Sea turtles have seven extant species, with five of the seven being at least 100 million years old (Anquetin et al. 2008). The group as a whole (save for the leatherback, which has no bony carapace) has limited intraspecies variance, with distinct digression in carapace pattern (Spotila 2004). So what is the most prominent transition theory for the development of paddle legs in sea turtles? No consensus cuts to the front of the line. A weak and unsubstantiated hypothesis: the land was too crowded a niche.
More extensive data is available on freshwater land turtles because of the plethora of extant species and the fossil record. The obvious differentiation between the two superfamilies is their respective digits / flippers, but subtle changes such as a modified hyoid bone exist (Jones et al. 2012).
Excluding fringe claims, there is no non-disputed fossil evidence any dinosaur lived solely in the water. One proponent surmises the load-bearing joints would be unable to support many larger dinosaurs (Ford 2012). On the other hand, the replies of paleontologists suspect spending long periods underwater would be impossible due the pressure exerted on the respiratory system, and assert structural integrity of the skeletal system has already been proven in modeling of extant species (Henderson 2012).
A great deal of evidence does exist with regards to the prevalence of dinosaurs proper in brackish areas or coastal fossils. This data accumulation was provided by oxygen isotope ratios in the available fossil record (Amiot et al. 2010).
Ichthyosaurs and plesiosaurs and other marine reptiles exhibited convergent evolution, inclusive of the less-discussed mososaurs and nothosaurus (Lindgren et al. 2010). Their morphology went from a lizard or fish shape during the early Jurassic period, and became similar to dolphins or tuna during the end of their reign (Cenomanian-Turonian EE). Icsythoaur
evolution has the benefit of transitive species within the group, ending at Ophthalmosaurus spp. (O’Keefe 2005).
How superbly icthyosaurs dominated the oceans is lucidly shown by their slim backbone, astounding eye development and ability to survive periods of deep sea diving without returning for air (Motani 2009). In a review of his work, the explanation of intermediate forms is overlooked. So, the “Why?” is more opaque. Thus, the author of this paper is forced into pure speculation. Geological events. There. Secondarily, a diminishing predation niche. Unfortunately, the ancestral fossils prior to this population explosion (ie punctuated equilibrium) have yet to be discovered.
The fossil record for the heretofore mentioned organisms shows they were on land with near-certitude, ultimately due to their bony hands. One could extrapolate assumptions about whales to that of their reptilian ghosts, similiar in mass. Such an assumption is risky, but the explanation offered by the New Scientist article by Le Page (2009) offers a solid qualitative structure-function relationship: For such energy-intensive predation, perhaps gills would have not extracted enough oxygen.
The lack of gills is a tradeoff that seems at face value to be detrimental, but it’s likely large tetrapods went extinct due to events unrelated to their morphology. Motani and other experts are so in awe of their predation attributes overcoming the seemingly vestigial apparatus like gills. They might have developed them, who is to say? This does not even take into consideration the vast rearrangement of the body plan that would be required.
Icthyosaurs ran into temperature-based bottlenecks (Thorne et al. 2011), culling their species multiple times. The Jurassic-Triassic extinction did not cause the transition to the seas, but it did allow them to flourish and demonstrate one of the few cases of Cope’s Rule.
This same rule may have applied to plesiosaurs transition during the Mesozoic era. Big
old reptiles were unable to stand on land due to their size. Moreover, their sheer foraging requirements might not have been sufficiently met by food available on land (O’Keefe and Carrano 2005).
All water-based reptilian species similar to dinosaurs went extinct during the Permian-Triassic event, many before that due to ecological changes. Starting with tens of adaptive tetrapods, three remained until the aforementioned event, all three of which were over a meter long (Thorne et al. 2011).