Electronic supporting information for: Evolution of posture in amniotes–diving into the trabecular architecture of the femoral head

Extant amniotes show remarkable postural diversity. Broadly speaking,
limbs with erect (strongly adducted, more vertically oriented) posture are
found in mammals that are particularly heavy (graviportal) or show good
running skills (cursorial), while crouched (highly flexed) limbs are found
in taxa with more generalized locomotion. In Reptilia, crocodylians have a
“semi-erect” (somewhat adducted) posture, birds have more crouched limbs
and lepidosaurs have sprawling (well-abducted) limbs. Both synapsids and
reptiles underwent a postural transition from sprawling to more erect
limbs during the Mesozoic Era. In Reptilia, this postural change is
prominent among archosauriforms in the Triassic Period. However, limb
posture in many key Triassic taxa remains poorly known. In Synapsida, the
chronology of this transition is less clear, and competing hypotheses
exist. On land, the limb bones are subject to various stresses related to
body support that partly shape their external and internal morphology.
Indeed, bone trabeculae (lattice-like bony struts that form the spongy
bone tissue) tend to orient themselves along lines of force. Here, we
study the link between femoral posture and the femoral trabecular
architecture using phylogenetic generalized least squares. We show that
microanatomical parameters measured on bone cubes extracted from the
femoral head of a sample of amniote femora depend strongly on body mass,
but not on femoral posture or lifestyle. We reconstruct ancestral states
of femoral posture and various microanatomical parameters to study the
“sprawling-to-erect” transition in reptiles and synapsids, and obtain
conflicting results. We tentatively infer femoral posture in several
hypothetical ancestors using phylogenetic flexible discriminant analysis
from maximum likelihood estimates of the microanatomical parameters. In
general, the trabecular network of the femoral head is not a good
indicator of femoral posture. However, ancestral state reconstruction
methods hold great promise for advancing our understanding of the
evolution of posture in amniotes.