Transition from synapsid reptiles to mammals

Transition from synapsid reptiles to mammals - Transition...

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Transition from synapsid reptiles to mammals This is the best-documented transition between vertebrate classes. So far this series is known only as a series of genera or families; the transitions from species to species are not known. But the family sequence is quite complete. Each group is clearly related to both the group that came before, and the group that came after, and yet the sequence is so long that the fossils at the end are astoundingly different from those at the beginning. As Rowe recently said about this transition (in Szalay et al., 1993), "When sampling artifact is removed and all available character data analyzed [with computer phylogeny programs that do not assume anything about evolution], a highly corroborated, stable phylogeny remains, which is largely consistent with the temporal distributions of taxa recorded in the fossil record." Similarly, Gingerich has stated (1977) "While living mammals are well separated from other groups of animals today, the fossil record clearly shows their origin from a reptilian stock and permits one to trace the origin and radiation of mammals in considerable detail." For more details, see Kermack's superb and readable little book (1984), Kemp's more detailed but older book (1982), and read Szalay et al.'s recent collection of review articles (1993, vol. 1). This list starts with pelycosaurs (early synapsid reptiles) and continues with therapsids and cynodonts up to the first unarguable "mammal". Most of the changes in this transition involved elaborate repackaging of an expanded brain and special sense organs, remodeling of the jaws & teeth for more efficient eating, and changes in the limbs & vertebrae related to active, legs-under- the-body locomotion. Here are some differences to keep an eye on: # Early Reptiles Mammals 1 No fenestrae in skull Massive fenestra exposes all of braincase 2 Braincase attached loosely Braincase attached firmly to skull 3 No secondary palate Complete bony secondary palate 4 Undifferentiated dentition Incisors, canines, premolars, molars 5 Cheek teeth uncrowned points Cheek teeth (PM & 6 Teeth replaced continuously Teeth replaced once at most 7 Teeth with single root Molars double-rooted 8 Jaw joint quadrate- articular Jaw joint dentary- squamosal (*) 9 Lower jaw of several bones Lower jaw of dentary bone only 10 Single ear bone (stapes) Three ear bones (stapes, incus,
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malleus) 11 Joined external nares Separate external nares 12 Single occipital condyle Double occipital condyle 13 Long cervical ribs Cervical ribs tiny, fused to vertebrae 14 Lumbar region with ribs Lumbar region rib- free 15 No diaphragm Diaphragm 16 Limbs sprawled out from body Limbs under body 17 Scapula simple Scapula with big spine for muscles 18 Pelvic bones unfused Pelvis fused 19 Two sacral (hip) vertebrae Three or more sacral vertebrae 20 Toe bone #'s 2-3-4-5- 4 Toe bones 2-3-3-3-3 21 Body temperature variable Body temperature constant (*) The presence of a dentary-squamosal jaw joint has been arbitrarily selected as the defining trait of a mammal. Paleothyris
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This note was uploaded on 11/22/2011 for the course GLY GLY1100 taught by Professor Jaymuza during the Spring '10 term at Broward College.

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Transition from synapsid reptiles to mammals - Transition...

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