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Ichthyosaur diapsid origin questioned
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Ben Creisler
Apr 26, 2025, 1:23:19 PMApr 26
to DinosaurMa...@googlegroups.com
Ben Creisler
A new paper:
Michael W.Maisch (2025)
Ichthyosaur ancestors – the case against a diapsid origin of ichthyosaurs revisited
Neues Jahrbuch für Geologie und Paläontologie – Abhandlungen (advance online publication)
DOI: 10.1127/njgpa/2025/1250
https://www.schweizerbart.de/papers/njgpa/detail/prepub/106737/Ichthyosaur_ancestors_the_case_against_a_diapsid_origin_of_ichthyosaurs_revisited
The origin of ichthyosaurs (Reptilia: Ichthyosauria), a major group of secondarily marine amniotes of the Mesozoic, is discussed from a purely anatomical and constructional morphological viewpoint. Whereas most Mesozoic marine reptile groups, particularly thalattosaurs, sauropterygians, and placodonts, are likely to be closely related forms within the Neodiapsida, which may form a monophylum (Enaliosauria), ichthyosaurs possibly do not form a part of it. An alternative hypothesis proposed here is that the closest known relatives of ichthyosaurs are owenettids, a low diversity group of small, lizard-like non-neodiapsid, procolophonomorph reptiles that existed from the Late Permian to the Middle Triassic. Arguments for and against such a scenario are discussed, and shared derived characters of procolophonomorphs, particularly owenettids, and ichthyosaurs are identified. Under this scenario, the supratemporal fenenstra of ichthyosaurs developed independently and convergently to that of neodiapsids, as did the upper temporal fenestrae of araeocelids, Orovenator and other taxa (including some owenettid procolophonians). Possible objections to the procolophonomorph hypothesis are also discussed. A novel evolutionary scenario for the development of the uniquely constructed ichthyosaur rostrum, and an explanation for the observed differences to enaliosaurian rostra is provided, as is a scenario for the loss of the ectopterygoid and particularly the configuration of the ichthyosaurian temporal region, occiput and other cranial peculiarities of the group. They can be interpreted as a result of different evolutionary pathways enforced by phylogenetic morphological constraints that have their reason in the divergent morphology of potential ancestral taxa. Deriving ichthyosaurs from procolophonomorphs eliminates problems encountered with numerous structural peculiarities in the cranial skeleton of ichthyosaurs that were hitherto difficult to explain under the neodiapsid hypothesis. If ichthyosaurs are closely related to owenettids, their ghost-lineage does not extend much further than the latest Permian, and they may have had their origin in Gondwana, just as it would be the case under the neodiapsid hypothesis. This is in line with the fossil record of the group and its explosive adaptive radiation during the Early Triassic. Despite phenetic similarities in the cranial and postcranial skeleton, neither mesosaurs nor hupehsuchids are considered here as closely related to ichthyosaurs. Hupehsuchids are more likely neodiapsid ichthyosaur-mimics that form part of the enaliosaurian radiation, although the group is still too little known to make a definite statement. Ichthyosaurs can be regarded as diapsids, if a wider concept of Diapsida (the last common ancestor of Petrolacosaurus, Passer, and all of its descendants) is employed, which is proposed here. Although recent phylogenetic work on basal reptiles has called for abandoning the term ‘Diapsida’ altogether, I advocate for it being retained. The results of the present study therefore provide a compromise solution between apparently irreconcilable concepts of ichthyosaur ancestry previously proposed.
Ichthyosaur ancestors – the case against a diapsid origin of ichthyosaurs revisited
Neues Jahrbuch für Geologie und Paläontologie – Abhandlungen (advance online publication)
DOI: 10.1127/njgpa/2025/1250
https://www.schweizerbart.de/papers/njgpa/detail/prepub/106737/Ichthyosaur_ancestors_the_case_against_a_diapsid_origin_of_ichthyosaurs_revisited
The origin of ichthyosaurs (Reptilia: Ichthyosauria), a major group of secondarily marine amniotes of the Mesozoic, is discussed from a purely anatomical and constructional morphological viewpoint. Whereas most Mesozoic marine reptile groups, particularly thalattosaurs, sauropterygians, and placodonts, are likely to be closely related forms within the Neodiapsida, which may form a monophylum (Enaliosauria), ichthyosaurs possibly do not form a part of it. An alternative hypothesis proposed here is that the closest known relatives of ichthyosaurs are owenettids, a low diversity group of small, lizard-like non-neodiapsid, procolophonomorph reptiles that existed from the Late Permian to the Middle Triassic. Arguments for and against such a scenario are discussed, and shared derived characters of procolophonomorphs, particularly owenettids, and ichthyosaurs are identified. Under this scenario, the supratemporal fenenstra of ichthyosaurs developed independently and convergently to that of neodiapsids, as did the upper temporal fenestrae of araeocelids, Orovenator and other taxa (including some owenettid procolophonians). Possible objections to the procolophonomorph hypothesis are also discussed. A novel evolutionary scenario for the development of the uniquely constructed ichthyosaur rostrum, and an explanation for the observed differences to enaliosaurian rostra is provided, as is a scenario for the loss of the ectopterygoid and particularly the configuration of the ichthyosaurian temporal region, occiput and other cranial peculiarities of the group. They can be interpreted as a result of different evolutionary pathways enforced by phylogenetic morphological constraints that have their reason in the divergent morphology of potential ancestral taxa. Deriving ichthyosaurs from procolophonomorphs eliminates problems encountered with numerous structural peculiarities in the cranial skeleton of ichthyosaurs that were hitherto difficult to explain under the neodiapsid hypothesis. If ichthyosaurs are closely related to owenettids, their ghost-lineage does not extend much further than the latest Permian, and they may have had their origin in Gondwana, just as it would be the case under the neodiapsid hypothesis. This is in line with the fossil record of the group and its explosive adaptive radiation during the Early Triassic. Despite phenetic similarities in the cranial and postcranial skeleton, neither mesosaurs nor hupehsuchids are considered here as closely related to ichthyosaurs. Hupehsuchids are more likely neodiapsid ichthyosaur-mimics that form part of the enaliosaurian radiation, although the group is still too little known to make a definite statement. Ichthyosaurs can be regarded as diapsids, if a wider concept of Diapsida (the last common ancestor of Petrolacosaurus, Passer, and all of its descendants) is employed, which is proposed here. Although recent phylogenetic work on basal reptiles has called for abandoning the term ‘Diapsida’ altogether, I advocate for it being retained. The results of the present study therefore provide a compromise solution between apparently irreconcilable concepts of ichthyosaur ancestry previously proposed.
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