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Permian Aquatic Reptiles

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Permian Aquatic Reptiles Permian Aquatic Reptiles Mark A. S. McMenamin 1,* 1 Department of Geology and Geography, Mount Holyoke College, South Hadley, Massachusetts, United States *Correspondence: [email protected]; Tel.: 413-538-2280 Copyright©2019 by author, all rights reserved. Author agrees that this article remains permanently open access under the terms of the Creative Commons Attribution License 4.0 International License Abstract Eight amniote genera (representing four clades) amniotes returning to the water. Although several genera became aquatic during the Permian. The four clades were (Barasaurus and Hovasaurus) survived the Permo-Triassic mesosaurids, tangasaurids, the neodiapsid Claudiosaurus, mass extinction, in spite of opportunities to do so none of and the procolophonid Barasaurus. Two of eight genera these Paleozoic aquatic reptile lineages underwent major survived the end-Permian mass extinction, but did not last diversification in marine habitats. long into the Mesozoic. A previously undescribed specimen of Claudiosaurus germaini, preserved in a lacustrine concretion from the Sakamena Formation, Madagascar, 2. Modifications for Aquatic Life bears seventeen vertebrae that has been split along an approximate horizontal plane to reveal sections of neural A series of repeated and predictable sclerotome canal casted in white calcite. Enlargement of the neural modifications occur in amniote tetrapods after they return to canal in the sacral region of this specimen of Claudiosaurus an aquatic lifestyle. Before listing these modifications, it is (vertebral segments 22-26) is more similar to that of important to note that several aquatic reptiles today (certain Tupinambis (segments 24-28) than it is to Testudo crocodilians, marine iguanas [Amblyrhynchus cristatus], (segments 16-23). Claudiosaurus skeletal anatomy provides nile monitors [Varanus niloticus]) may show few if any evidence for swim propulsion by both hind limbs and by discernible modification to their sclerotome resulting from undulation of a dorsal-ventrally flattened tail. Evidence for their aquatic lifestyle. That said, skeletal changes in support the latter includes elongate transverse processes on distal of an aquatic lifestyle include the following: 1, changes in tail vertebrae. Other Permian aquatic reptile genera neck length; 2, decreased ossification of limb joints; 3, (Mesosaurus, Hovasaurus, Barasaurus) used snake-like thickening of the neural canal associated with enervation side-to-side tail undulation, whereas Claudiosaurus used demands of swim propulsion; 4, modification of fore and/or cetacean-like up-down tail undulation in the vertical plane. hind limbs into flippers, with or without development of It seems unlikely that any of these animals were particularly hyperphalangy [1]; 5, thickening of the ribs for ballast fast swimmers. (pachyostosis, which involves additions of lamellar bone); 6, changes in the number of caudal vertebrae [2]; 7, Keywords aquatic reptiles, parareptiles, procolophonids, enlarged coracoid or bone fusion to form scapulocoracoid; neodiapsids, Permian, Barasaurus, mesosaurs, Hovasaurus, 8, changes in rib length; 9, enlarged humerus with expanded Tangasauridae, Claudiosaurus, neuroanatomy, swim entepicondyle and corresponding foramina; 10, elongate kinematics, neural canal, caudal vertebrae, transverse neural spines on the caudal vertebrae for enhanced tail process propulsion; 11, enlarged transverse processes (caudal ribs) in vertebrae at the base of the tail; 12, transverse processes at the base of the tail curve to the anterior; 13, changes in forelimb length; 14, reduced ossification in the carpals and 1. Introduction tarsals, ostensibly to provide greater swimming flexibility. Mesozoic aquatic reptiles were a diverse group that Expansion of the entepicondyle improves the efficiency of included ichthyosaurs, plesiosaurs, thallatosaurs, the forelimb swimming stroke, but this feature is not thallatosuchians such as Dakosaurus, nothosaurs, exclusive to marine reptiles, as it is also seen in the pachypleurosaurs, placodonts, the shovel-jawed marine terrestrial Permian neodiapsid Thadeosaurus. A role likely reptile Atopodentatus, the herbivorous marine comparable to that of the expanded entepicondyle was sphenodontian Ankylosphenodon, sea turtles, palaeophilid played by the enlarged and flattened ulna in Triassic sea snakes, and mosasaurs [1]. Keichousaurus. In the category of behavioral modifications Paleozoic aquatic reptiles are much less diverse, but for aquatic life, small stones may also be ingested for nevertheless interesting as they represent the vanguard of ballast, as in Hovasaurus. Scoliosis has been reported from Paleozoic aquatic skink-like ventral scales in a configuration that was well reptiles [3, 4]. Whether or not these deformities correlate to suited for existence in an aquatic habitat. The Barasaurus osteological modifications acquired during transition to an lifestyle was comparable to that of the crab-eating modern aquatic lifestyle is at present unknown, but it seems likely Madagascan skink (Amphiglossus astrolabi [3]) and the that the hydrostatic support provided by an aquatic habitat Cuban aquatic anole (Anolis vermiculatus [8]). Although would somewhat reduce the lethality of sclerotome Barasaurus had the potential to do, especially considering aberrations as they would not have the same biomechanical that it survived the Permo-Triassic mass extinction, the requirements as would be the case for land tetrapod genus did not diversify into a major group of Mesozoic skeletons. marine tetrapods. 3. Paleozoic Aquatic Reptiles Brazilosaurus Amniotes first appear in the Carboniferous at 312 Ma [5]. Mesosaurids of Mesosauridae, constituting a part of the There are no reported cases of return to aquatic lifestyle Sauropsida, form a sister group to all other reptiles among Carboniferous amniotes. By the Early Permian, (Parareptilia and Eureptilia, respectively). This puts however, several lineages had returned to the water. All of mesosaurids in a curious phylogenetic position between these consisted of relatively small animals, most less than a synapsids and Reptilia. Indeed, suggestions that meter in length. mesosaurids may be synapsids themselves have recently The three Early Permian genera belong to the resurfaced [9]. It remains unclear at this point whether Mesosauridae (Mesosaurus, Stereosternum and mesosaurids are basal Parareptilia, Synapsida, or basal Brazilosaurus). These are followed by Late Permian genera Sauropsida, an unsettled placement for this iconic group of belonging to the aquatic neodiapsid family Tangasauridae Permian aquatic creatures, famous for their role in (Tangasaurus, Acerosodontosaurus, and Hovasaurus). Thus establishing the Pangaean paleobiogeographic linkage the Permian sees two radiations of marine reptiles, an between South America and Africa. earlier one consisting of mesosaurids and a later one Brazilosaurus is one of the three described mesosaurid consisting of tangasaurids. genera, and along with Mesosaurus occurs in Brazil [10]. Two additional genera added to the Late Permian Brazilosaurus has considerably smaller teeth than either diversification include the neodiapsid Claudiosaurus and Stereosternum or Mesosaurus [11]. Barasaurus, the only known aquatic procolophonid. As is the case for Hovasaurus, Barasaurus survives for a short time into the Triassic. Claudiosaurus Known from the Sakamena Formation of Madagascar, Acerosodontosaurus Claudiosaurus is the most basal neodiapsid and thus occupies an important position in diapsid phylogeny, close Acerosodontosaurus is an aquatic neodiapsid, closely to the Carboniferous-Permian Order Araeoscelidia. The related to Hovasaurus. Acerosodontosaurus is thought to largest claudiosaur individuals reach up to 60 cm length. have reached a total length of 70 cm. Its skull has lost the The fibula has a gentle S-shaped curve, comparable that of quadratojugal and thus develops a spacious lower temporal its terrestrial presumed close relative Thadeosaurus. arcade [6]. Acerosodontosaurus’s humeri have enlarged In Claudiosaurus, the transverse vertebral processes entepicondyles with corresponding foramina. occurring at the tail base are large and elongate, and curved Acerosodontosaurus’ femur is thicker than that of the in an anterior direction, evidently as muscle attachments for related terrestrial neodiapsid Kenyasaurus. the powerful caudofemoralis musculature that powered the hind limbs. The width of the long, narrow trunk vertebrae in Barasaurus Claudiosaurus are roughly 50% of their length, as opposed to Thadeosaurus where the ratio is 70% [12]. The typical procolophonoid parareptile (such as In Claudiosaurus, the “centra of the two principal sacrals Hypsognathus) is a rather squat creature with a pair of and the first four caudals are as long as the longest trunk pointed projections that jut out from the back of its skull, centra, after which the length somewhat decreases” [12]. giving the animal a resemblance to a desert horned lizard. Also, in addition to two pairs of primary sacral ribs, a third Interestingly, there was one successful aquatic pair is partially incorporated into the sacrum [12]. Growth procolophonoid, Barasaurus. The existence of Barasaurus rings occur on the zygapophyses of the trunk vertebrae [12]. recalls the single known successful aquatic sphenodontian, The margins of contact in the carpals “are poorly Ankylosphenodon from the Early Cretaceous of México [7]. defined” and therefore retained considerable cartilage,
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