The Sauropods are a highly distinctive group of Dinosaurs, with a long fossil record, covering the entirety of the Jurassic and Cretaceous periods. Although the Sauropods were a morphologically conservative group, the popular view of these Animals can be a bit limited, due to the high profile of the Sauropods of the Late Jurassic Morrison Formation of North America and Tendaguru Formation of Tanzania, both of which have produced Sauropods of enormous sizes, with columnar legs, long necks and tails, small heads, and deep bodies, leading to an under-appreciation of the actual diversity present within the group.
The term Neosauropoda was coined by Argentinian palaeontologist José Bonaparte to cover these distinctive Late Jurassic forms and their Cretaceous descendants, with earlier branching Sauropods which did not fit this bodyplan being referred to as 'Eosauropods'. This term has now fallen from favour, as it is seen as polyphyletic, as the Neosauropoda are clearly descended from this group but excluded from it, and expanding the group to include the Neosauropoda would make it identical to the Sauropoda. Instead the Sauropods are seen as being made up of a series of nested clades, with all members belonging to the Saurapoda, within which the majority belong to a more derived group, the Eusauropoda, withing which the Neosauropoda arise. Sauropods which do not belong to the Eusaurapoda can therefore be referred to as 'basal Sauropods', and Eusauropods which do not belong to the Neosauropoda as 'basal Neosauropods'.
Since the Neosauropods were clearly an established group by the Late Jurassic, understanding their origin requires examination of the (rather limited) fossil record of Early and Middle Jurassic Sauropods. Of the 1094 Sauropod body fossils are known globally from the Jurassic, 139 of these (12.7%) from the Middle Jurassic, and only 65 (5.9%) from the Early Jurassic, these fossils extremely rare, and each one important to our understanding of the history of the group.
Of the 65 known Early Jurassic Sauropod body fossils, twenty are from Africa, nineteen from Asia, nine each from India and South America, seven from Europe, and one from Madagascar. These remains are almost all from a very low number of sites, for example all bar one of the South American Early Jurassic Sauropod fossils come from Chubut and Santa Cruz provinces in southern Argentina. The one exception to this is an isolated vertebra, currently housed in the University of Michigan Museum of Paleontology, where it has been given the specimen number UCMP 37689, recovered from the redbeds of the Early-Middle Jurassic La Quinta Formation in the Department of Cesar, Colombia, by an oil exploration team in 1943.
The La Quinta redbeds extend from Colombia into Venezuela, and have produced a number of other Dinosaur remains, but no further Sauropod material. This paucity of Sauropod material from northern South America extends throughout the remainder of the Mesozoic, with one other Sauropod known from Colombia, the Early Cretaceous Padillasaurus leivaensis, a single specimen from Ecuador, the Late Cretaceous Yamanasaurus lojaensis, as well as two poorly preserved and unidentified specimens from Cretaceous localities in northern Peru and northern Brazil.
This absence of Sauropod specimens from northern South America is puzzling. Southern South America is considered to have been somewhat of a biodiversity hotspot for Sauropods, driven by the extinction of other large herbivores in the region during the volcanic break-up of Pangea, and the global distribution of Sauropods suggests they were capable of inhabiting a wide variety of habitats, including those likely to have been found in northern South America. Many tropical environments are considered to have low potential for the preservation of fossils today, due to high erosion rates and acid soils, and this is likely to have been the case in the Mesozoic too, but the absence of Sauropods from formations such as the La Quinta redbeds, which have produced numerous other Dinosaur fossils, is harder to explain.
In a paper published in the Journal of Vertebrate Paleontology on 10 August 2022, Aldo Rincón and Daniel Raad Pájaro of the Departamento de Física y Geociencias at the Universidad del Norte, Harold Jiménez Velandia of the Departamento de Geología at Universidad EAFIT, and the Departamento de Ciencias Geológicas at the Universidad de Caldas, Martín Ezcurra of the Sección Paleontología de Vertebrados at the Museo Argentino de Ciencias Naturales 'Bernardino Rivadavia', and Jeffrey Wilson Mantilla of the Museum of Paleontology and Department of Earth & Environmental Sciences at the University of Michigan, redescribe specimen UCMP 37689, formally naming it and examining its importance for our understanding of the history of Sauropods as a group.
UCMP 37689 comprises two sections of a dorsal vertebra interpreted as that of a medium-sized Sauropod, reconstructed as having been roughly 55 cm high and 45 cm wide. The two sections do not fit, but it has been possible to reconstruct the original size by virtually reversing the upper neural arch.
Although the material is extremely limited, Rincón et al. were able to carry out a phylogenetic analysis using a character matrix developed by Kevin Leonel Gomez, Jose Luis Carballido, and Diego Pol, and published in a paper on an Early Jurassic Sauropod in 2021. This suggested that the species closest to specimen UCMP 37689 were species for which the dorsal vertebrae were known; while there are several South American Sauropod species for which the dorsal vertebrae are not known, these can be shown on the basis of other parts of their anatomy to be closely related to other species with dorsal vertebrae quite different to UCMP 37689. On which basis Rincón et al. feel comfortable formally naming the specimen as a new species, and giving it the name Perijasaurus lapaz, where 'Perijasaurus' derives from Serranía del Perijá, a mountain range on the border between northeast Colombia and northwest Venezuela, where the specimen was found, and 'lapaz' refers both the the town of La Paz, close to where the specimen was found, and the 2016 Acuerdos de Paz peace agreement, which enabled Rincón et al. to carry out fieldwork in this area.
Sauropod vertebrae show a range of features which change across the cervico-dorsal transition and throughout the dorsal series, enabling fairly accurate estimation of the position of even a lone vertebra on the spine. These include the degree of opisthocoely, position of the parapophysis, development of spinodiapophyseal laminae, and presence of hyposphene-hypantrum articulations. Furthermore, in mature Sauropods the cervical vertebrae have fused cervical ribs, and a parapophysis on the ventrolateral edge of the centrum, whereas on dorsal vertebrae the ribs are unfused and the parapophysis positioned on the ventrolateral edge of the centrum over the course of the first few vertebrae. Based upon this, Rincón et al. believe that UCMP 37689 is probably a fifth dorsal vertebrae.
Specimen UCMP 37689 only includes 8.75% of the features used to classify Sauropods in Gomez et al.'s matrix, nevertheless this was enough to make some estimations about its relationships, with Perijasaurus lapaz either being the sister taxon to Cetiosaurus, from the Middle Jurassic of England, Patagosaurus and closely related forms from southern South America, Africa, and Asia, and the Eursaurapod line that leads to the Neosauropods, or Cetiosaurus and the Eursaurapod line that leads to the Neosauropods, or the Mamenchisaurids and the Eursaurapod line that leads to the Neosauropods.
Sauropods achieved a global distribution in the Early Jurassic, with numerous groups being found in the temperate zones of both hemispheres. Sauropods from tropical regions, however, remain rare, despite the Animals clearly having passed through these regions. Perijasaurus lapaz provides a rare example of such a tropical Sauropod, and demonstrates that basal Eusauropods had reached South America by the Early Jurassic, less than 10 million years after the End Triassic extinction of the earlier Prosauropods.
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