Sarcopterygii (; ) is a clade (traditionally a class or subclass) of vertebrate animals which includes a group of bony fishes and all four legged land animals, including humans. The common name for this clade is lobe-finned fish.
Lobe-finned fish have muscular limb buds (lobes) within their fins, which are supported by appendicular skeletons. This is in contrast to the other clade of bony fishes, the Actinopterygii, which have only skin-covered bony spines supporting the fins.
Sarcopterygii emerged in the Silurian and early Devonian periods consisting entirely of aquatic species, but today the vast majority of its species are land animals called tetrapods. This mostly terrestrial clade of vertebrates are now recognized as having evolved from sarcopterygian ancestors and are most closely related to lungfishes. The lobed fins evolved into limbs, and foregut diverticulum eventually evolved into air-breathing lungs. These changes made it possible for tetrapods to explore and eventually permeate terrestrial ecosystems. While it is technically correct to include tetrapod species with Sarcopterygii, the phrase "lobe-finned fish" normally refers to the aquatic species that are not tetrapods.
Most species of lobe-finned fish are now extinct, but during the Carboniferous and Permian periods these non-tetrapod sarcopterygians were dominant predators of freshwater ecosystems. The entire clade suffered significant decline after the Permian–Triassic extinction event known as the Great Dying. It is estimated that at least two thirds of tetrapod species went extinct during this time. The lobe-finned fish were hit even harder. Today, the only living non-tetrapod sarcopterygians are the two species of coelacanths and six species of lungfishes.
Characteristics
thumb|253px|left|[[Guiyu oneiros, the earliest-known bony fish, lived during the Late Silurian, 419 million years ago).. Among the two groups of living species, the coelacanths and the lungfishes, the largest species is the West Indian Ocean coelacanth, reaching in length and weighing up . The largest lungfish is the marbled lungfish which can reach 2 m (6.6 ft) in length and weigh up to . The fin-limbs found in lobe-finned fishes like the coelacanths display a strong resemblance to the presumed ancestral form of tetrapod limbs. Lobe-finned fishes seemingly underwent two distinct evolutionary paths, leading to their classification into two clades: the Rhipidistia (comprising the Dipnoi, or lungfish, and the Tetrapodomorpha, which includes the Tetrapoda) and the Actinistia (represented by coelacanths).
The extensive fossil record and numerous morphological and molecular studies have shown that lungfish and some fossil lobe-finned fish ("rhipidistians") are more closely related to tetrapods than they are to coelacanths; as a result tetrapods are nested within Sarcopterygii. This abides to cladistics in that in order for a group to be valid, it must have an ancestral species and all descendants of that common ancestor based on shared characteristics. As such mammals, sauropsids (birds and "reptiles"), and amphibians are highly derived sarcopterygians despite superficially looking nothing like the standard lobe-finned fish anatomically speaking. However, similarities can be noticed in their limb bones and tooth enamel. Additionally, lungfish and tetrapods share a divided atrium.
Multiple Linnean classifications have been proposed with the explicit intent to incorporate Sarcopterygii as a monophyletic taxon instead of maintaining its traditional paraphyletic definition.
Ahlberg (1991)
Class Osteichthyes
- Subclass Actinopterygii
- Subclass Sarcopterygii
- Plesion Onychodontia
- Infraclass Actinistia
- Infraclass Rhipidistia
- Superdivision Tetrapodomorpha
- Plesion Rhizodontida
- Plesion Osteolepiformes
- Plesion Panderichthyidae
- Division Tetrapoda
- Superdivision Dipnomorpha
- Plesion Porolepiformes
- Plesion Powichthys
- Plesion Youngolepis
- Plesion Diabolepis
- Division Dipnoi
Nelson et al. (2016)
Class Osteichthyes
- Subclass Actinopterygii
- Subclass Sarcopterygii
- Infraclass Actinistia
- †Infraclass Onychodontida
- Infraclass Dipnomorpha
- †Infraclass Rhizodontida
- †Infraclass Osteolepidida
- †Infraclass Elpistostegalia
- Infraclass Tetrapoda
Betancur-Rodrigues et al. (2017)
Superclass Sarcopterygii
- Class Coelacanthimorpha
- Class Dipnotetrapodomorpha
- Subclass Dipnomorpha
- Subclass Tetrapodomorpha
Other classifications do not use Sarcopterygii as a ranked taxon but still reject traditional paraphyletic assemblages. In the scheme below, sarcopterygian groups are marked in bold letters.
Tedersoo (2017)
Phylum Craniata
- Class Cyclostomata
- Class Chondrichthyes
- Class Cladistia
- Class Actinopteri
- Class unspecified
- Class Ceratodontimorpha
- Class Amphibia
- Class Mammalia
- Class Sphenodontea
- Class Squamatea
- Class Testudinea
- Class Crocodylea
- Class Aves
Taxonomy
The classification below follows Benton (2004),
| image2 = Fishapods.svg
| alt2 =
| caption2 = 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.
thumb|Tooth from the sarcopterygian Onychodus from the [[Devonian of Wisconsin]]
Lobe-finned fishes and their sister group, the ray-finned fishes, make up the clade Osteichthyes, characterized by the presence of swim bladders (which share ancestry with lungs) as well as the evolution of ossified endoskeleton instead of cartilages like the skeletons of acanthodians, chondrichthyians and most placoderms. There are otherwise vast differences in fin, respiratory and circulatory structures between the Sarcopterygii and the Actinopterygii, such as the presence of cosmoid layers in the scales of sarcopterygians. The earliest sarcopterygian fossils were found in the uppermost Silurian, about 418 Ma. They closely resembled the acanthodians (the "spiny fish", a taxon that became extinct at the end of the Paleozoic). In the early–middle Devonian (416–385 Ma), while the predatory placoderms dominated the seas, some sarcopterygians came into freshwater habitats.
In the Early Devonian (416–397 Ma), the sarcopterygians, or lobe-finned fishes, split into two main lineages: the coelacanths and the rhipidistians. Coelacanths never left the oceans and their heyday was the late Devonian and Carboniferous, from 385 to 299 Ma, as they were more common during those periods than in any other period in the Phanerozoic.
Actinistians, a group within the lobe-finned fish, have been around for almost 380 million years. Over time, researchers have identified 121 species spread across 47 genera. Some species are well-documented in their evolutionary placement, while others are harder to track. The greatest boom in actinistian diversity happened during the Early Triassic, just after the Great Dying. The surviving tetrapods then underwent adaptive radiation on dry land and become the dominant terrestrial animals during the Carboniferous and the Permian periods.
Hypotheses for means of pre-adaptation
There are three major hypotheses as to how lungfish evolved their stubby fins (proto-limbs).
;Shrinking waterhole: The first, traditional explanation is the "shrinking waterhole hypothesis", or "desert hypothesis", posited by the American paleontologist Alfred Romer, who believed that limbs and lungs may have evolved from the necessity of having to find new bodies of water as old waterholes dried up.
Non-tetrapod sarcopterygians continued until towards the end of Paleozoic era, suffering heavy losses during the Permian–Triassic extinction event (251 Ma).
Phylogeny
The cladogram presented below is based on studies compiled by Janvier et al. (1997) for the Tree of Life Web Project,
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- Cloutier, R., & Forey, P. L. (1991). Diversity of extinct and living actinistian fishes (Sarcopterygii). In J. A. Musick, M. N. Bruton, & E. K. Balon (Eds.), The biology of Latimeriachalumnae and evolution of coelacanths (pp. 59–74). Springer Netherlands.