Name Meaning: Fish Lizard
Geologic Era: Late Triassic – Late Cretaceous
Location Found: Dorset, England
Size: 2-4 meters long on average; largest known species 21 meters long (Shonisaurus)
Extinction Date: 93 million years ago
Icthyosaurs were among the first animals to spark a true interest in paleontology. The first Icthyosaur was discovered by Mary Anning ( who also discovered the first complete plesiosaur and pterosaur in England), and helped put England on the map in geological and paleontological terms. At first mistaken for a type of crocodile, further study revealed that, while Icthyosaurus was a reptile, it was not terribly similar to crocodiles. Superficially, it was probably the most fish-like of the marine reptiles. Our specimen is a vertebra, recovered from Dorset, England (the same location as many of Mary Anning’s finds), and has some degree of weathering. The cliffs of this region can be dangerous to collect fossils from, as landslides are not uncommon.
At first glance, Icthyosaurs looks not unlike a modern dolphin. They possess many of the same skeletal features, if in modified forms. This is an excellent example of convergent evolution. Convergent evolution is when organisms that are not closely related develop similar structures (often due to similar environments or habits). Another example of convergent evolution is wings (insects vs. birds).
Icthyosaurs in general were very well adapted to being completely marine creatures. It was once thought that they laid eggs on land, but several fossils have been found that show partially developed young in the abdominal cavity of females. One specimen even appears to show the moment of birth. It is now known that icthyosaurs were viviparous, meaning they bore live young. Most modern aquatic mammals do this, with the young being born tail first to prevent drowning.
I count at least 6 fetuses still inside the female (based on trying to count the vertebral columns). One fetus was expelled either at the mother’s death or shortly before, and is below her. It is not known what caused their demise, but it’s possible that one of the fetuses became lodged in the birth canal, and the whole group drowned. Such incidents occasionally happen with modern dolphins, though they generally have the assistance of their pod to push them to the surface to breathe. It is not known if icthyosaurs were gregarious animals. If they were not, such birthing accidents may have been rather common.
Burial in a marine environment sometimes allows for the outline of soft tissue to be preserved, and sometimes the soft tissue itself. Some icthyosaurs have been found with dorsal fins preserved,, while others were not. This leads to the assumption that not all icthyosaurs had dorsal fins, and some had a shape closer to monitor lizards. In a few cases, pigment cells have been preserved.
These cells, called melanosomes, reveal that icthyosaurs had an overall darker pigmentation (probably browns, dark greys, and blacks), which would have both camouflaged them and allowed for better thermoregulation. This is in contract to sharks and mosasaurs, which tended to have countershading (light on bottom, dark on top).
“Birthing” Fossil: “Ichthyosauria offspring in Vienna” by Tommy from Arad – Ichthyosauria offspringsUploaded by FunkMonk. Licensed under CC BY 2.0 via Commons – https://commons.wikimedia.org/wiki/File:Ichthyosauria_offspring_in_Vienna.jpg#/media/File:Ichthyosauria_offspring_in_Vienna.jpg
Comparison: “Ichthyosaur vs dolphin” by Sceptic view – Own work. Licensed under CC BY-SA 4.0 via Commons – https://commons.wikimedia.org/wiki/File:Ichthyosaur_vs_dolphin.svg#/media/File:Ichthyosaur_vs_dolphin.svg
Icthyosaur Sampling: http://spinops.blogspot.com/2013/12/ichthyosaurs-of-lower-lias-of-england.html?view=flipcard (art by Nobu Tamura)