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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.
The terms "primitive" and "advanced", etc., are not properly used in referring to a species or an organism as any species or organism is a mosaic of primitive and derived traits. Using "primitive" and "advanced" may lead to "ladder thinking" (compare the Latin term scala naturae 'ladder of nature'), [ 8 ] which is the thought that all species ...
The type material of Najash is the only possible madtsoiid specimen retaining evidence of pelvic and hindlimb elements, which are claimed to be more plesiomorphic than other Cretaceous limbed snakes, such as Pachyrhachis, Haasiophis or Eupodophis, in retaining a sacro-iliac contact and well-developed limbs, with a huge and well-defined trochanter.
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.
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 ...
Estimated size, compared to a human. Animated restoration. Polcyn and Bell diagnose Dallasaurus as follows: "Small, plesiopedal mosasauroid possessing the following autapomorphies: posterior maxillary teeth strongly recurved posteriorly, slightly inflated at the crown and bearing only posterior carinae that is slightly offset laterally; atlas neural arch mediolaterally compressed but not ...
Omomyidae is a group of early primates that radiated during the Eocene epoch between about (mya). Fossil omomyids are found in North America, Europe & Asia, making it one of two groups of Eocene primates with a geographic distribution spanning holarctic continents, the other being the adapids (family Adapidae).
The pelvic girdle of Achillobator features plesiomorphic (primitive) saurischian characteristics compared to other dromaeosaurids. For instance, the pubis is aligned vertically and has a relatively large pubic boot (a wide expansion at the end), unlike most other dromaeosaurids, where a generally a much smaller boot is seen.