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Apomorphic and synapomorphic characteristics convey much information about evolutionary clades and can be used to define taxa. However, plesiomorphic and symplesiomorphic characteristics cannot. The term symplesiomorphy was introduced in 1950 by German entomologist Willi Hennig.
What counts as a synapomorphy for one clade may well be a primitive character or plesiomorphy at a less inclusive or nested clade. For example, the presence of mammary glands is a synapomorphy for mammals in relation to tetrapods but is a symplesiomorphy for mammals in relation to one another—rodents and primates, for example.
Trihecaton is an extinct genus of microsaur from the Late Pennsylvanian of Colorado.Known from a single species, Trihecaton howardinus, this genus is distinctive compared to other microsaurs due to possessing a number of plesiomorphic ("primitive") features relative to the rest of the group.
Facial flatness as a morphological clade feature has been rejected by many anthropologists since it is found on many early African Homo erectus fossils, and is therefore considered plesiomorphic, [50] but Wu has responded that the form of facial flatness in the Chinese fossil record appears distinct to other (i.e. primitive) forms. Toetik ...
Nexuotapirus shows both plesiomorphic and derived features compared to other early tapirs, making its exact relationship with them difficult to place. [3]Primitive traits of the genus include less molar-like premolars and incisive foramina that extend posteriorly through the postcanine diastema, as well as a braincase that tapers towards the back.
These features may be plesiomorphic ("primitive") for alligatorids. Culebrasuchus also has a straighter lower jaw than most other alligatorids, it lacks the ridges on the frontal bone between the eye sockets that are common among crocodylians, and the fourth tooth of the maxilla (rather than third, as in almost all other alligatorids) is the ...
Temnospondyli (from Greek τέμνειν, temnein 'to cut' and σπόνδυλος, spondylos 'vertebra') or temnospondyls is a diverse ancient order of small to giant tetrapods—often considered primitive amphibians—that flourished worldwide during the Carboniferous, Permian and Triassic periods, with fossils being found on every continent.
A few advanced archosauriforms reacquired the plesiomorphic ("primitive") state present in other reptiles, that being a short or absent posterodorsal process of the premaxilla, with the rear edge of the nares formed primarily by the maxilla bones instead. As for the nares themselves, they were generally large and oval-shaped, positioned high ...