Eusthenopteron

Eusthenopteron is a genus of prehistoric sarcopterygian (often called lobe-finned fishes) which has attained an iconic status from its close relationships to tetrapods. The name derives from two Greek stems—eustheno- "strength" and -pteron "wing" and thus "strongly developed fins".[1] Early depictions of this animal show it emerging onto land; however, paleontologists now widely agree that it was a strictly aquatic animal.[2] The genus Eusthenopteron is known from several species that lived during the Late Devonian period, about 385 million years ago. Eusthenopteron was first described by J. F. Whiteaves in 1881, as part of a large collection of fishes from Miguasha, Quebec.[3] Some 2,000 Eusthenopteron specimens have been collected from Miguasha, one of which was the object of intensely detailed study and several papers from the 1940s to the 1990s by paleoichthyologist Erik Jarvik.[4]

Eusthenopteron
Temporal range: Late Devonian, 385 Ma
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life restoration of Eusthenopteron foordi
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Clade: Eotetrapodiformes
Family: †Tristichopteridae
Genus: †Eusthenopteron
Whiteaves, 1881
Species
  • E. foordi (type)
  • E. jenkinsi
  • E. savesoderberghi

Description
In Late Devonian vertebrate speciation, descendants of pelagic lobe-finned fish—like Eusthenopteron—exhibited a sequence of adaptations: * Panderichthys, suited to muddy shallows; * Tiktaalik with limb-like fins that could take it onto land; * Early tetrapods in weed-filled swamps, such as: * Acanthostega which had feet with eight digits, * Ichthyostega with limbs. Descendants also included pelagic lobe-finned fish such as coelacanth species.
Reconstruction of Eusthenopteron
Head of Eusthenopteron (model)
Eusthenopteron foordi
Eusthenopteron foordi

Anatomically, Eusthenopteron shares many unique features in common with the earliest-known tetrapods. It shares a similar pattern of skull roofing bones with forms such as Ichthyostega and Acanthostega. Eusthenopteron, like other tetrapodomorph fishes, had internal nostrils, (or a choana) which are one of the defining traits of tetrapodomorphs (including tetrapods). It also had labyrinthodont teeth, characterized by infolded enamel, which characterizes all of the earliest known tetrapods as well. Like other fish-like sarcopterygians, Eusthenopteron possessed a two-part cranium, which hinged at mid-length along an intracranial joint. Eusthenopteron's notoriety comes from the pattern of its fin endoskeleton, which bears a distinct humerus, ulna, and radius (in the fore-fin) and femur, tibia, and fibula (in the pelvic fin). These appendicular long bones had epiphyseal growth plates that allowed substantial longitudinal growth through endochondral ossification, as in tetrapod long bones.[5] These six appendicular bones also occur in tetrapods and are a synapomorphy of a large clade of sarcopterygians, possibly Tetrapodomorpha (the humerus and femur are present in all sarcoptergyians). Similarly, its elasmoid scales lack superficial odontodes composed of dentine and enamel; this loss appears to be a synapomorphy with more crownward tetrapodomorphs.[6]

The earliest-known fossilised evidence of bone marrow has been found in Eusthenopteron, which may be the origin of bone marrow in tetrapods.[7]

Eusthenopteron differs significantly from some later Carboniferous tetrapods in the apparent absence of a recognized larval stage and a definitive metamorphosis.[8] In even the smallest known specimen of Eusthenopteron foordi (at 29 mm), the lepidotrichia cover all of the fins, which does not happen until after metamorphosis in genera like Polyodon. This might indicate that Eusthenopteron developed directly, with the hatchling already attaining the general body form of the adult (Cote et al., 2002).

See also

References
  1. http://www.merriam-webster.com/dictionary/Eusthenopteron
  2. M. Laurin, F. J. Meunier, D. Germain, and M. Lemoine 2007. A microanatomical and histological study of the paired fin skeleton of the Devonian sarcopterygian Eusthenopteron foordi. Journal of Paleontology 81: 143–153.
  3. Whiteaves, Joseph Frederick (1881). "On some remarkable fossil fishes from the Devonian rocks of Scaumenac Bay, in the Province of Quebec". Annals and Magazine of Natural History. 8 (44): 159–162. doi:10.1080/00222938109487434.
  4. Geological Survey of Canada (2008-02-07). "Past lives: Chronicles of Canadian Paleontology: Eusthenopteron - the Prince of Miguasha". Archived from the original on 2004-12-11. Retrieved 2009-02-10.
  5. M. Laurin, F. and J. Meunier 2012. A microanatomical and histological study of the fin long bones of the Devonian sarcopterygian Eusthenopteron foordi. Acta Zoologica 93: 88–97.
  6. Zylberberg, L., Meunier, F. J. and Laurin, M. 2010. A microanatomical and histological study of the postcranial dermal skeleton in the Devonian sarcopterygian Eusthenopteron foordi. Acta Palaeontologica Polonica 55: 459–470.
  7. Sanchez S, Tafforeau P and Ahlberg P E (2014) "The humerus of Eusthenopteron: a puzzling organization presaging the establishment of tetrapod limb bone marrow" Proceedings of the Royal Society B: Biological Sciences, 281 (1782): 20140299. doi:10.1098/rspb.2014.0299
  8. Schultze, H.-P. 1984. Juvenile specimens of Eusthenopteron foordi Whiteaves, 1881 (Osteolepiform rhipidistian, Pisces) from the Late Devonian of Miguasha, Quebec, Canada. Journal of Vertebrate Paleontology 4: 1-16.

Other references
  • S. Cote; R. Carroll; R. Cloutier; L. Bar-Sagi (September 2002). "Vertebral development in the Devonian Sarcopterygian fish Eusthenopteron foordi and the polarity of vertebral evolution in non-amniote tetrapods". Journal of Vertebrate Paleontology. 22 (3): 487–502. doi:10.1671/0272-4634(2002)022[0487:VDITDS]2.0.CO;2. ISSN 0272-4634.
  • R. Cloutier (1996). "Taxonomic review of Eusthenopteron foordi.". Devonian Fishes and Plants of Miguasha, Quebec, Canada. Dr. Friedrich Pfeil, München. pp. 487–502.
  • E. Jarvik (1980). Basic Structure and Evolution of Vertebrates. Academic Press.
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