| SYSTEMATIC BIOLOGY |
|
| CLASS EODIAPSIDA+[i] |
Eodiapsida (e-o-di-AP-si-da) is derived from three Greek roots meaning "dawn animals with two arches" [dawn- early (έως); two- dyo (δύο); arches- apse (αψίδα)]. The name is a reference to the double fenestral arches in the early forms. Many reptilian groups had this type of skull, including the birds, dinosaurs, crocodilians, and lizards.
The eodiapsids are made up of two very different groups that might not be related at all. The two subclasses in this group are very different from each other. The Younginomorpha (named for a common genus, Youngina) were lizard-like animals that were carnivores or insectivores. They exploited terrestrial and aquatic environments, and one group evolved elongate ribs, which likely allowed them to glide.
The ichthyosaurs (from two Greek roots meaning "fish lizards") appeared in the early to mid Triassic as shallow water animals that resembled large lizards with legs modified slightly into four paddles, and a whip-like tail with a small fin. As the ichthyosaurs began to exploit other marine environments, their bodies became more fish-like and tear-drop shaped. True giants that attained lengths of up to 21 meters evolved by the end of the Triassic. The Jurassic Period, however, saw the greatest diversity of ichthyosaurs. Most had a long snout and a dorsal fin with a reverse heterocercal (the vertebral column bent down into the lower lobe of the tail; also called hypocercal) tail. These taxa began to exploit more open water and very deep waters. Evidence from fossilized stomach contents suggests that the diets of some species consisted chiefly of cephalopods, and likely they fed much like sperm whales do today (100-1,000 meters deep). In addition, they had very large eyes. The more surface-feeding taxa likely fed on fish. They began to decline at the beginning of the Cretaceous and were reduced to a single global genus by the mid Cretaceous. What led to their demise likely was a conjunction of evolutionary events that included the rise of teleost fishes and ambush predators like the plesiosaurs and mosasaurs. Also, the decline of the open water taxa seemed to be related to the rise of modern sharks.
| HIERARCHICAL TAXONOMY OF THE CLASS EODIAPSIDA+. The following descriptions come from Benton (2005). The structure of the following system is also based, in part, on the phylogenetic treatment by Mikko's Phylogeny Web (from the Finnish Museum of Natural History). Taxa in red are extinct. |
|
Petrolacosaurus (Left), an early lizard-like younginiform, and a dolphin-like ichthyosaur (Right). Drawing of Petrolacosaurus from: http://taggart.glg.msu.edu/isb200/carbfor.htm Drawing of an ichthyosaur from: http://www.museum.vic.gov.au/dinosaurs/images/mn005579_lg.gif CLASS EODIAPSIDA[i] (THE STEM DIAPSIDS) These
animals had a pair of temporal fenestrae, though a secondary loss of the lower
temporal fenestra ( a condition called euryapsid) seems to have occurred in the
ichthyosaurs. The basal diapsids, a
class that I call Eodiapsida, as defined here is based on Benton (2005), but the
relative placements of the taxa here are in question (Benton, 2005 and Laurin
and Gauthier, 2000). Because this
group has the sisters to the Archosauromorpha and to the Lepidosauromorpha, it
is necessarily paraphyletic, a condition typical of stem groups.
The alternative would be to place all three into a single class which
would include all living reptiles (except turtles), dinosaurs, and birds.
Such a diverse taxon would not be useful in a catalog and retrieval
sense. I have chosen to separate the
three groups. The inclusion of the
ichthyosaurs into this taxon is problematic and likely will be revised. The first diapsids appeared in the Pennsylvanian and the ichthyosaurs disappeared in the Cretaceous. SUBCLASS YOUNGINOMORPHA+ ORDER
ARAEOSCELIDIA Variable.
Some were aquatic and other terrestrial.
They had two temporal fenestrae and a suborbital fenestra that penetrated
the palate. The teeth were small,
sharp, and indicate that it was an insectivore.
Upper
Carboniferous (Pennsylvanian) Spinoaequalis, Petrolacosaurus ORDER
UNNAMED Very
unusual gliding animals. They had
elongate ribs that could be folded back when running.
Late
Permian Coelourosauravus ORDER
YOUNGINIFORMES This
group was successful in the upper Permian and lower Triassic.
They were lizard-like carnivores and insectivores.
Later forms developed the deep tail and paddle-like feet of semi-aquatic
reptiles. Upper
Permian to lower Triassic Youngina,
Petrolacosaurus SUBCLASS
ICHTHYOSAUROMORPHA (INCERTAE SEDIS)+ The origins of this group are unknown. They were euryapsids and aquatic. However, they were not closely related to the sauropterygians (members of the Lepidosauromorpha), which also have those characters. These dolphin-like animals lived through most of the Mesozoic with only subtle changes in their overall forms. The earliest ichthyosaurs had forelimbs and hind limbs of roughly the same size. Later taxa had larger forelimbs. Also, the snout tended to elongate and the eyes become larger (a later taxon had an eyeball that was 300mm in diameter). They had a fish-like tail with a reverse heterocercal caudal fin (the vertebral column bent downward). The feet had become modified as paddles with many phalanges, much like a whale. The giants of this group appeared in the upper Triassic (up to 15m long). Middle
Triassic to upper Cretaceous. BASAL Hupehsuchus. SUPERORDER
ICHTHYOPTERYGIA Thaisaurus, Isfjordosaurus, Parvinatator. SUPERORDER
EOICHTHYOSAURIA Grippia,
Chaohusaurus. ORDER
ICHTHYOSAURIA UNASSIGNED
GENERA Mikadocephalus, Shastasaurus, Wimanius,
Cymbospondylus. SUBORDER
MIXOSAURIA Phalarodon,
Mixosaurus. SUBORDER
MERRIAMOSAURIFORMES BASAL
FAMILIES Guanlingsaurus Besanosaurus Merriamosaurus. INFRAORDER
SHASTASAURIA Pessosaurus,
Pessopteryx, Besanosaurus, Himalayasaurus, Shastasaurus, Callawayia, Shonisaurus. INFRAORDER
EUICHTHYOSAURIA Qianichthyosaurus,
Toretocnemus. Californosaurus. INFRAORDER
PARVIPELIA Macgovania Hudsonelpidia. Suevoleviathan Leptonectes,
Eurhinosaurus. Temnodontosaurus INFRAORDER
THUNNOSAURIA Ichthyosaurs
that had a tuna-like caudal fin. Chacaicosaurus,
Mollesaurus, Stenopterygius. Ichthyosaurus. INFRAORDER
OPHTHALMOSAURIA Aegirosaurus, Nannopterygius, Simbirskiasaurus, Paraophthalmosaurus, Otschevia, Plutoniosaurus, Undorosaurus, Brachypterygius, Ophthalmosaurus, Caypullisaurus, Platypterygius. [i] I created this term to refer to the stem diapsids. The name literally means dawn diapsids. |
