Patterns of Ossification in Southern Versus Northern Placental Mammals

Abstract

Consensus on placental mammal phylogeny is fairly recent compared to that for vertebrates as a whole. A stable phylogenetic hypothesis enables investigation into the possibility that placental clades differ from one another in terms of their development.Here, we focus on the sequence of skeletal ossification as a possible source of developmental distinctiveness in "northern" (Laurasiatheria and Euarchontoglires) versus "southern" (Afrotheria and Xenarthra) placental clades. We contribute data on cranial and postcranial ossification events during growth in Afrotheria, including elephants, hyraxes, golden moles, tenrecs, sengis, and aardvarks. We use three different techniques to quantify sequence heterochrony: continuous method, sequence-ANOVA (analysis of variance) and event-paring/Parsimov. We show that afrotherians significantly differ from other placentals by an early ossification of the orbitosphenoid and caudal vertebrae. Our analysis also suggests that both southern placental groups show a greater degree of developmental variability; however, they rarely seem to vary in the same direction, especially regarding the shifts that differ statistically. The latter observation is inconsistent with the Atlantogenata hypothesis in which afrotherians are considered as the sister clade of xenarthrans. Interestingly, ancestral nodes for Laurasiatheria and Euarchontoglires show very similar trends and our results suggest that developmental homogeneity in some ossification sequences may be restricted to northern placental mammals (Boreoeutheria). K E Y W O R D S :Afrotheria, development, evolution, heterochrony, ossification, phylogeny, skeletogenesis.Until the close of the 20th century, many high-level nodes in the placental mammal Tree of Life were not well understood. The last 15 years have changed this state of affairs dramatically. Although most individual orders and some interordinal groups have been accurately recognized based on comparative anatomy for over a century, few zoologists would have predicted the now-stable phylogeny consisting of four major clades: Afrotheria, Xenarthra, Laurasiatheria, and Euarchontoglires (Murphy et al. 2001). Certain elements of these clades have a long history in comparative anatomy, including elephant-sea cow-hyrax as afrotherians; primate-tree shrew-dermopteran and rodent-rabbit as euarchontoglires; sloth-armadillo-anteater as xenarthrans; pangolin-carnivoran and artiodactyl-whale as laurasiatheres. Other groups are quite novel to morphologists (e.g., tenrec-golden

Timing of cranial suture closure in placental mammals: Phylogenetic patterns, intraspecific variation, and comparison with marsupials

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Mammalian skull heterochrony reveals modular evolution and a link between cranial development and brain size

“…Microtomographic imaging allows documentation of the onset of individual bone ossification, a powerful marker for tracing perinatal anatomy. This approach has served to identify fundamental differences in postcranial osteogenesis between marsupials and placentals10, and unsuspected variation in placental development11. However, the ancestral patterns of craniogenesis timing and factors behind the cranial heterochrony remain largely unknown.…”

Prenatal allometric trajectories and the developmental basis of postcranial phenotypic diversity in bats (Chiroptera)

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