Wikipedia reports: “Ichthyosaurs thrived during much of the Mesozoic era; based on fossil evidence, they first appeared approximately 245 million years ago (mya) and disappeared about 90 million years ago, about 25 million years before the dinosaurs became extinct. During the middle Triassic Period, ichthyosaurs evolved from as-yet unidentified land reptiles that moved back into the water, in a development parallel to that of the ancestors of modern-day dolphins and whales.”
Well, here we’re going to identify those “unidentified” land animals as the ancestors of ichthyosaurs: They’re mesosaurs.
It’s not difficult to figure out which reptiles were the closest to ichthyosaurs. All you have to do is include representatives from all the major groups and run their characters through phylogenetic analysis. The ones most like ichthyosaurs will nest on a tree closest to ichthyosaurs. Such a study has never been undertaken before. Typically a few suprageneric (above the level of genus) taxa are chosen. This gives no opportunity for individual genera within a larger clade to step up to the plate and nest as sister taxa. This also allows cheating, the ability to cherry pick traits from several taxa within a clade to create a chimaera taxon.
Here is the tree I used (and it keeps growing and growing):
Figure 2. The Reptile Family Tree and the Evolution of Ichthyosaurs. Here you can trace the evolution of ichthyosaurs (represented by Utatsusaurus) all the way back to Ichthyostega.
Past Mistakes Based on Using Suprageneric Taxa
Motani (1998) linked ichthyosaurs to Younginiforms or to Coelurosauravus (a gliding reptile) through Saurosternon. Unfortunately, Motani used suprageneric taxa and did not include mesosaurs.
Maisch (2010) nested ichthyosaurs either with mesosaurs within the diapsida (which is generally correct) or with Procolophon, an anapsid turtle sister. The connection was never spelled out, perhaps because of his use of suprageneric taxa. In fact, Maisch threw up his hands when he reported, “In the case of the ichthyosaurs we know that they are amniotes, and that they are not synapsids… Whether ichthyosaurs are diapsids, and if so, where exactly they have to be placed within the Diapsida, or whether they are parareptiles, and if so, whether they are related to mesosaurs or not, these are questions that remain as unresolved as one hundred years ago.”
Figure 2. Click to enlarge. The origin of ichthyosaurs and thalattosaurs from basal diapsids and basal mesosaurs. Relationships are rather apparent when seen in this context.
Here is the Ancestral Lineage of Ichthyosaurs
Starting with the basal diapsid, Petrolacosaurus and moving toward the basal enaliosaur, Claudiosaurus, the skull size was reduced (but probably not to the extents seen in Claudiosaurus) and the limbs become smaller and probably webbed. Hovasaurus was a sister to Claudiosaurus. Small-skulled Pachypleurosaurus was another sister to Claudiosaurus that nested at the base of the Sauropterygia.
Stereosternum is a somewhat forgotten taxon, but it represents a basal mesosaur, before all the temporal fenestra were closed off. The skull was not so small, the rostrum was elongated and the ribs became thicker. Mesosaurus was a derived sister, but too specialized in a different direction.
Wumengosaurus was considered an odd pachypleurosaur (Wu et al. 2011), but Wumengosaurus nests closer to the base of the ichthyosauria including Hupehsuchus. The quadratojugal was greatly reduced, eliminating the lower temporal bar, as in sauropterygians and ichthyosaurs.
Askeptosaurus was a thalattosaur, a sister taxa to the ichthyosaurs. Overall it retained the shape of a mesosaur, but the skull openings were reduced on top and expanded to the sides.
Hupehsuchus is a basal ichthyosauriform specialized without teeth. Even so it is the closest sister taxon known to basal ichthyosaurs represented by Utatsusaurus, a basal ichthyosaur.
The long term trend in the evolution of ichthyosaurs was to enlarge the body overall, elongate the rostrum, shorten the neck, deepen the torso, reduce the thickness of the ribs, shorten the tail and give it a dorsal kink, reduce the limbs and turn them into paddles.
There is no more parsimonious solution among the tested reptiles. And the resemblances between each sister taxon are in line with the small steps that evolution takes. You can read more details starting here.
Evidence and support in the form of nexus, pdf and jpeg files will be sent to all who request additional data.
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