Evolution of Marine Reptiles

Amphibia-Amniota: Basic anatomical and reproductive differences Reptiles Skin covered by scales. Eggs are laid in ground, developing embryonary membranes and [blank_start]apergaminate[blank_end] (soft) or calcareous (hard) shells. Some groups can develop embryo retention and [blank_start]viviparity[blank_end]. Larval stages [blank_start]do not exist[blank_end].

Key terms Reptiles: all members of the amniote clade that includes living turtles, lizards, crocs [blank_start]Diapsids[blank_end]: reptiles with two skull openings behind orbit: the lizard-croc clade Neodiapsid: diapsid clade that includes [blank_start]Younginiformes[blank_end] + Sauria Lepidosaurs: tuatara and kin (rhynchocephalians) and squamates [blank_start]Squamates[blank_end]: snakes, lizards, amphisbaenians ( = ‘worm-lizards’) Archosauromorphs: archosaurs and close relatives (like protosaurs & [blank_start]rhynchosaurs[blank_end]) Archosaurs: [blank_start]crocodiles[blank_end] and kin, pterosaurs, dinosaurs and kin (including birds)

Earliest reptiles have an [blank_start]anapsid[blank_end] skull (= where there are no post-orbital openings) The diapsid skull evolved from an anapsid ancestor… The diapsid skull (or a modified version of it) is present in lizards, snakes, crocs, birds

Turtles have an anapsid skull and hence have conventionally been placed outside of [blank_start]Diapsida[blank_end]. However, molecular data places them deep within Diapsida. If correct, this means that they evolved their [blank_start]anapsid condition from a diapsidone[blank_end].

Archosauromorphs: archosaurs and their close relatives Non-archosaurian archosauromorphs include [blank_start]protorosaurs[blank_end], trilophosaurs and rhynchosaurs. Mostly terrestrial [blank_start]Triassic[blank_end] animals; include small, lizard-like predators and omnivores, long-necked amphibious forms, specialised herbivores.

[blank_start]Lepidosaurs[blank_end]: lizards, snakes, tuatara and their relatives Mostly small diapsids with flexible skulls: key events in evolution include appearance of different [blank_start]kinetic zones[blank_end] in skull. Post-Triassic group, many key divergences in [blank_start]Jurassic and Cretaceous.[blank_end]

[blank_start]Tuatara[blank_end] (Sphenodon): two species of chunky, lizard-like diapsids, endemic to New Zealand. Endangered, now restricted to offshore Islands. Enlarged [blank_start]premaxillary[blank_end] teeth and marginal teeth all [blank_start]fusedto jaw edges[blank_end]. Propaliny present; halves of lower jaw rotate about symphysis. [blank_start]Shearing or sawing[blank_end] motion used to break up prey. Complete LTB long thought to be a ‘primitive’ feature; contributed to the idea that Sphenodon is an archaic relict.

Squamates: over [blank_start]7500[blank_end] species ‘lizards’ [blank_start]paraphyletic[blank_end] to amphisbaenians and snakes.

Reptile physiology and anatomy makes the transition to marine life relatively easy. However, to take to marine life a reptile still has to… --- move efficiently in the water --- feed effectively in the water --- [blank_start]regulate salt concentrations[blank_end] ….. oh, and cope with [blank_start]buoyancy[blank_end], reproduction and heat loss too…

The evolution of locomotion in water, examples from the marine reptile fossil record [blank_start]Discontinuous locomotion[blank_end]: Where propulsion is only generated during the [blank_start]power stroke[blank_end], e.g., paddling, rowing (bears, paddling seabirds, swimming rodents, diving ducks)

Continuous locomotion: there’s [blank_start]axial locomotion[blank_end], powered by body and/or tail. Axial locomotors practising continuous locomotion can be axial [blank_start]undulatory[blank_end] swimmers (using wave-like sweeps of tail) or axial [blank_start]oscillatory[blank_end] swimmers (using swivelling of propulsive structure, like tail fin). Another category within continuous locomotion: [blank_start]paraxial locomotion[blank_end] (using lift- or drag-based limbs).

Trend seen frequently in evolutionary history of marine reptiles. Transition from axial [blank_start]undulatory[blank_end] swimmers to axial [blank_start]oscillatory[blank_end] swimmers. Seen in ichthyosaurs, [blank_start]crocodyliforms[blank_end], mosasaurs.

Trend seen rarely in evolutionary history of marine reptiles. Transition from axial undulatory swimmers to paraxial swimmers. Seen in [blank_start]sauropterygians[blank_end].

Salt and sea-water Seawater contains about 35g of salt per liter. Marine mammals void unwanted salt via [blank_start]efficient kidneys[blank_end] that produce concentrated [blank_start]urine[blank_end]. Reptiles don’t have such efficient kidneys: instead they use salt-excreting skull glands. Marine iguana salt glands are in the [blank_start]nasal cavity[blank_end] Crocodile salt glands are on the tongue Sea snake salt glands are [blank_start]under the tongue[blank_end] Sea turtle salt glands are between the eyes Seabird salt glands are set in bony depressions above the eyes

Sauropterygians Major clade of Mesozoic marine reptiles, initially mostly [blank_start]Tethyan[blank_end] but worldwide by Jurassic. [blank_start]Placodonts[blank_end], pachypleurosaurs, [blank_start]nothosaurs[blank_end], pistosaurs and plesiosaurs. Key characters: [blank_start]euryapsid[blank_end] condition, retracted external nostrils, closed palate extending to braincase, absence of several skull bones (lacrimal, tabular, postparietal, supratemporal), large retroarticular process, scapula superficial to clavicle, small ilium etc.

Placodonts Triassic sauropterygians from Europe, Middle East and China, famous for their [blank_start]crushing[blank_end] dentition. Evolved an elaborate, turtle-like armour composed of interlocking [blank_start]scutes[blank_end].

[blank_start]Pachypleurosaurs[blank_end] Small amphibious Triassic sauropterygians (to 2m) known from 100s of specimens. Some suction-feeding features. Embryos show that [blank_start]viviparity[blank_end] present already.

Viviparity evolved early on within Sauropterygia – before these animals evolved [blank_start]giant size[blank_end] and pelagic habit

Nothosauroids The best known nothosauroid: Nothosaurus from Middle and Upper Triassic Europe, Middle East and China. Several species ranging from 1.25 to 3.5 m long. Fantastic [blank_start]teeth[blank_end]!

Pistosaurs: ancestors of plesiosaurs Several Late Triassic taxa appear to be ‘[blank_start]proto-plesiosaurs[blank_end]’. Resemble plesiosaurs in having [blank_start]retracted external[blank_end] nostrils, parietal crest, a simplified humerus shape and other characters. Enlarged, wing-shaped hands suggest [blank_start]paraxial Locomotion[blank_end] (presumably underwater ‘flight’).

Examples of non-plesiosaurian sauropterygians [blank_start]nothosaur[blank_end] [blank_start]pistosaur[blank_end]

Plesiosaurs Carnivorous Mesozoic marine reptiles Approx. [blank_start]200-65 million[blank_end] years ago Plesiosaurs are derived sauropterygians Sauropterygia – including placodonts, pachypleurosaurs, nothosaurs, pistosaurs

Quick history of plesiosaurs Plesiosaurs evolved during the Late [blank_start]Triassic[blank_end] from [blank_start]pistosaurs[blank_end]. Several lineages evolved giant size (10m+, 5tons+) during Jurassic & Cretaceous. ‘Pliosauromorphs’ ([blank_start]big[blank_end] heads, [blank_start]short[blank_end] necks) evolved several times from ‘plesiosauromorphs’ ([blank_start]small[blank_end] heads, [blank_start]long[blank_end] necks). Most ‘pliosauromorphs’ belong within Pliosauroidea. Plesiosauroids include very long- necked elasmosaurids as well as long-snouted polycotylids. Plesiosaurs mostly pelagic ocean- going animals, but there were [blank_start]estuarine and freshwater[blank_end] lineages within Leptocleidia. Mass extinction end of Cretaceous.

The skull and dentition Modified diapsid (‘euryapsid’) condition: [blank_start]loss of lower temporal fenestrae[blank_end] Position of the intenal and external nares Single large temporal fenestra

Polycotylid debate Traditional Classifications treat them as [blank_start]Pliosaurs[blank_end] But apart from the [blank_start]large head[blank_end], most of the characters are typically plesiosauroid. So, [blank_start]neck length and size of head[blank_end] are unreliable It now makes more sense to refer to plesiosaurs as either pliosauromorph or plesiosauromorph.

Mesozoic plesiosaur ‘pregnant female’ with a [blank_start]foetus inside the body cavity[blank_end]

Ichthyosaurs Carnivorous Mesozoic marine reptiles Approx. [blank_start]200-90 million[blank_end] years ago (before [blank_start]K-T extinction[blank_end]) Exact origin of ichthyosaurs unknown Teeth set in [blank_start]groove[blank_end] (not sockets) Ichthyopterygia – increasingly [blank_start]fish-like[blank_end] through time Huge [blank_start]eyes[blank_end]

The big question about all the Mesozoic marine reptile lineages we’ve looked at (thalattosaurs, hupehsuchians, sauropterygians, also ichthyosaurs) is: where do their affinities lie within Reptilia? Difficult to resolve because all have a strongly modified morphology: in the skull, diapsid condition has been modified to [blank_start]euryapsid condition[blank_end] (where there’s a supratemporal fenestra, but no obvious laterotemporal fenestra). Seems that all of these lineages are within [blank_start]Neodiapsida[blank_end]. Sauropterygians have sometimes been argued to be close to lepidosaurs; other studies suggest that all of these lineages are archosauromorphs.

Cretaceous lizards invade the seas During the ‘mid’ Cretaceous (from c [blank_start]100Ma[blank_end]), several lineages of anguimorph lizards (related to [blank_start]monitorsand gila monsters[blank_end]) took to the seas as mid-sized (c1m), long-bodied, shallow-water foragers on reefs etc. From shallow marine sediments of Europe, Middle East, N America. Dolichosaurs and aigialosaurs. [blank_start]Viviparity[blank_end] was present, even in these mid-sized amphibious lineages. Carsosaurus with 4 embryos in body cavity.

Mosasaurs Large to gigantic aquatic anguimorphs (mostly marine). Flexible zone in [blank_start]lower jaw[blank_end], evidence for [blank_start]ratchet[blank_end] feeding. Evolutionary trends: longer snout, retracted Nostrils, longer and wider paddle-like limbs, more paddle-like tail, with some evolving [blank_start]heterocercal[blank_end] tail.