tetrapod

/te"treuh pod'/, n.

1. any vertebrate having four limbs or, as in the snake and whale, having had four-limbed ancestors.

2. an object, as a caltrop, having four projections radiating from one central node, with each forming an angle of 120° with any other, so that no matter how the object is placed on a relatively flat surface, three of the projections will form a supporting tripod and the fourth will point directly upward.

adj.

3. having four limbs or descended from four-limbed ancestors.

[1820-30; < NL tetrapodus < Gk tetrapod- (s. of tetrápous) four-footed. (see TETRA-, -POD) + NL -us adj. suffix]

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▪ animal

      a superclass of animals (animal) that includes all limbed vertebrates (vertebrate) (backboned animals) constituting the classes Amphibia (amphibians (amphibian)), Reptilia (reptiles (reptile)), Aves (birds (bird)), Mammalia (mammals (mammal)), and their direct ancestors that emerged between roughly 365 million and 390 million years ago during the Devonian Period. In a strict evolutionary sense, all tetrapods are essentially “limbed fish,” because their ultimate vertebrate ancestor is a fish. All tetrapods share a variety of morphological features. These include a pair of bones (bone) (the ulna and radius and the tibia and fibula) in the epipodial segments of the forelimbs and hind limbs, digits on the end of each limb, an oval window (fenestra ovalis) in the skull opening into the middle ear, a stapes (ear bone), and several other skeletal features. Undoubtedly, the early tetrapods also shared unique physiological, behavioral, and soft anatomical features; however, only skeletal features are preserved in the fossil record and thus are used for classification.

      There is near universal agreement that tetrapods originated somewhere within the fleshy-finned or lobed-finned fishes (Sarcopterygii), although total agreement does not exist on which sarcopterygian group is ancestral to them. The difficulty in deciding tetrapod ancestry stems from the inability to determine conclusively which traits are ancestral and which traits arose after one group diverged from another. Furthermore, the diversity of skeletal anatomies among the early tetrapods confuses this issue; when comparing the skeletal features of one group with those of another, it is unclear whether the comparison is between the same elements or ones that appear the same but arose from different ancestral structures. Nevertheless, Ventastega curonica is the first creature whose limb and skull anatomy share most of the features characteristic of early tetrapods. Fossil fragments of V. curonica—which included parts of a pelvis, a shoulder girdle, and a braincase—have been unearthed in Latvia and dated to 365 million years ago.

George R. Zug

 

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