Mesopodium and anatomical terms

The first scheme to divide the mesopodium was introduced by Gegenbaur (2, 73), who proposed three transverse rows based on the spatial relationships with the zeugopodium and metapodium: the proximal row (radiale/tibiale, intermedium, and ulnare/fibulare), central row (element y, centralia, and element m), and the distal row (basale commune, distal carpals/tarsals, prehallux, and postminimus). However, the prevailing scheme of the mesopodium, created by Goette (74), is to divide it along the proximodistal axis of the limb into three longitudinal and paralleling columns: the preaxial column (radiale/tibiale and element y), central column (intermedium, centralia, and element m), and postaxial column (ulnare/fibulare) (3, 14). The digital arch was coined by Schmalhausen (15) to represent series of branching and segmentation events from the basale commune that gives rise to metapodials and distal carpals/tarsals and was later refined [(14), pp. 339 and 361] to include the basale commune and distal carpals/tarsals in the mesopodium. The digital arch remains independent from the longitudinal columns in most taxa (14, 50) but occasionally joins the postaxial column in certain species, in which the posterior distal tarsals and/or postminimus segment from the ulnare/fibulare [e.g., T. marmoratus in (48), S. keyserlingii in (18), and Dicamptodon ensatus in (17, 42)]. Goette’s scheme patterned with the refined concept of digital arch is commonly used to date because it matches well with the early skeletogenesis of the limb (14): The radius/tibia distally segments to form radiale/tibiale, element y, and, if any, the prehallux; the ulna/fibula distally bifurcates to form the intermedium in the central column and the ulnare/fibulare in the postaxial column. In the central column, the centrale either segments from the intermedium or condense independently. The basale commune is a de novo condensation and segments into the distal carpals/tarsals and, if any, the postminimus to form the digital arch.

Here, we follow both schemes because we found that it is efficient for morphological descriptions by Gegenbaur’s scheme and it is coherent when referring developmental patterns by Goette’s scheme. Many anatomical terms have been introduced for the mesopodium over the past century, and we herein follow Shubin and Alberch (14) and Borkhvardt (75): element y [or centrale 1 in (3); mediale 1 in (54)], centrale close to intermedium as centrale 1 [or central proximal in (3)], centrale close to basale commune as centrale 2 [or centrale distal in (3)], and basale commune [or carpale/tarsale commune in (3)]. We also follow previous studies to use digits to include both the metapodium and phalanges [e.g., (21, 42)].