, ''Mussaurus'' (which was also bipedal) was likely able to pronate its arms to some degree, and the mobility of its elbow joint was likely much greater than in earlier prosauropods. This was a precursor to the evolution of the fuller pronation which was necessary for quadrupedality to evolve.

The size of the arms relative to the body in sauropodomorphs increased generally during their early evolution. Even fully bipedal taxa like ''Aardonyx'' had arms which were similar in length to their hind legs. This saw its culmination with the evolution of ''Melanorosaurus'' and the true sauropods, which were believed to have been an obligate quadrupeds and would have used their limbs more exclusively for locomotion. However, even in the largest Early Jurassic sauropodomorphs, such as '' Ledumahadi'', the forelimbs were not fully columnar, and they still retained a degree of mobility in their elbows and wrists that would have been impossible for the true sauropods.

Paleobiology and behavior

Sauropodomorphs reached the age of sexual maturity well before they were fully-grown adults. A study by Griebeler et al. (2013) concluded that the maximum growth rates of sauropodomorphs were comparable to those of precocial birds and the black rhinoceros but lower than the growth rates of average mammals.

Senses

Inferring the sensory capabilities of extinct animals is difficult due to the lack of soft tissue preservation, so any study of these fields must necessarily rely on skeletal proxies for these sensory systems. One of the methods available is the study of the braincases of taxa with well-preserved skull remains. The brains of archosaurs generally have large olfactory bulbs which comprise the front-most portion of the brain. Well-preserved braincases are rare in fossil vertebrates, but in cases where they do exist, researchers can create endocasts—3D models of the brain based on the internal structure of the braincase. Rodrigo Müller conducted one such study in 2022 on sauropodomorphs specifically. He sampled the braincases of over 30 dinosaur species including several primitive sauropodomorphs, core prosauropods, and derived true sauropods in order to measure the "olfactory ratios" (ORs) of these taxa (an established estimate for the volume of the brain dedicated to smell). The absolute and proportional size of this region of the brain varied considerably among the sampled taxa. Notably, ''Buriolestes'' had very large olfactory bulbs, which is a condition shared by ''Saturnalia'', '' Panphagia'', and '' Pampadromaeus''. ''Plateosaurus'' had very small olfactory bulbs, but ''Massospondylus'' (which lived more recently that ''Plateosaurus'') had a very high OR, similar to ''Macrocollum'' (which lived much earlier than both), which suggests that the evolution of olfactory capacity in sauropodomorphs was complex and did not exhibit a linear progression.

For the taxa which had published mass estimates (''Buriolestes'', ''Macrocollum'', ''Diplodocus'', ''Camarasaurus'', and ''Nigersaurus''), Müller compared these ORs to their total body mass, and these relationships were in turn compared to the OR-to-body-mass ratios for theropods and ornithischians. He found that ''Butiolestes'', ''Macrocollum'', and ''Camarasaurus'' had very high OR-to-body-mass ratios compared to most of the other dinosaurs sampled. This suggests that smell was an important sense for a wide variety of sauropodomorph taxa regardless of absolute body size. The earliest sauropodomorph taxa are believed to have been carnivores, and smell likely served a predatory function taxa like ''Buriolestes''. However, the continued prevalence of the olfactory bulbs in fully herbivorous taxa suggests that it remained important for other reasons. Suggested functions include distinguishing edible plants from inedible plants, detecting predators, or using smell in social interactions.

A study in 2011 by Lars Schmitz and Ryosuke Motani examined another skeletal feature believed to correlate with sensory function, the scleral rings in the eyes of extinct archosaurs. Schmitz and Motani examined 33 taxa including five sauropodomorphs from which the scleral morphology is known (''Diplodocus'', ''Lufengosaurus'', ''Nemegtosaurus'', ''Plateosaurus'', and ''Riojasaurus''). The thickness and diameter of the scleral ring is correlated strongly to both the absolute and relative sizes of the pupils and lenses in the eyes of modern animals, and it can therefore be used to approximate the ability for an extinct animal to see in the dark. Schmitz and Motani discovered that the scleral morphology of the sauropodomorphs studied indicated that they were likely capable of seeing in most light conditions. This suggests that they were likely active for long periods both during the day and night. They hypothesized that this was a result of their large size and herbivorous, because they found a similar in large herbivorous ornithischians. The absolute size of even early sauropodomorphs (exceeding one metric ton) necessitated that they would have likely had to feed for 12 or more hours every day, which likely meant they had to be habitually active in low-light conditions as well as during the day. One of the sauropodomorphs studied, ''Lufengosaurus'', may have even been primarily active during the night, but this finding was ambiguous.

The hearing abilities of sauropodomorphs have not been the subject of extensive study. Michael Hanson and colleagues published a study of reptile inner ear morphology in 2021 in which they examined the bony ear structure of numerous reptiles including the sauropodomorph ''Thecodontosaurus''. Archosaur cochlear shape elongated in a relatively linear fashion on the line towards birds, which was suggested by Hanson and colleagues to be a paedomorphic adaptation to hear the high-pitched vocalizations of juveniles of the same species. This line of evidence is also used to suggest that parental care evolved early in the evolution of archosaurs, and therefore would have been present in sauropodomorphs. The high-pitched calls of juvenile sauropodomorphs would have been distinct and differentiable to the parents' ears from other ambient noise. ''Thecodontosaurus'' fit into the general category that Hanson and colleagues called "Semicircular Canal Morphotype 2", with dimensions similar to other non-avian dinosaurs and palaeognaths, suggesting the general hearing capabilities of these groups were broadly similar.

Diet and digestion

Sauropodomorphs exhibited a wide variety of diets over the 160 million years during which they existed. Sauropodomorphs are believed to have been ancestrally carnivorous, with later genera evolving to be omnivorous. The largest prosauropods and the true sauropods are believed to have been the first group of dinosaurs to become fully herbivorous, with this dietary shift contributing to their general increase in body size. There were exceptions to this trend including the prosauropod '' Jingshanosaurus'', which had slender recurved teeth, pointing to a possibly carnivorous diet. Herbivory in most prosauropods is well-attested by a suite of tooth adaptations many sauropodomorphs possessed. These include spatulate (leaf-shaped) teeth with serrations similar to extant herbivorous lizards, the orientation of the teeth to form a continuous cutting edge well-suited for slicing vegetation, and a marked decrease in tooth size along the length of the jaw. These adaptations are seen in most of the core prosauropods (e.g. ''Massospondylus'', ''Plateosaurus'', and their relatives) and are broadly conserved in more advanced prosauropods as well as the true sauropods.

Throughout the course of their evolution, sauropodomorphs never evolved the ability to chew. Scientists have inferred this from the relatively simple jaw joints they possessed. Chewing requires a relatively complex jaw morphology to allow the jaws to flex along multiple planes of motion in order to achieve the grinding action that makes chewing possible, and no sauropodomorph skulls appear to preserve the ability to perform this range of motion. This inability to chew distinguishes sauropodomorphs from large herbivorous mammals and also from the large herbivorous ornithischians with which they coexisted. Some scientists have suggested that the long necks of sauropodomorphs imposed constraints on the evolution of their heads which prevented the robust jaws and musculature necessary to evolve the ability to chew. Over the course of their evolution, sauropodomorphs evolved other methods of assisting with the digestion of large amounts of plant matter.

One method that may have evolved to circumvent the inability to chew was the use of gastroliths (literally "stomach stones") in which animals deliberately swallow small stones to grind and process plant matter after it has been swallowed. Gastroliths are known to have been used by a variety of animal groups including plesiosaurs, birds, and other non-avian dinosaurs. Gastroliths can be difficult to distinguish from other rocks in the fossil record, but they are generally smooth and relatively uniform in size within a single animal. The presence of gastroliths in sauropodomorphs had evolved by the Early Jurassic because several taxa including ''Massospondylus'' and ''Ammosaurus'' (possibly a junior synonym of ''Anchisaurus'') have been found with stones preserved inside their body cavities that are believed to be gastroliths. However, this adaptation was not necessarily widespread among prosauropods. Members of Plateosauridae are well-represented in the fossil record—being known from hundreds of specimens—but there are few unambiguous examples of gastroliths preserved in these fossils. Some authors have suggested that the use of gastroliths did not become widespread until after the common ancestor of Massospondylidae and Sauropoda had already diverged from their common ancestor with Plateosauridae. Subsequent review of the function of gastroliths in a wide variety of extant and extinct taxa has called into question the interpretation of gastroliths in non-avian taxa as serving a digestive function. Oliver Wings, in a 2007 publication, suggested they may have served to assist in balance for bipedal sauropodomorphs or may have been swallowed accidentally during feeding, which explains their rarity and inconsitence in their association with sauropodomorph fossils.

Metabolism and thermoregulation

Metabolic strategies in extant animals are quite varied and cannot be neatly categorized. The general distinction between ectothermy ("cold-bloodedness") and endothermy ("warm-bloodedness") is based on the dramatic metabolic differences between extant mammals and birds (which have very high metabolic rates) and modern reptiles, which have very slow metabolic rates by comparison. Some researchers have suggested that this simple dichotomy does not account for the full range of possible variation, especially in the fossil record, because they both represent derived metabolic conditions. Archosaurs (and possibly amniotes more generally) may have had significantly higher metabolic rates than modern crocodilians and squamates—a metabolic condition which is sometimes called mesothermy. It is unclear when the avian metabolic condition evolved, but it is believed to have been present in most theropods based on histological data.

The presence of feathers is often used as a proxy measure of an endothermic metabolism. Pterosaurs, theropods, and ornithischians are all known to have possessed feathers or feather-like filaments, which has led some researchers to suggest that feathers may have been an ancestral trait for bird-line archosaurs. If this is true, it would imply that sauropodomorphs were ancestrally feathered, which itself may have implied an endothermic (or at least moderately high) metabolism. However, researchers who have studied the question concluded that it is not likely, given the current evidence, that feathers were an ancestral trait for bird-line archosaurs, and it is more probable that feathers or filaments evolved independently in pterosaurs, theropods, and ornithischians. This implies that sauropodomorphs were ancestrally scaly.

Other proxies for metabolism in sauropodomorphs inclued inferences based on their paleobiogeography. Palynology and other plant fossils can be used to infer climatic data for various paleoenvironments in which sauropodomorphs were present. Global sampling of fossil data indicates that sauropodomorphs generally were more common at lower latitudes, which may be indicative of lower metabolic rates and a higher reliance on environmental conditions to mantain a high internal temperature. However, this correlation may also be non-causal because plant matter is also generally more abundant in lower latitudes and sauropodomorphs (due to their generally large size) would have needed much more plant matter to sustain their size regardless of their metabolic proclivities. This apparent sauropodomorph preference for lower latitudes could also reflect a sampling bias in the fossil record, and should not necessarily be interpreted as indicative of anything by itself. Some researchers have also used regression analyses of metabolic rates in extant vertebrates based on body mass to estimate that most dinosaurs, including sauropodomorphs, were mesotherms.

Some researchers have suggested that it would be impossible for animals as large as sauropods to exhibit endothermy because they were too large and would overheat. This was subsequently called into question by Eva Maria Griebeler, who used data from extant animals and from dinosaurs with known ontogenetic series and published mass estimates to approximate the internal temperatures of dinosaurs at various points in their lives. While her findings were not meant to determine precise body temperature estimates, she did find that the body temperature of large dinosaurs did not scale directly with their size beyond a certain mass, meaning that endothermic sauropods would not necessarily overheat. This finding did not conclusively determine whether or not sauropodomorphs were endotherms, it did disprove the overheating hypothesis. Other authors have examined the skull vasculature of sauropods and found that the large present in most sauropod genera could have served as efficient sites for heat exchange. This meant that the brains of sauropods could be kept cooler than the rest of the body, and this temperature differential could have allowed for higher body temperatures to be non-lethal.

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''; as well as birds. However, sauropodomorphs also share a number of characteristics with the Ornithischia, so a small minority of palaeontologists, like Bakker, have historically placed both sets of herbivores within a group called "Phytodinosauria" or "Ornithischiformes". A summary of the possible classification schemes is shown below.

;Conventional relationships

;Ornithoscelida hypothesis

;Phytodinosauria hypothesis

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.

Phylogenetic analyses by Adam Yates (2004, 2006) and others firmly placed Sauropoda within a paraphyletic "Prosauropoda". Recent cladistic analyses suggest that the clade Prosauropoda, which was named by Huene in 1920 and was defined by Sereno, in 1998, as all animals more closely related to ''Plateosaurus engelhardti'' than to '' Saltasaurus loricatus'', is a junior synonym of Plateosauridae as both contain the same taxa.

Most modern classification schemes break the prosauropods into a half-dozen groups that evolved separately from one common lineage. 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.

The phylogenetic analysis of Otero et al., 2015 found Sauropodomorpha to be in a polytomy with '' Agnosphitys'' and Theropoda within Eusaurischia, with Herrerasauridae and ''Eoraptor'' external to it within Saurischia. A large phylogenetic analysis of early dinosaurs published by Matthew Baron, David Norman and Paul Barrett (2017) in the journal ''Nature'' redefined Sauropodomorpha and Saurischia and recovered Herrerasauridae as the sister group to Sauropodomorpha within Saurischia. This resulted from the proposed removal of Theropoda from Saurischia and the formation of Ornithoscelida, a clade containing Theropoda and Ornithischia.

Phylogeny

Within Sauropodomorpha, there is a large clade named Plateosauria. The name Plateosauria was first coined by Gustav Tornier in 1913. The name afterwards fell out of use until the 1980s. Plateosauria is a node-based taxon. In 1998, Paul Sereno defined Plateosauria as the last common ancestor of ''Plateosaurus engelhardti'' and ''Massospondylus carinatus'', and its descendants. Peter Galton and Paul Upchurch in 2004 used a different definition: the last common ancestor of ''Plateosaurus engelhardti'' and '' Jingshanosaurus xinwaensis'', and its descendants. In their cladistic analysis the Plateosauria belonged to the Prosauropoda, and included the Plateosauridae subgroup. In Galton's and Upchurch's study also '' Coloradisaurus'', ''Euskelosaurus'', '' Jingshanosaurus'', ''Massospondylus'', ''Mussaurus'', '' Sellosaurus'', and ''Yunnanosaurus'' proved to be plateosaurians.

However, recent cladistic analyses suggest that the Prosauropoda as traditionally defined is paraphyletic to sauropods. Prosauropoda, as currently defined, is a synonym of Plateosauridae as both contain the same taxa by definition. The phylogenetic analysis of 2021 recovered ''Issi'' and ''Plateosaurus'' as the basal-most plateosaurs. Below is a cladogram of basal sauropodomorpha after Apaldetti and colleagues, 2021.

Subgroups

Below are the various subgroups of sauropodomorphs alongside their accompanying definitions.

Evolutionary history and paleobiogeography

Among the first dinosaurs to evolve in the Late Triassic period, about 230 million years ago (Mya), they became the dominant herbivores by halfway 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, but sauropods were still the dominant herbivores in the Gondwanan landmasses. 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 (birds), the sauropodomorphs became extinct 66 Mya, during the Cretaceous–Paleogene extinction event.

The earliest and most basal sauropodomorphs known are '' Chromogisaurus novasi'' and '' Panphagia protos'', both from the Ischigualasto Formation, dated to 231.4 million years ago (late Carnian age of the Late Triassic according to the ICS (''Warning:'' abstract is 12 kb PDF)). Some studies have found '' Eoraptor lunensis'' (also from the Ischigualasto Formation), traditionally considered a theropod, to be an early member of the sauropodomorph lineage, which would make it the most basal sauropodomorph known.

Sauropodomorph remains have been found on every continent, including Antarctica. They evolved during the existence of the supercontinent Pangaea, and are believed to have been widespread on this landmass prior to its separation into multiple continents. However, multiple intercontinental dispersal events are believed to have occurred after Pangaea began to separate, resulting in the widespread proliferation of true sauropods after they evolved. Sauropods themselves are also known from every continent, including Antarctica.

Timeline of groups

ImageSize = width:950px height:auto barincrement:15px
PlotArea = left:10px bottom:50px top:10px right:10px

Period = from:-251 till:-48
TimeAxis = orientation:horizontal
ScaleMajor = unit:year increment:10 start:-251
ScaleMinor = unit:year increment:1 start:-251
TimeAxis = orientation:hor
AlignBars = justify

Colors =
#legends
id:SMP value:green
id:SAU value:blue
id:black value:black
id:white value:white
id:mesozoic value:rgb(0.54,0.54,0.258)
id:triassic value:rgb(0.51,0.17,0.57)
id:earlytriassic value:rgb(0.6,0.22,0.61)
id:middletriassic value:rgb(0.73,0.53,0.71)
id:latetriassic value:rgb(0.78,0.65,0.8)
id:jurassic value:rgb(0.2,0.7,0.79)
id:earlyjurassic value:rgb(0,0.69,0.89)
id:middlejurassic value:rgb(0.52,0.81,0.91)
id:latejurassic value:rgb(0.74,0.89,0.97)
id:cretaceous value:rgb(0.5,0.78,0.31)
id:earlycretaceous value:rgb(0.63,0.78,0.65)
id:latecretaceous value:rgb(0.74,0.82,0.37)
id:paleogene value:rgb(0.99,0.6,0.32)
id:paleocene value:rgb(0.99,0.65,0.37)
id:eocene value:rgb(0.99,0.71,0.42)

BarData=
bar:eratop
bar:space
bar:periodtop
bar:space
bar:NAM1
bar:NAM2
bar:NAM3
bar:NAM4
bar:NAM5
bar:NAM6
bar:NAM7
bar:NAM8
bar:NAM9
bar:NAM10
bar:NAM11
bar:NAM12
bar:NAM13
bar:NAM14

bar:space
bar:period
bar:space
bar:era

PlotData=
align:center textcolor:black fontsize:M mark:(line,black) width:25
shift:(7,-4)

bar:periodtop
from: -251 till: -245 color:earlytriassic text:Early
from: -245 till: -228 color:middletriassic text:Middle
from: -228 till: -199.6 color:latetriassic text:Late
from: -199.6 till: -175.6 color:earlyjurassic text:Early
from: -175.6 till: -161.2 color:middlejurassic text:Middle
from: -161.2 till: -145.5 color:latejurassic text:Late
from: -145.5 till: -99.6 color:earlycretaceous text:Early
from: -99.6 till: -65.5 color:latecretaceous text:Late
from: -65.5 till: -55.8 color:paleocene text: Pal.
from: -55.8 till: -48 color:Eocene text: Eo.

bar:eratop
from: -251 till: -199.6 color:triassic text:Triassic
from: -199.6 till: -145.5 color:jurassic text:Jurassic
from: -145.5 till: -65.5 color:cretaceous text:Cretaceous
from: -65.5 till: -48 color:paleogene text:Paleogene

PlotData=
align:left fontsize:M mark:(line,white) width:5 anchor:till align:left

color:SMP bar:NAM1 from:-233.3 till:-201 text:Primitive sauropodomorphs
color:SMP bar:NAM3 from:-225 till:-190 text:Plateosauridae
color:SMP bar:NAM5 from:-227 till:-176 text:Massospondylidae
color:SMP bar:NAM7 from:-216 till:-185 text: Non-sauropod sauropodiformes
color:SAU bar:NAM9 from:-206 till:-167 text: Primitive sauropods
color:SAU bar:NAM11 from:-184 till:-114 text: Non-neosauropod eusauropods
color:SAU bar:NAM13 from:-168 till:-65.5 text: Neosauropods

PlotData=
align:center textcolor:black fontsize:M mark:(line,black) width:25

bar:period
from: -251 till: -245 color:earlytriassic text:Early
from: -245 till: -228 color:middletriassic text:Middle
from: -228 till: -199.6 color:latetriassic text:Late
from: -199.6 till: -175.6 color:earlyjurassic text:Early
from: -175.6 till: -161.2 color:middlejurassic text:Middle
from: -161.2 till: -145.5 color:latejurassic text:Late
from: -145.5 till: -99.6 color:earlycretaceous text:Early
from: -99.6 till: -65.5 color:latecretaceous text:Late
from: -65.5 till: -55.8 color:paleocene text: Pal.
from: -55.8 till: -48 color:Eocene text: Eo.

bar:era
from: -251 till: -199.6 color:triassic text:Triassic
from: -199.6 till: -145.5 color:jurassic text:Jurassic
from: -145.5 till: -65.5 color:cretaceous text:Cretaceous
from: -65.5 till: -48 color:paleogene text:Paleogene

References

Sources

*

External links

Sauropodomorpha: Overview
from Palæos.


, from When Dinosaurs Ruled Texas, by Jon A. Baskin.


by Thomas R. Holtz Jr., from the University of Maryland.
*

{{Taxonbar, from=Q133012

Dinosaur clades