Elpistostegalia Explained

Elpistostegalia is a clade containing Panderichthys and all more derived tetrapodomorph taxa. The earliest elpistostegalians, combining fishlike and tetrapod-like characters, such as Tiktaalik, are sometimes called fishapods. Although historically Elpistostegalia (referred to as Panderichthyida) was considered an order of prehistoric lobe-finned fishes, it was cladistically redefined to include tetrapods.^[1]

Paleobiology

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.^[2] Several groups evolved to fill these niches, the most successful were the elpistiostegalians. In such environments, they would have been challenged by periodic oxygen deficiency.^[3] 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.^{[4] [5]}

Being shallow-water fishes, the elpistostegalians evolved many of the basic adaptions that later allowed the tetrapods to become terrestrial animals. The most important ones were the shift of main propulsion apparatus from the tail fin to the pectoral and pelvic fins, and a shift to reliance on lungs rather than gills as the main means of obtaining oxygen.^[6] Both of these appear to be a direct result of moving to an inland freshwater mode of living.^[7]

Fossils of Panderichthys are dated to the Givetian (around 385 million years old) or the Frasnian (around 380–375 million years old^[8]). Thus, the oldest bone remains of elpistostegalians appear in the late Middle Devonian or in the early Late Devonian. However, a series of trace fossils from the early Middle Devonian of Poland suggests that tetrapod-limbed animals may have existed as early as in the Eifelian, around 395 million years ago.

Traits

Paleontologist and professor Per E. Ahlberg has identified the following traits as synapomorphic for Elpistostegalia (and thus Tetrapoda):

• The endocranium is hinged, the hinge forming the profundus nerve foramen. The cranial kinesis is also visible in the skull roof, between the parietal bones and the postparietal bones.
• A rather small shoulder girdle is present.
• The anal and posterior dorsal fin supported by a basal plate and three unjointed radials.
• The pectoral fin skeleton is composed of bones homologous to the tetrapod humerus, ulna, and radius, followed by a host of smaller bones anchoring the fin rays; the pelvic fin skeleton similarly has femur, tibia, and fibula.

Phylogeny

The name, originally coined around the genus Elpistostege, later become a synonym for Panderichthyida.^[6] In most analyses, the group as traditionally imagined is actually an evolutionary grade, the last "fishes" of the tetrapod stem line, though Chang and Yu (1997) treated them as the sister clade to Tetrapoda.^{[9] [10]} Elpistostegalia was re-defined as a clade containing Panderichthys and tetrapods.^[1]

Below is a cladogram from Swartz, 2012.^[1]

The 2020 study by Cloutier et al. revealed that the paired fins of Elpistostege contained bones homologous to the phalanges (digit bones) of modern tetrapods. The analysis carried out in this study recovered Elpistostege as the sister taxon of all unequivocally digited vertebrates.^[11]

Notes and References

1. Swartz . B. . 2012 . A marine stem-tetrapod from the Devonian of Western North America . PLOS ONE . 22448265 . 7 . 3 . 3308997 . e33683 . 10.1371/journal.pone.0033683 . 2012PLoSO...733683S . free .
2. Dahl TW, Hammarlund EU, Anbar AD . 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 . October 2010 . 20884852 . 2964239 . 10.1073/pnas.1011287107 . 2010PNAS..10717911D . etal. free .
3. Lewis . William M. Jr. . Morphological Adaptations of Cyprinodontoids for Inhabiting Oxygen Deficient Waters . Copeia . 1 June 1970 . 2 . 2 . 319–326 . 1441653 . 10.2307/1441653 . dead . https://web.archive.org/web/20110813153431/http://ciresweb.colorado.edu/limnology/pubs/pdfs/Pub001.pdf . 2011-08-13 .
4. 10.1111/j.1502-3931.1990.tb01357.x . Long, J.A. . Heterochrony and the origin of tetrapods . Lethaia . 23 . 2 . 157–166 . 1990 .
5. Modifications of the Digestive Tract for Holding Air in Loricariid and Scoloplacid Catfishes. Copeia. 1998. 1998. 3. 663–675. 25 June 2009. 10.2307/1447796 . 1447796. Armbruster. Jonathan W..
6. 10.1086/425183 . Gordon, M.S. . Long, J.A. . The Greatest Step In Vertebrate History: A Paleobiological Review of the Fish-Tetrapod Transition . Physiological and Biochemical Zoology . 77 . 5 . 700–719 . 2004 . 15547790. 1260442 .
7. 10.1111/j.1096-3642.1998.tb02526.x . Ahlberg, P.E. . Postcranial stem tetrapod remains from the Devonian of Scat Craig, Morayshire, Scotland . . 122 . 1–2 . 99–141 . 1998 . free .
8. Web site: Clack . J. A. . 2005 . Getting a Leg Up on Land . 28 March 2024 . Scientific American.
9. Ahlberg, P.E . Johanson, Z. . Osteolepiforms and the ancestry of tetrapods . . 395 . 6704 . 792–4 . 1998 . 10.1038/27421 . 1998Natur.395..792A . 4430783 . 2011-03-12 . https://web.archive.org/web/20141124103604/http://www.biology.ualberta.ca/courses.hp/biol606/papers/Ahlberg+1998.pdf . 2014-11-24 . dead .
10. Chang, M.-M. . Yu, X. . Reexamination of the relationship of Middle Devonian osteolepids–fossil characters and their interpretations . American Museum Novitates . 3189 . 1–20 . 1997 .
11. Cloutier . R. . Clement . A. M. . Lee . M. S. Y. . Noël . R. . Béchard . I. . Roy . V. . Long . J. A. . Elpistostege and the origin of the vertebrate hand . Nature . 2020 . 579 . 7800 . 549–554 . 10.1038/s41586-020-2100-8. 32214248 . 2020Natur.579..549C . 213171029 .