Circumventing the polydactyly ‘constraint’: the mole's ‘thumb’

Abstract

Talpid moles across all northern continents exhibit a remarkably large, sickle-like radial sesamoid bone anterior to their five digits, always coupled with a smaller tibial sesamoid bone. A possible developmental mechanism behind this phenomenon was revealed using molecular markers during limb development in the Iberian mole ( Talpa occidentalis ) and a shrew ( Cryptotis parva ), as shrews represent the closest relatives of moles but do not show these conspicuous elements. The mole's radial sesamoid develops later than true digits, as shown by Sox9 , and extends into the digit area, developing in relation to an Msx2 -domain at the anterior border of the digital plate. Fgf8 expression, marking the apical ectodermal ridge, is comparable in both species. Developmental peculiarities facilitated the inclusion of the mole's radial sesamoid into the digit series; talpid moles circumvent the almost universal pentadactyly constraint by recruiting wrist sesamoids into their digital region using a novel developmental pathway and timing.

Rate of evolutionary change in cranial morphology of the marsupial genus Monodelphis is constrained by the availability of additive genetic variation

“…Examples span skeletal elements such as the scapulae (Sears, 2004), forelimbs (Kelly & Sears, 2011), and mandible/maxillae (Bennett & Goswami, 2013). Criticism regarding this hypothesis comes from the observation that developmental constraints have often been circumvented by species during their diversification (Sanchez-Villagra, 2012), such as human limb proportions (Young et al , 2010) and birth canal (Grabowski, 2012), and talpid moles’ digits (Mitgutsch et al , 2012). …”

Extraordinarily preserved talpids (Mammalia, Lipotyphla) and the evolution of fossoriality

“…The broad hand is enlarged by an additional bone, the prepollex or os falciforme, which sits alongside the radius, carpals, and first metacarpal. This bone is present in most extant talpids (S anchez-Villagra and Menke, 2005;Mitgutsch et al, 2012). The only fossil evidence of the prepollex is found in a partial skeleton of Geotrypus antiquus (de Blainville, 1840) from Enspel, Germany (Schwermann and Martin, 2012).…”

What limits the morphological disparity of clades?

“…Many amniote groups, by contrast, have reduced digit numbers as adults (e.g. horses, non-avian dinosaurs, birds [179]), but few lineages have attained higher numbers, often evolving a variety of digit-like structures rather than extra digits per se [194,195]. Ichthyosaurs furnish the best-known exception: opthalmosaurians added digits anterior to digit one and posterior to digit five [196], whereas non-opthalmosaurians may have achieved polydactyly by interdigital or postaxial phalangeal bifurcation [197].…”