Saturday, 21 March 2020
DIGITS AND PHALANGES
In 2018, Benjamin Kear and his team delved into a new are of study through technology that allows us to look at ichthyosaur remains at the molecular level. Their findings suggest ichthyosaurs had skin and blubber quite similar to our modern dolphins.
While ichthyosaurs evolved from land-dwelling, lung-breathing reptiles, they returned to our ancient seas and evolved into the fish-shaped creatures we find in the fossil record today. Their limbs fully transformed into flippers, sometimes containing a very large number of digits and phalanges.
Their flippers tell us they were entirely aquatic as they were not well-designed for use on land. And it was their flippers that first gave us the clue that they gave birth to live young; a find later confirmed by fossil embryo and wee baby ichy finds. They thrived during much of the Mesozoic era; based on fossil evidence, they first appeared around 250 million years ago (Ma) and at least one species survived until about 90 million years ago into the Late Cretaceous.
During the early Triassic period, ichthyosaurs evolved from a group of unidentified land reptiles that returned to the sea. They were particularly abundant in the later Triassic and early Jurassic periods before being replaced as a premier aquatic predator by another marine reptilian group, the Plesiosauria, in the later Jurassic and Cretaceous.
In the Late Cretaceous, ichthyosaurs were hard hit by the Cenomanian-Turonian anoxic event. As the deepest benthos layers of the seas became anoxic, poisoned by hydrogen sulphide, deep water marine life died off. This caused a cascade that wreaked havoc all the way up the food chain. At the end of that chain were our mighty predaceous marine reptiles.
Bounty turned to scarcity and a race for survival began. The ichthyosaurs lost that race as the last lineage became extinct. It may have been their conservative evolution as a genus when faced with a need for adaptation to the world in which they found themselves and/or being outcompeted by early mosasaurs.
There are promising discoveries coming out of strata from the Cretaceous epeiric seas of Texas, USA from Nathan E. Van Vranken. His published paper from 2017, "An overview of ichthyosaurian remains from the Cretaceous of Texas, USA," looks at ichthyosaurian taxa from the mid-Cretaceous (Albian–Cenomanian) time interval in North America with an eye to ichthyosaurian distribution and demise.
Photo: This beautifully preserved Ichthyosaur paddle with its incredible detail is from Early Jurassic (183 Million Years) deposits in the Ohmden, Posidonia Shale Formation, Baden-Württemberg, east of the Rhine, southwestern Germany.