The Stem Tetrapoda are a cladistically defined group, consisting of all animals more closely related to extant four-legged vertebrates than to their closest extant relatives (the lungfish ), but excluding the crown group Tetrapoda . They are thus paraphyletic , though acceptable in phylogenetic nomenclature as the group is defined by strict reference to phylogeny rather than to traits as in traditional systematics. Thus, some finned sarcopterygians are considered to be stem tetrapods.
25-413: Acanthostega (meaning "spiny roof") is an extinct genus of stem-tetrapod , among the first vertebrate animals to have recognizable limbs . It appeared in the late Devonian period ( Famennian age) about 365 million years ago, and was anatomically intermediate between lobe-finned fishes and those that were fully capable of coming onto land. The fossilized remains are generally well preserved, with
50-405: A crown group , or as an apomorphy-based group, using the limb with digits), making the actual content of the group uncertain. [REDACTED] [REDACTED] [REDACTED] [REDACTED] [REDACTED] [REDACTED] Suture (anatomy) In anatomy , a suture is a fairly rigid joint between two or more hard elements of an organism, with or without significant overlap of
75-655: A lot less differences from juveniles to adults than the latter, it has been suggested that Acanthostega might be descended from a neotenic lineage. Although it appears to have spent its whole life in water, its humerus also exhibits traits that resemble those of later, fully terrestrial stem-tetrapods (the humerus in Ichthyostega being somewhat derived from, and homologous with the pectoral and pelvic fin bones of earlier fishes). This could indicate that vertebrates evolved terrestrial traits earlier than previously assumed, and numerous times independently from another. Muscle scars on
100-521: A sequence of adaptations: Panderichthys , suited to muddy shallows; Tiktaalik with limb-like fins that could take it onto land; stem-tetrapods in weed-filled swamps, such as Acanthostega , which had eight-digited feet; and Ichthyostega , with full limbs. Their descendants also included pelagic lobe-finned fish such as coelacanth species. It has been inferred that Acanthostega probably lived in shallow, weed-choked swamps, its legs apparently being adapted for these specific ecosystems. Apart from
125-399: A suture, and the joint bends around a peg on the astragalus, which fits into a socket in the calcaneum . The shells of most molluscs are made of calcium carbonate (the main constituent of limestone and chalk ), and of conchiolin , a protein. For more information, see Mollusc shell . In cephalopod mollusks , which have external shells (e.g. Nautilus , ammonites ), the shell
150-436: Is divided into compartments by septa (partitions). The septa are joined to the external shell by sutures formed by repeated invagination (they interlock like pieces of a jigsaw puzzle ). The sutures are visible from the outside and often form complex and elaborate patterns. Nearly all snail shells (except for the shells of limpets , abalone , sea hares , etc.) can be visualized as a tube of increasing diameter, closed at
175-472: Is divided into three major sections: a cephalon (head section) with eyes, mouthparts and sensory organs such as antennae; a thorax of multiple segments which are similar to each other; and a pygidium, or tail section. In many species, the cephalon had sutures running from back to front round the outside edges of the eyes. These sutures divided the cephalon into three pieces. The sutures in trilobites' cephalons were unusual because it seems their main function
200-422: The apex of the shell to the aperture ; this line is the suture. Details of the suture are often useful in discriminating one species from another, for example, sometimes the suture is channeled. The suture also provides a sort of geographic marker from which one can refer to the positioning of patterning or sculpture , where that is relevant: for example some species have a darker or lighter subsutural band on
225-454: The pectoral girdle to the pelvic girdle. There are many morphological changes that allowed the pelvic girdle of Acanthostega to become a weight-bearing structure. In more ancestral states the two sides of the girdle were not attached. In Acanthostega there is contact between the two sides and fusion of the girdle with the sacral rib of the vertebral column. These fusions would have made the pelvic region more powerful and equipped to counter
250-536: The elements. Sutures are found in the skeletons or exoskeletons of a wide range of animals, in both invertebrates and vertebrates . Sutures are found in animals with hard parts from the Cambrian period to the present day. Sutures were and are formed by several different methods, and they exist between hard parts that are made from several different materials. The skeletons of vertebrate animals (fish, amphibians, reptiles, birds, and mammals) are made of bone , in which
275-467: The famous fossil by which the significance of this species was discovered being found by Jennifer A. Clack in East Greenland in 1987, though fragments of the skull had been discovered in 1933 by Gunnar Säve-Söderbergh and Erik Jarvik . The 60 cm (24 in) Acanthostega had eight digits on each hand (the number of digits on the feet is unclear) linked by webbing, it lacked wrists, and
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#1732876621231300-473: The force of gravity when not supported by the buoyancy of an aquatic environment. It had internal gills that were covered like those of fish. It also had lungs, but its ribs were too short to support its chest cavity out of water. Acanthostega is seen as part of widespread evolutionary radiation in the late Devonian period, starting with purely aquatic finned tetrapodomorphs, with their successors showing increased air-breathing capability and related adaptions to
325-472: The forelimbs of Acanthostega were similar to those of crown-tetrapods, suggesting that it evolved from an ancestor that had more terrestrial adaptations than itself. A histological study of Acanthostega humeri, assisted by synchrotron scans, indicates that the animal matured slowly. Some individuals reached sexual maturity (based on a fully ossified humerus) at more than six years of age, and adult fossils are much rarer than juveniles. Late ossification of
350-423: The humerus supports a fully aquatic lifestyle for Acanthostega . There is barely any correlation between humerus size and maturity, suggesting that there was significant size variation among individuals of the same age. This may be due to competitive pressures, differing adaptive strategies, or even sexual dimorphism . However, the small sample size prevents recognition of a bimodal distribution which could confirm
375-470: The jaws and gills, as well as more muscular neck allowing freer movement of the head than fish have, and use of the fins to raise the body of the fish. These features are displayed by the earlier Tiktaalik , which like Ichthyostega showed signs of greater abilities to move around on land, but is thought to have been primarily aquatic. In Late Devonian vertebrate speciation, descendants of pelagic lobe-finned fish –like Eusthenopteron – exhibited
400-577: The latter hypothesis. [REDACTED] [REDACTED] [REDACTED] [REDACTED] [REDACTED] [REDACTED] Stem tetrapoda Stem tetrapods are members of Tetrapodomorpha , the total group and clade that also includes their descendants, the crown tetrapods: The stem Tetrapoda encompass three distinct grades successively closer to crown group Tetrapoda: Both Ichthyostegalia and Labyrinthodontia constitute paraphyletic evolutionary grades rather than clades, with amniotes and modern amphibians branching off at some point from
425-417: The latter. The stem tetrapods may also include one or both of Temnospondyli and Lepospondyli , depending on author. This is due to the uncertain origin of the modern amphibians , whose position in the phylogenetic tree dictates what lineages go in the crown group Tetrapoda. Neither is there for the moment a consensus of the phylogeny of stem tetrapods, nor how Tetrapoda itself should be defined (i.e. as
450-410: The main rigid ingredient is calcium phosphate . The skulls of most vertebrates consist of sets of bony plates held together by cranial sutures . These sutures are held together mainly by Sharpey's fibers which grow from each bone into the adjoining one. In the type of crurotarsal ankle, which is found in crocodilians and some other archosaurs , the astragalus is fixed to the tibia by
475-484: The presence of limbs, it was not adapted in any way for walking on land. Jennifer A. Clack interprets this as showing that Acanthostega was primarily an aquatic animal descended from fish that never left the sea, and that the specializations of the tetrapod lineage were exaptations : features which would later be useful for terrestrial life, even if they originated for a different purpose. At that period, deciduous plants were flourishing and annually shedding leaves into
500-449: The shell. When an angulation of the whorls occurs, the space between it and the suture above it (i.e. the abaxial edge of the sutural ramp) constitutes the area known as the "shoulder" of the shell. The shoulder angle may be simple or keeled, and may sometimes have nodes or spines A trilobite's carapace consisted of calcite and calcium phosphate deposited on a lattice (framework) of chitin (a polysaccharide ). The trilobite body
525-408: The small end, and spirally wrapped around a central axis. For more information, see Gastropod shell . Each complete rotation of this spirally-arranged tube is called a whorl . The whorls of a snail shell usually overlap one another, forming a spire . Where the whorls overlap, there is usually a clear (if narrow) indentation. This indentation forms a visible line, which is continuous and reaches from
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#1732876621231550-489: The water to feeding with the head above water or on land. Research based on analysis of the suture morphology in the skull of Acanthostega indicates that the species was able to bite prey at or near the water's edge. Markey and Marshall compared the skull with the skulls of fish, which use suction feeding as the primary method of prey capture, and creatures known to have used the direct biting on prey typical of terrestrial animals . Their results indicate that Acanthostega
575-435: The water, attracting small prey into warm oxygen-poor shallows that were difficult for larger fish to swim in; Clack remarks on how the lower jaw of Acanthostega shows a change from those of fish that have two rows of teeth, with a large number of small teeth in the outer row, and two large fangs and some smaller teeth in the inner row. This difference likely corresponds to a shift in stem-tetrapods from feeding exclusively in
600-477: Was adapted for what they call terrestrial-style feeding, strongly supporting the hypothesis that the terrestrial mode of feeding first emerged in aquatic animals. If correct, this shows an animal specialized for hunting and living in shallow waters in the line between land and water. While normally considered more basal than Ichthyostega , it is possible that Acanthostega was actually more derived. Since Acanthostega resembles juvenile Ichthyostega and shows
625-420: Was generally poorly adapted for walking on land. It also had a remarkably fish-like shoulder and forelimb. The front limbs of Acanthostega could not bend forward at the elbow, and therefore could not be brought into a weight bearing position, appearing to be more suitable for paddling or for holding on to aquatic plants. Acanthostega is the earliest stem-tetrapod to show the shift in locomotory dominance from
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