Definition of synapsid in US English:



  • A fossil reptile of a Permian and Triassic group, the members of which show increasingly mammalian characteristics and include the ancestors of mammals.

    Subclass Synapsida; includes the pelycosaurs and the therapsids

    • ‘These creatures succeeded the pelycosaurian synapsids as the rulers of the land, until they in turn were supplanted by the archosaurs during the early Triassic.’
    • ‘It has therefore been suggested that the mammalian class II gene clusters arose after the separation of the synapsids and the therapsids.’
    • ‘Conversely, if the fossil record does not accurately portray the first appearances of synapsids because preservation rates vary widely, then phylogenetic measures might yield a more reliable sequence of branching events.’
    • ‘The hypothesis that disparate groups of synapsids independently acquired mammallike characteristics has a long pedigree.’
    • ‘The possible parallel evolution of a jaw joint that allows extensive sliding in anomodonts is not unprecedented, because other synapsids independently evolved similar features.’
    • ‘In the dry late Permian environment many types of synapsids and reptiles flourished.’
    • ‘The early tetrapods of this time were amphibian-like animals that eventually gave rise to the reptiles and synapsids by the end of the Paleozoic.’
    • ‘Land amniotes continued to diversify, and by the middle Pennsylvanian had split into several taxa, two of which would go on to dominate the Mesozoic and Cenozoic: the diapsids and the synapsids.’
    • ‘As Rowe points out, the early cynodonts were the first synapsids in which the brain filled the endocranial cavity.’
    • ‘The temporal openings at the back of the skull that are shared by all synapsids were greatly enlarged, so that the remaining bone formed long arches.’
    • ‘The amniotes in turn have two main groups: the synapsids (including mammals) and the sauropsids (including reptiles and their fossil relatives).’
    • ‘Indeed, most workers recognized a geographical and temporal gap between Permo-Carboniferous ‘pelycosaurs’ and therapsid synapsids.’
    • ‘In reptiles and primitive synapsids, the right and left lower jaws are each made up of a number of bones, one of which is the dentary, or tooth-bearing, bone.’
    • ‘This is an example of convergent evolution; synapsids and archosaurs evolved these features independently.’
    • ‘The earliest occurring and phylogenetically most primitive synapsids are the ‘pelycosaurs’ of traditional terminology.’
    • ‘Tall neural spines are also a characteristic of early synapsids.’
    • ‘Assuming (likely, but not certain) that turtles diverged from Eureptilia after synapsids, there remains the problem of whether turtles have turbinals.’
    • ‘Dimetrodon is neither a reptile nor a mammal, but a basal synapsid - that is, an early relative of the ancestors of mammals.’
    • ‘The second major group of reptiles, the synapsids, have only a single temporal opening.’
    • ‘Paleozoic synapsids have been the focus of many large-scale treatments over the past century and their anatomy and interrelationships are well known.’


Early 20th century: from modern Latin Synapsida, from Greek sun- ‘together’ + apsis, apsid- ‘arch’.