Elpistostegalia

Elpistostegalia
Temporal range: 385–374 Ma
Late Devonian
Panderichthys
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Subphylum: Vertebrata
Superclass: Osteichthyes
Class: Sarcopterygii
Superorder: Osteolepiformes
Order: †Elpistostegalia
Camp & Allison, 1961
Familia

Elpistostegalia or Panderichthyida is an order of prehistoric lobe-finned fishes which lived during the Late Devonian period (about 385 to 374 million years ago).[1] They represent the advanced tetrapodomorph stock, the fishes more closely related to tetrapods than the osteolepiform fishes. The elpistostegalians, combining fishlike and tetrapod-like characters, are sometimes called fishapods, a phrase coined for the advanced elpistostegalian Tiktaalik.

Contents

Palaeobiology of the elpistostegalians

The elpistostegalids were shallow-water fishes. Many of the basic adaptions that later allowed the tetrapods to become terrestrial animals took place in the group. The most important ones were the shift of main propulsion apparatus from the tailfin to the pectoral and pelvic fins, and a shift to reliance on lungs rather than gills as the main means of obtaining oxygen.[2] Both of these appear to be a direct result of moving to an inland freshwater mode of living.[3]

A rise in global oxygen content allowed for the evolution of large, predatory fish that were able to exploit the shallow tidal areas and swamplands as top predators.[4] Such environments would have had periodic oxygen deficiency.[5] In comparable modern aquatic environments like shallow eutrophic lakes and swampland, modern lungfish and some genera of catfish also rely on the more stable, atmospheric source of oxygen.[6][7]

The elpistostegalids gave rise to the labyrinthodonts in the Eifelian (early middle Devonian) around 395 million years ago. While the early tetrapods flourished and diversified over the next 30 million years, the elpistostegalians disappear from the fossil record fairly quickly in the early Frasnean around 380 million years ago, leaving the labyrinthodonts the sole survivors of their line.[8]

Traits

Professor Per Ahlberg has identified the following traits as synapomorphic for Elpistostegalia (and thus Tetrapoda):[9]

Phylogeny

The name, originally coined around the genus Elpistostege has become a synonym for Panderichthyida.[2] In most analysis the group is an evolutionary grade, the last "fishes" of the tetrapod stem line, though Chang and Yu (1997) treat them as the sister clade to Tetrapoda.[9][10]

The elpistostegalians encompasses a number of well known transitional fossils such as Tiktaalik, Panderichthys and Elginerpeton.

References

  1. ^ Elpistostegidae on the Taxonomicon
  2. ^ a b Gordon, M.S.; Long, J.A. (2004). "The Greatest Step In Vertebrate History: A Paleobiological Review of the Fish-Tetrapod Transition" (PDF). Physiological and Biochemical Zoology 77 (5): 700–719. doi:10.1086/425183. PMID 15547790. http://usf.usfca.edu/fac_staff/dever/tetrapod_review.pdf. 
  3. ^ Ahlber, P.E. (1998). "Postcranial stem tetrapod remains from the Devonian of Scat Craig, Morayshire, Scotland". Zoological Journal of the Linnean Society 122: 99–141. doi:10.1111/j.1096-3642.1998.tb02526.x. 
  4. ^ Dahl TW, Hammarlund EU, Anbar AD, et al. (October 2010). "Devonian rise in atmospheric oxygen correlated to the radiations of terrestrial plants and large predatory fish". Proc. Natl. Acad. Sci. U.S.A. 107 (42): 17911–5. doi:10.1073/pnas.1011287107. PMC 2964239. PMID 20884852. http://www.pnas.org/content/107/42/17911.full. 
  5. ^ Lewis Jr., William M.. "Morphological Adaptations of Cyprinodontoids for Inhabiting Oxygen Deficient Waters" (PDF). Copeia 2: 319–326. JSTOR 1441653. http://ciresweb.colorado.edu/limnology/pubs/pdfs/Pub001.pdf. 
  6. ^ Long, J.A. (1990). "Heterochrony and the origin of tetrapods". Lethaia 23 (2): 157–166. doi:10.1111/j.1502-3931.1990.tb01357.x. 
  7. ^ "Modifications of the Digestive Tract for Holding Air in Loricariid and Scoloplacid Catfishes" (PDF). Copeia (3): 663–675. 1998. http://www.auburn.edu/academic/science_math/res_area/loricariid/fish_key/Air.pdf. Retrieved 25 June 2009. 
  8. ^ Niedźwiedzki & al. (2010): Tetrapod trackways from the early Middle Devonian period of Poland. Nature no 463, pp 43–48 DOI: 10.1038/
  9. ^ a b Ahlberg, P.E; Johanson, Z. (1998). "Osteolepiforms and the ancestry of tetrapods" (PDF). Nature 395 (6704): 792–4. Bibcode 1998Natur.395..792A. doi:10.1038/27421. http://www.biology.ualberta.ca/courses.hp/biol606/papers/Ahlberg+1998.pdf. 
  10. ^ Chang, M.-M.; Yu, X. (1997). "Reexamination of the relationship of Middle Devonian osteolepids–fossil characters and their interpretations". American museum novitates (3189): 1–20.