DESCRIPTION OF THE CLASS EODIAPSIDA+[i]
| EUKARYA> UNIKONTA> OPISTHOKONTA> ANIMALIA> BILATERIA> DEUTEROSTOMATA> VERTEBRATA> GNATHOSTOMATA> TETRAPODA> AMNIOTA> EODIAPSIDA |
CLASS EODIAPSIDA LINKS
| 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)]. We coined the name as 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. |
| INTRODUCTION TO THE EODIAPSIDA The basal diapsids, a class that we 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. We have chosen to separate the three groups. The inclusion of the ichthyosaurs into this taxon is problematic and likely will be revised. The eodiapsids are the earliest animals to have displayed a diapsid condition in the skull. That is, they appeared to have had two arches (one over the other) on the skull behind the orbit (Figure 1). The diapsid condition is common among amniotes and found in other classes like the Archosauromorpha and Lepidosauromorpha. Also, the secondary loss of the lower temporal fenestra, a condition called euryapsid, seems to have occurred in the ichthyosaurs. |
FIGURE 1. A stylized diapsid skull with two temporal fenestrae behind the orbit. The apparent holes are formed by two arches.
Image from Biodidac.
FIGURE 2. The relationship between the two groups that are merged here to create the Eodiapsida, a sister group to the Lepidosauromorpha and Archosauromorpha. In general, this figure is based on Benton (2005) and shows the relationship between this group and the other higher taxa of gnathostomes.
| YOUNGINOMORPHA The two subclasses in this group are very different from each other. The Younginomorpha (named for a common genus, Youngina) were lizard-like animals. The earliest taxa like Petrolacosaurus (Figure 3) from the upper Pennsylvanian were terrestrial animals with small, needle-like teeth, indicating insectivory. In the upper Permian, one group (e.g. Coelosauravus, Figure 4) evolved remarkably elongate ribs, which likely allowed them to glide. Later taxa like Spinoequalis of the lower Triassic, which was discovered only in 1995 (deBraga and Reisz. 1995) showed clear signs of an aquatic life-style with deep tails and paddle-like feet. The lizard-like Younginomorpha persisted into the lower Triassic when they became extinct (Falcon-Lang et al. 2007). |
| ICHTHYOSAUROMORPHA The ichthyosaurs (the name comes 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 (e.g. Utatsusaurus Figure 5). The origins of this group are unknown and the relationships between the Triassic and Jurassic taxa are uncertain (Motani et al. 1998, Nicholls and Manabe 2001). These dolphin-like animals lived through most of the Mesozoic with only subtle changes in their overall forms. Although they were euryapsids and aquatic, ichthyosaurs were not closely related to the sauropterygians (members of the Lepidosauromorpha), which also had those characters. As the ichthyosaurs began to exploit other marine environments, their bodies became more fish-like and tear-drop shaped. Furthermore, the feet became modified as paddles with many phalanges, much like a whale. The earliest ichthyosaurs had forelimbs and hind limbs of roughly the same size, but later taxa had larger forelimbs and reduced hind limbs. Also, the snout tended to elongate and the eyes become larger (a later taxon had an eyeball that was 300mm in diameter). True giants like Shonisaurus (Figure 6) evolved by the end of the Triassic (199-251) and attained lengths of up to 21 meters and may have represented a specialized offshoot. Not only were they enormous, but the adults seem to have been toothless (Motani 1997). How they might have lived and what their diets may have been are unresolved questions. The Jurassic Period (145-199) 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, similar to Ichthyosaurus (Figure 7), began to exploit more open water and very deep waters (Motani 2000). 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). The more surface-feeding taxa likely fed on fish. Ichthyosaurs began to decline at the beginning of the Cretaceous and were reduced to a single global genus, Platypterygius (Figure 8), 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 (Lingham-Soliar 1999). Also, the decline of the open water taxa seemed to be related to the rise of modern sharks. |
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| FIGURE 3. Petrolacosaurus, the earliest known diapsid. Image from: http://taggart.glg.msu.edu/isb200/carbfor.htm | FIGURE 4. Coelosauravus of the upper Permian evolved remarkably long ribs that articulated and could be extended as fixed wings for gliding. Image by: © Arthur Weasley; Creative Commons | FIGURE 5. Utatsusaurus of the early Triassic was one of the earliest known ichthyosaurs. Note the elongate body, paddle-like limbs, and the small fin on the tail. Image by: © Arthur Weasley; Creative Commons |
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| FIGURE 6. Shonisaurus of the upper Triassic was the largest known ichthyosaur with a maximum length of 21 meters. The animal was whale-like and may have been toothless as an adult. Image by: © Arthur Weasley | FIGURE 7. Ichthyosaurus, a typical Jurassic ichthyosaur, had evolved a hydrodynamic dolphin-like body and a broad hypocercal tail. The animal was built for speed and the relatively large eyes suggest that it fed at great depths. Drawing from: http://www.museum.vic.gov.au/dinosaurs/images/mn005579_lg.gif | FIGURE 8. Platypterygius, the last known ichthyosaur, lived in the Cretaceous. They had very large pectoral paddles, powerful tails, and streamlined bodies. Platypterygius grew to 7 meters long and hunted in the open ocean. Image by: Dimitry Bogdanov; Creative Commons |
| [i] We created this term to refer to the stem diapsids. The name literally means dawn diapsids. |
| LITERATURE CITED Benton, M. J. 2005. Vertebrate Paleontology. Third Edition. Blackwell Publishing, Malden, MA. deBraga, M. and R. R. Reisz. 1995. A new diapsid reptile from the uppermost Carboniferous (Stephanian) of Kansas. Palaeontology. 38: 199-212. Falcon-Lang, H. J., M. J. Benton, and M. Stimson. 2007. Ecology of early reptiles inferred from Lower Pennsylvanian trackways. Journal of the Geological Society, London. 164(6): 1113-1118. Laurin, M. and J. A. Gauthier. 2000. Diapsida. In: Maddison, D. R., ed. The Tree of Life Web Project. [http://tolweb.org/tree?group=Diapsida&contgroup=Amniota Lingham-Soliar, T. 1999. A functional analysis of the skull of Goronyosaurus nigeriensis (Squamata: Mosasauridae) and its bearing on the predatory behavior and evolution of the enigmatic taxon. N. Jb. Geol. Palaeont. Abh. 2134 (3): 355-74. Martill D. M. 1993. Soupy Substrates: A Medium for the Exceptional Preservation of Ichthyosaurs of the Posidonia Shale (Lower Jurassic) of Germany. Kaupia – Darmstädter Beiträge zur Naturgeschichte. 2: 77-97. Motani, R. 1997. Temporal and spatial distribution of tooth implantation in ichthyosaurs. In: J. M. Callaway and E. L. Nicholls, eds. Ancient Marine Reptiles. Academic Press. pp. 81–103. Motani, R., N. Minoura, and T. Ando. 1998. Ichthyosaurian relationships illuminated by new primitive skeletons from Japan. Nature. 393: 255-257. Motani, R. 2000. Is Omphalosaurus ichthyopterygian? –A phylogenetic perspective. Journal of Vertebrate Paleontology 20:295-301. Nicholls, E. L. and M. Manabe. 2001. A new genus of ichthyosaur from the Late Triassic Pardonet Formation of British Columbia: bridging the Triassic-Jurassic gap. Canadian Journal of Earth Sciences. 38: 983-1002. |
| By Jack R. Holt and Carlos A. Iudica. Last revised: 04/07/2013 |
