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Revision of basal sauropods from the Middle Jurassic of Patagonia and the early evolution of sauropods
Revision of basal sauropods from the Middle Jurassic of Patagonia and the early evolution of sauropods
The Early and Middle Jurassic are widely regarded as the critical time for sauropod radiation and worldwide dispersal. The accepted theory is that the common ancestor of Early Jurassic sauropods had its provenance in South Africa, South Gondwana at the time. The major sauropod diversification and dispersal out of South Africa happened between the Early and Middle Jurassic (±180-160 Ma), with major clades firmly established by the Late Jurassic (±150 Ma). The Early and Middle Jurassic, however, have a generally poor sauropod body fossil record with the exception being a few taxa from Gondwana (e.g. Patagosaurus, Volkheimeria, Amygdalodon, Vulcanodon, Tazoudasaurus, Spinophorosaurus, Barapasaurus, Bothriospondylus, Lapparentosaurus) and even fewer from Laurasia (e.g. Cetiosaurus, Cetiosauriscus, Shunosaurus, Klamelisaurus, Mamenchisaurus, Omeisaurus). The Gondwanan sauropod with the most material preserved is Patagosaurus fariasi, from the Cañadón Asfálto Formation of west-central Chubut province, Patagonia, Argentina. It has a holotype consisting of axial and appendicular elements, and associated material consisting of cranial, axial, appendicular and dermal bones. The completeness of the material makes it valuable for Middle Jurassic sauropod research. Moreover, recent dating of the sediments belonging to the Cañadón Asfálto Formation resulted in a much older age range for the latter, redating it and all vertebrates found there from a Callovian to a much older Aalenian- Bajocian age, and thus placing Patagosaurus in a critical time for sauropod evolution; the latest Early to the early Middle Jurassic. Patagosaurus has been found in two bonebeds, not far apart; the holotype and several other associated specimens from Cerro Condor Norte, north of the village of Cerro Cóndor by the Chubut river, and more associated material, as well as another sauropod taxon, Volkheimeria chubutensis, from Cerro Condor Sur, a site close to the Chubut river. Both beds are lacustrine deposits. However, the original 1986 description of Patagosaurus consisted of a blend of the holotype and associated material, without clearly separating the former from the latter. Moreover, some material from Cerro Condor Sur, specimen MACN-CH 934, has since been considered to be a taxon other than Patagosaurus or Volkheimeria. Thus, a revision of all material has been done. The holotype has been revised and the alpha taxonomy of Patagosaurus fariasi has been established, confirming or discarding old diagnostic characters as appropriate, and generating new diagnostic characters for the new assignment of associated material to Patagosaurus fariasi, Volkheimeria, sauropod indet., or a new taxon. After the revision of all material, the Cerro Condor Norte bonebed was found to be monospecific, and includes an ontogenetic series of a small juvenile Patagosaurus, an intermediate juvenile, a subadult, the holotype (being a subadult to adult specimen) and another adult specimen. Cerro Condor Sur proved to be more problematic; yielding Patagosaurus material of an adult and a fully grown large adult, material that is sauropod indet., due to the fragmentary nature, and potentially two new taxa, one of which is more likely than the other: MACN-CH 934 and MACN-CH 230. A phylogenetic analysis using all confirmed material of Patagosaurus, and the MACN-CH 934 as OTU, as well as a recoding of Volkheimeria, Spinophorosaurus, Tazoudasaurus, Cetiosaurus oxoniensis and the Rutland Cetiosaurus, Lapparentosaurus, Bothriospondylus, confirms Patagosaurus to be a derived eusauropod, sister taxon to the Rutland Cetiosaurus, and nested within Cetiosaurus, both from the UK, thus confirming the existence of Cetiosauridae. Barapasaurus, an Indian taxon, which is traditionally closely associated with Patagosaurus, was retrieved as being more basal than the Cetiosauridae, and is thus considered a more basal eusauropod, and not closely related to Patagosaurus. Interestingly, the new, unnamed taxon MACN-CH 934 is retrieved as a derived eusauropod or even a basal neosauropod, and sister-taxon to Lapparentosaurus, a taxon from Madagascar. Volkheimeria comes out as sister taxon to the North African Spinophorosaurus. The addition of MACN-CH 230 as a potential new OTU, however, destabilizes the tree, and forces MACN-CH 230, and Bothriospondylus and Lapparentosaurus from Madagascar together, while MACN-CH 934 remains a basal neosauropod. This means there is probably not enough information on Lapparentosaurus, Bothriospondylus, or MACN-CH 230, and future research will establish their phylogenetic relationships better. The grouping together of taxa from across the Gondwanan continent, and even together with Laurasian taxa, shows a greater mobility for sauropods in the Middle Jurassic than previously assumed. The evolution of Gondwanan sauropods was thought to be bracketed by their diversification wave after the Toarcian mass extinction, and by a physical barrier in the form of the Central Gondwanan Desert, cutting taxa off from dispersing, and thus creating endemic radiation patterns in South Africa and Argentina. However, this study shows that sauropods would be able to migrate through the physical barrier, and questions the existence of a real barrier, or whether the desert was periodically (in)accessible. Future research on the more unstable taxa of the tree, and more revisions of poorly known basal eusauropods will hopefully clarify this. Chapter 1 introduces the thesis, Chapters 2 and 3 revise the holotype and associated material, and Chapter 4 shows the phylogenetic study. Chapter 5 shows geometric morphometric analysis on adult and juvenile Patagosaurus elements. Thus, the revision of Patagosaurus aids sauropod taxonomic, evolutionary, ontogenetic, and phylogenetic investigation.
Middle Jurassic, Sauropoda, Patagonia, evolution, phylogeny
Holwerda, Femke Marleen
2019
English
Universitätsbibliothek der Ludwig-Maximilians-Universität München
Holwerda, Femke Marleen (2019): Revision of basal sauropods from the Middle Jurassic of Patagonia and the early evolution of sauropods. Dissertation, LMU München: Faculty of Geosciences
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Abstract

The Early and Middle Jurassic are widely regarded as the critical time for sauropod radiation and worldwide dispersal. The accepted theory is that the common ancestor of Early Jurassic sauropods had its provenance in South Africa, South Gondwana at the time. The major sauropod diversification and dispersal out of South Africa happened between the Early and Middle Jurassic (±180-160 Ma), with major clades firmly established by the Late Jurassic (±150 Ma). The Early and Middle Jurassic, however, have a generally poor sauropod body fossil record with the exception being a few taxa from Gondwana (e.g. Patagosaurus, Volkheimeria, Amygdalodon, Vulcanodon, Tazoudasaurus, Spinophorosaurus, Barapasaurus, Bothriospondylus, Lapparentosaurus) and even fewer from Laurasia (e.g. Cetiosaurus, Cetiosauriscus, Shunosaurus, Klamelisaurus, Mamenchisaurus, Omeisaurus). The Gondwanan sauropod with the most material preserved is Patagosaurus fariasi, from the Cañadón Asfálto Formation of west-central Chubut province, Patagonia, Argentina. It has a holotype consisting of axial and appendicular elements, and associated material consisting of cranial, axial, appendicular and dermal bones. The completeness of the material makes it valuable for Middle Jurassic sauropod research. Moreover, recent dating of the sediments belonging to the Cañadón Asfálto Formation resulted in a much older age range for the latter, redating it and all vertebrates found there from a Callovian to a much older Aalenian- Bajocian age, and thus placing Patagosaurus in a critical time for sauropod evolution; the latest Early to the early Middle Jurassic. Patagosaurus has been found in two bonebeds, not far apart; the holotype and several other associated specimens from Cerro Condor Norte, north of the village of Cerro Cóndor by the Chubut river, and more associated material, as well as another sauropod taxon, Volkheimeria chubutensis, from Cerro Condor Sur, a site close to the Chubut river. Both beds are lacustrine deposits. However, the original 1986 description of Patagosaurus consisted of a blend of the holotype and associated material, without clearly separating the former from the latter. Moreover, some material from Cerro Condor Sur, specimen MACN-CH 934, has since been considered to be a taxon other than Patagosaurus or Volkheimeria. Thus, a revision of all material has been done. The holotype has been revised and the alpha taxonomy of Patagosaurus fariasi has been established, confirming or discarding old diagnostic characters as appropriate, and generating new diagnostic characters for the new assignment of associated material to Patagosaurus fariasi, Volkheimeria, sauropod indet., or a new taxon. After the revision of all material, the Cerro Condor Norte bonebed was found to be monospecific, and includes an ontogenetic series of a small juvenile Patagosaurus, an intermediate juvenile, a subadult, the holotype (being a subadult to adult specimen) and another adult specimen. Cerro Condor Sur proved to be more problematic; yielding Patagosaurus material of an adult and a fully grown large adult, material that is sauropod indet., due to the fragmentary nature, and potentially two new taxa, one of which is more likely than the other: MACN-CH 934 and MACN-CH 230. A phylogenetic analysis using all confirmed material of Patagosaurus, and the MACN-CH 934 as OTU, as well as a recoding of Volkheimeria, Spinophorosaurus, Tazoudasaurus, Cetiosaurus oxoniensis and the Rutland Cetiosaurus, Lapparentosaurus, Bothriospondylus, confirms Patagosaurus to be a derived eusauropod, sister taxon to the Rutland Cetiosaurus, and nested within Cetiosaurus, both from the UK, thus confirming the existence of Cetiosauridae. Barapasaurus, an Indian taxon, which is traditionally closely associated with Patagosaurus, was retrieved as being more basal than the Cetiosauridae, and is thus considered a more basal eusauropod, and not closely related to Patagosaurus. Interestingly, the new, unnamed taxon MACN-CH 934 is retrieved as a derived eusauropod or even a basal neosauropod, and sister-taxon to Lapparentosaurus, a taxon from Madagascar. Volkheimeria comes out as sister taxon to the North African Spinophorosaurus. The addition of MACN-CH 230 as a potential new OTU, however, destabilizes the tree, and forces MACN-CH 230, and Bothriospondylus and Lapparentosaurus from Madagascar together, while MACN-CH 934 remains a basal neosauropod. This means there is probably not enough information on Lapparentosaurus, Bothriospondylus, or MACN-CH 230, and future research will establish their phylogenetic relationships better. The grouping together of taxa from across the Gondwanan continent, and even together with Laurasian taxa, shows a greater mobility for sauropods in the Middle Jurassic than previously assumed. The evolution of Gondwanan sauropods was thought to be bracketed by their diversification wave after the Toarcian mass extinction, and by a physical barrier in the form of the Central Gondwanan Desert, cutting taxa off from dispersing, and thus creating endemic radiation patterns in South Africa and Argentina. However, this study shows that sauropods would be able to migrate through the physical barrier, and questions the existence of a real barrier, or whether the desert was periodically (in)accessible. Future research on the more unstable taxa of the tree, and more revisions of poorly known basal eusauropods will hopefully clarify this. Chapter 1 introduces the thesis, Chapters 2 and 3 revise the holotype and associated material, and Chapter 4 shows the phylogenetic study. Chapter 5 shows geometric morphometric analysis on adult and juvenile Patagosaurus elements. Thus, the revision of Patagosaurus aids sauropod taxonomic, evolutionary, ontogenetic, and phylogenetic investigation.