Data from: Mechanics of evolutionary digit reduction in fossil horses (Equidae)

Digit reduction is a major trend that characterizes horse evolution, but
its causes and consequences have rarely been quantitatively tested. Using
beam analysis on fossilized centre metapodials, we tested how locomotor
bone stresses changed with digit reduction and increasing body size across
the horse lineage. Internal bone geometry was captured from 13 fossil
horse genera that covered the breadth of the equid phylogeny and the
spectrum of digit reduction and body sizes, from Hyracotherium to Equus.
To account for the load-bearing role of side digits, a novel, continuous
measure of digit reduction was also established—toe reduction index (TRI).
Our results show that without accounting for side digits, three-toed
horses as late as Parahippus would have experienced physiologically
untenable bone stresses. Conversely, when side digits are modelled as
load-bearing, species at the base of the horse radiation through Equus
probably maintained a similar safety factor to fracture stress. We
conclude that the centre metapodial compensated for evolutionary digit
reduction and body mass increases by becoming more resistant to bending
through substantial positive allometry in internal geometry. These results
lend support to two historical hypotheses: that increasing body mass
selected for a single, robust metapodial rather than several smaller ones;
and that, as horse limbs became elongated, the cost of inertia from the
side toes outweighed their utility for stabilization or load-bearing.