Reconstructing Locomotor Behaviors: Cross‐sectional Property Analysis Brings More to the Story of How Earliest Euprimates Moved

Abstract

As a group, modern primates demonstrate an impressive degree of ecological and behavioral flexibility. It is likely that the earliest euprimates were similarly diverse in how they moved through their environments. Attempts to reconstruct the locomotor behaviors of Eocene primates have relied heavily on comparing the external morphologies of fossilized limb bones with those of extant taxa. Though broadly informative, the application of this method may be limited by the phylogenetic constraints associated with bone length and articular surface size and shape. Alternatively, the internal structure of a long bone's diaphysis is functionally plastic during life, responding to changes in loading behaviors. Therefore, diaphyseal cross‐sectional shape analyses may provide a more accurate account of how that individual actually moved. We apply this technique to a sample of ~54.5 million year old euprimates from Vastan Lignite Mine, India. This sample of fossils falls within 2 clades of Eocene euprimates, Omomyidae and Adapoidea, and represents at least 3 described genera.Our unpaired fossil sample of 6 femora (2 Asiadapis, 2 Marcgodinotius, 2 Vastanomys) and 4 humeri (2 probable Marcgodinotius, 2 unidentified euprimates) was compared with 20 species of extant prosimians (< 1 kilogram), spanning 3 locomotor classifications (leapers, slow climbers, and non‐specialists). Vastan specimens were scanned using a Nikon XT H 225 ST micro x‐ray computed tomography (CT) scanner. From these scans, diaphyseal cross‐sectional geometric properties were calculated at 50% of femoral length and 40% of humeral length (from distal end) using MomentMacro for ImageJ, which calculates polar second moment of area (J), second moments of area about the coronal (Ix) and sagittal (Iy) planes, cortical area (CA), and total subperiosteal area (TA). Each fossil was compared to the distributions of the prosimian locomotor groups for each of the cross‐sectional properties, as well as bone length and proximal articular surface areas (estimated from linear measurements).Results of structural analyses include an unusual mosaic of locomotor indicators. The relative thickness of bone cortices (CA/TA) is high in all humeri, which is best matched among modern slow climbers. Humeri are moderately long relative to humeral head articular surface area, similar to extant slow climbers and non‐specialists. Cross‐sectional shape (Ix/Iy) of femora is variable, with the adapoids (Asiadapis and Marcgodinotius) mostly grouping with leapers and non‐specialists, while the omomyids (Vastanomys) group with slow climbers, in contrast to inferences based on some other aspects of femoral articular morphology. Femoral length relative to bending rigidity (J) or femoral head surface area is moderate to high, primarily overlapping with extant leapers and non‐specialists. These results illustrate a combination of features not found in modern prosimians and may demonstrate a unique generalist regime of movement in our earliest antecedents.Support or Funding InformationNational Geographic and Leakey FoundationThis abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.