53.4 Saturday, Jan. 5 Osteohistological differences between marsupials and placental mammals reflect both growth rates and life history strategies WERNING, S; Univ California, Berkeley email@example.com
Bone microstructure is influenced by many factors, including body size, growth rate, and phylogeny. The literature acknowledges no great differences between marsupial and placental bone histology, leading some to infer a common histological signature for therian mammals. Histological similarity is reasonable for small marsupials and placentals (< ~40g ), which have similar growth rates and durations, but larger marsupials grow at lower rates and delay epiphyseal fusion for several years compared to placentals of similar body size and ecology. Given these growth differences, larger marsupials should show histological evidence of extended slow growth, contrasting the fast-growing bone tissues described for placentals. However, the mammalian osteohistological sample is biased toward placentals of economic importance, and only two marsupials have been usefully described. I sampled the mid-diaphyseal femora of 42 extant and extinct marsupial species, as well as afrotherian, xenarthran, and laurasiatherian placentals. My marsupial sample encompasses all extant orders, spans a 10g-2500kg size range, and comprises mainly wild-caught animals. Small therians do show a common histology of nearly avascular lamellar bone. Marsupials >50g typically produce well-vascularized woven bone early in life, but after 1-2 years deposit poorly vascularized lamellar bone for several years. This pattern also occurs in afrotheres (except elephants), xenarthrans, Solenodon, and bats; but differs from those of the large-bodied ungulates (exclusively well-vascularized woven bone) and primates (heavily remodeled bone) that dominate the literature. I propose that the first condition is plesiomorphic for therians, and that sampling biases have obscured both size and phylogenetic signals in the distribution of mammalian bone growth patterns.