Osteolepiform
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 The basal tetrapodomorph Tiktaalik.
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A cladistic classification which is a subgroup of the Sarcopterygian fishes.
[edit] History
Terrestrial vertebrates have a worldwide distribution. The earliest members of this group were moderately large (1-2.5 m body length). The oldest known skeletal remains of terrestrial vertebrates were found in the Upper Devonian of East Greenland (Clack, 1994). The presence of Lower to Middle Devonian trackways in Australia has led to suggestions that this group may have originated in the Lower Devonian, at least 400 million years ago (Warren et al., 1986), but digits are not visible in these impressions, so these trackways may have been left by other sarcopterygians.
The largest group of terrestrial vertebrates is Tetrapoda (see the section "Classification of Terrestrial Vertebrates", below). Tetrapoda means "four feet", and the group was so-named as its members primitively had four limbs, as opposed to fins. This taxon includes about 3000 extant species of amphibians (frogs, salamanders, and caecilians) and approximately 18100 extant species of amniotes (mammals, reptiles, and birds). The number of extinct species of tetrapods is of course unknown, but about half of the currently known species of tetrapods are extinct (Carroll, 1988).
Tetrapods originated no later than the Mississippian (about 350 million years ago), the period from which the oldest known relatives of living amphibians are known. Relatives of amniotes must have been present at that time, but they have not been found so far. The fossil previously reported (Smithson et al., 1994; Carroll, 1995) as an early Mississippian amniote or anthracosaur (Westlothiana) is probably either a stem-tetrapod or an early amphibian (Laurin, 1998a). Stegocephalians (see section on classification below for a definition of this group) originated no later than the Upper Devonian.
Tetrapods range from 9.8 mm (in the frog Psyllophryne didactyla) to 30 m (in the blue whale) in overall length. They have a worldwide distribution and inhabit all major habitats. Most are terrestrial, but several have returned to the aquatic environment in which our distant ancestors lived. Aquatic tetrapods include various salamanders (sirenids, cryptobranchids, proteids, etc.), frogs (pipids), some caecilians (typhlonectids), leatherback turtles, sea snakes, pinnipeds (seals and walruses), and whales. Some tetrapods are capable of flight (birds and bats), while others glide, such as flying squirrels, dermopterans (sometimes called "flying lemurs", even though they are not primates), and the flying dragons (Draco volans).
The page Life History of stegocephalians contains information on this complex topic.
The main breathing organ of most stegocephalians is the lung, but other respiratory organs exist in many groups. More detail is available on the Breathing in stegocephalians page.
Many stegocephalians have a tympanum for hearing high-frequence, air-borne sounds, and a lateral-line organ is found in many aquatic amphibians. For more information, see the Hearing in stegocephalians page.
[edit] Characteristics
Stegocephalians have an extensive fossil record (Carroll, 1988). Phylogenetic studies have revealed several derived characteristics (synapomorphies) of stegocephalians:
Loss of several cranial bones. In panderichthyids (the group of sarcopterygians most closely related to stegocephalians), the skull was rigidly linked to the shoulder girdle by several bones that disappeared early in the evolution of terrestrial vertebrates. The loss of these bones also allowed the appearance of a mobile neck that allows the head to be moved relative to the trunk. This decoupling allows the head to remain relatively stable while walking. Loss of the opercular bones that cover the gill chamber in bony fishes. The operculum was no longer needed in early choanates because they had lost the internal gills of their early ancestors. However, the operculum may have disappeared before the internal gills (Coates and Clack, 1991). A reduction of the notochord and a rigid spine. The vertebral centra of osteolepiforms are thin and surround the notochord (a rigid rod present in all chordates and that persists in man as the intervertebral disks) without constricting it greatly. In stegocephalians, the centra are thick and they constrict the notochord. Special articulatory surfaces (the zygapophyses) link the neural arches to each other. A shorter notochord that does not extend into the braincase. The notochord of osteolepiforms extended up to the vicinity of the pituitary. Four muscular limbs with discrete digits (fingers and toes). Osteolepiforms had fleshy fins with elements homologous to the humerus, radius, ulna, intermedium, ulnare, femur, tibia, fibula, and fibulare, but the homology of more distal limb elements is uncertain, and no digits were present. A sacral rib connecting the axial skeleton (the spine) to the pelvic girdle (the hip). This allows the weight of the body of tetrapods to be transmitted to the hind limb. There was no bony connection between the pelvic girdle of osteolepiforms and their axial skeleton. The loss of dermal fin rays (the modified scales that support the fins). This simply represents the elimination of a structure that was no longer needed and may even have been harmful on land. These characters did not appear simultaneously and suddenly. The oldest known stegocephalians, such as Ichthyostega and Acanthostega, possess intermediate conditions for some of these characters and lack others. For instance, Ichthyostega retained a subopercular, a bone that was part of the opercular complex that covered the gill chamber of osteolepiforms. Acanthostega retained an anocleithrum, which is one of the elements that linked the shoulder girdle to the skull in osteolepiforms (Coates and Clack, 1991). The notochord of Ichthyostega and Acanthostega extended deeply into the braincase, and most of its caudal vertebrae lacked zygapophyses (Jarvik, 1952). The connection between the sacral rib and the pelvic girdle of Acanthostega was still poorly defined. Finally, both Ichthyostega and Acanthostega retain lepidotrichia in the tail, indicating that these taxa still had a caudal fin.
The previous list includes only skeletal characters because all the earliest groups of stegocephalians are extinct, and soft anatomical characters can only be studied in extant taxa. The following characters are found in tetrapods, but not in other extant vertebrates:
A layer of dead, horny cells that reduces evaporative water loss. This layer is present in amniotes and in most lissamphibians. A well-developed muscular tongue with glands. However, some lissamphibians have only a primary tongue, like fishes. A primary tongue is simply a fleshy fold on the floor of the mouth that lacks intrinsic muscles and with limited mobility. A parathyroid gland involved in controlling the level of calcium in the blood. A Harderian gland located anterior to the eye. This gland secretes an oily liquid that lubricates the eye. A vomeronasal (Jacobson's) organ. This olfactory organ is located in the palate and is probably used to smell the food in the mouth. The loss of the internal gills. The external gills present in many aquatic and larval lissamphibians are new structures and are not homologous with the internal gills of fishes. It is difficult to determine exactly when these characters appeared because they are not preserved in fossils, except for indirect clues about the internal gills, and the closest known relatives of tetrapods are extinct. However, these characters are not found in lungfishes (the closest extant relatives of tetrapods). Acanthostega, a Devonian stegocephalian, still had internal gills (Coates and Clack, 1991), but no other stegocephalian is known to have had them. Therefore, internal gills were probably lost early in the evolution of stegocephalians, in the Devonian or the Mississippian (about 360 million years ago), and no tetrapod ever had internal gills.
[edit] Classification of Terrestrial Vertebrates
In the past, most terrestrial choanates were included in Tetrapoda (Gaffney, 1979). Recently, Tetrapoda was formally defined as a crown-group (Gauthier et al., 1989). A crown-group is a clade that includes the last common ancestor of two or more extant taxa, and all its descendants. In this case, Tetrapoda was defined as the clade that includes the last common ancestor of lissamphibians and amniotes, and all its descendants.
According to Gauthier et al. (1989), Tetrapoda included most known fossil terrestrial vertebrates because temnospondyls were thought to be the stem-group of amphibians, whereas embolomeres, gephyrostegids, and seymouriamorphs were thought to be more closely related to amniotes than to lissamphibians. Therefore, only a few very early terrestrial vertebrates, such as Ichthyostega and Acanthostega, were excluded from Tetrapoda.
The choanate phylogeny presented here suggests that temnospondyls, embolomeres, gephyrostegids, and seymouriamorphs are not part of the crown-group. If it is accurate, these taxa are not tetrapods and the origin of the "tetrapod limb" predates the origin of Tetrapoda. In the first versions of this page, all sarcopterygians bearing digits were simply called terrestrial vertebrates because no formal phylogenetic taxonomy of this whole clade had been proposed. Such a taxonomy was recently published (Laurin, 1998a). The taxon Stegocephali (that included for a long time all the vertebrates bearing a chiridium, except for lissamphibians and amniotes) was defined as all choanates more closely related to Temnospondyli than to Panderichthys (the closest relative of tetrapods known to have retained paired fins). Therefore, Stegocephali includes all taxa that bear digits, and a few (Elginerpeton, Metaxygnathus, Ventastega, and Hynerpeton) that may retain paired fins. Contrary to the old usage of this term, Stegocephali now refers to a clade. The term stegocephalian will be used below because it does not convey the hypothetical and probably somewhat erroneous interpretation that all digit-bearing vertebrates are terrestrial. As explained below, the earliest members of that clade were probably primitively aquatic.
Note about the node marked Amphibia: Others restrict the name Amphibia to descendents of the most recent common ancestor of extant amphibians (the terminal taxon Living Amphibians in this tree). The author prefers the usage indicated in the tree above, and the definition of Amphibia as all tetrapods more closely related to extant amphibians than to amniotes has historical precedence (Laurin, 1998a), but the second usage has been fairly widespread and cannot be ignored. In this page, the term Amphibia always refers to the node indicated in the tree above, and extant amphibians are referred to as lissamphibians.
Note about the node marked Tetrapoda: Others expand the use of this name to include all vertebrates that possess limbs with digits (Laurin and Anderson, 2004). However, in this page, Tetrapoda always refers to the node indicated in the tree above.
