Endochondral bones ossify directly from an embryonic cartilaginous precursor. These are the bones we examine and measure in order to identify land mammal remains. We expect uniformity and consistency in morphology. There is, however, a second category of bone.
"Intermembranous bones form directly from the connective tissue late in embryological development and after birth through intramembranous ossification. Some intermembranous bones, such as the kneecap (patella), are almost always ossified in adult mammals (with minor exceptions). Other intermembranous bones, known as sesamoids, occur only in areas where a tendon passes over a joint, and ossify in irregular and unpredictable patterns.
"The number and shape of intermembranous bones vary greatly within the Mammalia, and are highly taxon-dependent. Humans have only one sesamoid (the pisiform) in the carpus. In many mammals, such bones include the patella and large sesamoids in the manus and pes.
"In ungulates, on the other hand, the only [relatively] large sesamoid element is the patella. The sesamoids in the manus or pes are small nodular ossifications in the digital flexor tendons, both at the metapodial-phalangeal joint and the distal interphalangeal joint; suids have as many as 13 sesamoids in the manus alone."
adapted from Kristina R. Raymond and Donald R. Prothero
Palaeo-Electronica (March 2010)
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