Sauropodomorpha
Template:Short description Template:Automatic taxobox
Sauropodomorpha (Template:IPAc-enTemplate:Refn Template:Respell; from Greek, meaning "lizard-footed forms") is an extinct clade of saurischian dinosaurs that includes the long-necked, herbivorous 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 prosauropods, which preceded the sauropods, 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 studyEdit
Early studyEdit
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.<ref name="hallett2016"/> 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.<ref name="sereno2007"/> 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.<ref name="hallett2016"/> 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.<ref name="sereno2007"/> 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.<ref name="hallett2016"/>
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.<ref name="hallett2016"/><ref name="wilson2006"/> 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.<ref name="sereno2007"/><ref name="wilson2006"/> Many of the most iconic sauropods-Apatosaurus, Brontosaurus, Camarasaurus, and Diplodocus-were all named during this time.<ref name="hallett2016"/> 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.<ref name="hallett2016"/> 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.<ref name="sereno2007"/><ref name="huene1932"/>
DescriptionEdit
Sauropodomorphs were adapted to browsing higher than any other contemporary herbivore, giving them access to high tree foliage. This feeding strategy is supported by many of their defining characteristics, such as: a light, tiny skull on the end of a long neck (with ten or more elongated cervical vertebrae) and a counterbalancing long tail (with one to three extra sacral vertebrae).Template:Citation needed
Their teeth were weak, and shaped like leaves or spoons (lanceolate or spatulate). Instead of grinding teeth, they had stomach stones (gastroliths), similar to the gizzard stones of birds and crocodiles, to help digest tough plant fibers. The front of the upper mouth bends down in what may be a beak.Template:Citation needed
One of the earliest known sauropodomorphs, Saturnalia, was small and slender (1.5 metres, or 5 feet long); but, by the end of the Triassic, they were the largest dinosaurs of their time, and throughout the Jurassic and Cretaceous they kept on growing. Ultimately the largest sauropods, like Supersaurus, Diplodocus hallorum, Patagotitan, and Argentinosaurus, reached Template:Convert in length, and 60,000–100,000 kilograms (65–110 US short tons) or more in mass.Template:Citation needed
Initially bipedal, as their size increased they evolved a four-legged graviportal gait adapted only to walking slowly on land, like elephants. The early sauropodomorphs were most likely omnivores as their shared common ancestor with the other saurischian lineage (the theropods) was a carnivore. Therefore, their evolution to herbivory went hand in hand with their increasing size and neck length.Template:Citation needed
They also had large nostrils (nares), and retained a thumb (pollex) with a big claw, which may have been used for defense — though their primary defensive adaptation was their extreme size.Template:Citation needed
Distinguishing anatomical featuresEdit
Sauropodomorphs can be distinguished as a group on the basis of some of the following synapomorphies:<ref>Martin, A.J. (2006). Introduction to the Study of Dinosaurs. Second Edition. Oxford, Blackwell Publishing. pg. 299-300. Template:ISBN.</ref>
- The presence of large nares.
- The distal part of the tibia is covered by an ascending process of the astragalus.
- Their hind limbs are short when compared to their torso length.
- The presence of three or more sacral vertebrae.
- The teeth are thin, flat and are spatula-like, with bladed and serrated crowns.
- The presence of a minimum of 10 cervical vertebrae that are typically elongated
- The presence of 25 presacral vertebrae
- The manus had a large digit I.
Evolutionary historyEdit
Among the first dinosaurs to evolve in the Late Triassic period,<ref>Template:Cite journal</ref> 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 continentsTemplate:Citation needed. 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<ref name=leal2004>Template:Cite journal (Warning: abstract is 12 kb PDF)</ref><ref name=RMOA09>Template:Cite journal</ref>). 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.<ref name="Martinez2011">Template:Cite journal</ref>
ClassificationEdit
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".Template:Citation needed
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.Template:Citation needed
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,<ref name="sereno1998">Template:Cite journal</ref> is a junior synonym of Plateosauridae as both contain the same taxa.<ref name=Y07 /><ref name=Indiansaupo/>
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.<ref name=poletal2011>Template:Cite journal</ref>
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.<ref name="otero">Template:Cite journal</ref> 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.<ref name="Ornithoscelida">Template:Cite journal</ref>
PhylogenyEdit
Within Sauropodomorpha, there is a large clade named Plateosauria. The name Plateosauria was first coined by Gustav Tornier in 1913.<ref>Tornier, G., 1913, "Reptilia (Paläontologie)" In: Handwörterbuch Naturwissenschaften 8: 337-376</ref> 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.<ref name="sereno1998" /> 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.<ref name=GU04>Template:Cite journal</ref>
However, recent cladistic analyses suggest that the Prosauropoda as traditionally defined is paraphyletic to sauropods.<ref name=Y03>Template:Cite journal</ref><ref name=Y07>Template:Cite journal</ref><ref name=poletal2011 /><ref name=Indiansaupo>Template:Cite journal</ref><ref name=Leyesaurus>Template:Cite journal</ref> 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.<ref>Template:Cite journal</ref>
Cladogram after Novas et al., 2011:<ref name=Indiansaupo />
Below is a cladogram of basal sauropodomorpha after Müller, 2019.<ref name="Muller2019">Template:Cite journal</ref>
PaleobiologyEdit
Comparisons between the scleral rings of several sauropodomorph genera (Diplodocus, Lufengosaurus, Nemegtosaurus, Plateosaurus, and Riojasaurus) and modern birds and reptiles suggest that they may have been cathemeral, active throughout the day at short intervals.<ref>Template:Cite journal</ref>
Sauropodomorphs reached the age of sexual maturity well before they were fully-grown adults.<ref>Template:Cite journal</ref><ref name=Griebeler13>Template:Cite journal</ref> 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.<ref name=Griebeler13/>
A long-standing hypothesis has been that early sauropodomorphs were carnivorous, as expected for most early dinosaurs. This hypothesis is supported by the current basalmost sauropodomorph, Buriolestes, and its plesiomorphic, theropod-like teeth.<ref name="cabreira2016">Template:Cite journal</ref> The teeth of more derived sauropodomorphs such as Eoraptor, Panphagia, and Pampadromaeus are better-suited for herbivorous (or possibly omnivorous) diets.<ref name=sereno13>Template:Cite journal</ref> Regardless of the phylogenetic position of herrerasaurids or Eoraptor (which are in flux),<ref name=sereno13/><ref name=sues11>Template:Cite journal</ref><ref name=theropodphylogeny2015>Template:Cite journal</ref> ancestral state reconstructions recover carnivory as being ancestral to sauropodomorphs.<ref name="cabreira2016"/>
ReferencesEdit
SourcesEdit
External linksEdit
- Sauropodomorpha: Overview, from Palæos.
- Sauropodmorpha Template:Webarchive, from When Dinosaurs Ruled Texas, by Jon A. Baskin.
- Geol 104 Dinosaurs: A natural history: Sauropodomorpha: Size matters, by Thomas R. Holtz Jr., from the University of Maryland.
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