Abstract
Evidence for terrestriality in early tetrapods is fundamentally contradictory. Fossil trackways attributed to early terrestrial tetrapods long predate the first body fossils from the Late Devonian. However, the Devonian body fossils demonstrate an obligatorily aquatic lifestyle. Complicating our understanding of the transition from water to land is a pronounced gap in the fossil record between the aquatic Devonian taxa and presumably terrestrial tetrapods from the later Early Carboniferous. Recent work suggests that an obligatorily aquatic habit persists much higher in the tetrapod tree than previously recognized. Here, we present independent microanatomical data of locomotor capability from the earliest Carboniferous of Blue Beach, Nova Scotia. The site preserves limb bones from taxa representative of Late Devonian to mid-Carboniferous faunas as well as a rich trackway record. Given that bone remodels in response to functional stresses including gravity and ground reaction forces, we analysed both the midshaft compactness profiles and trabecular anisotropy, the latter using a new whole bone approach. Our findings suggest that early tetrapods retained an aquatic lifestyle despite varied limb morphologies, prior to their emergence onto land. These results suggest that trackways attributed to early tetrapods be closely scrutinized for additional information regarding their creation conditions, and demand an expansion of sampling to better identify the first terrestrial tetrapods.
Highlights
The evolution of digited limbs once appeared intimately tied with the transition from water to land, but the water-to-land and fin-to-limb transitions are no longer assumed to be synonymous [15,16,17]
A more recent study [6] used tetrapod humeral shape and traits, and possible load-bearing capabilities based on external morphology to predict when tetrapods were first capable of terrestrial locomotion
We studied the compactness profiles of eight extant tetrapods and five femoral fossils from Blue Beach using a previously published method [7,8]
Summary
The evolution of digited limbs once appeared intimately tied with the transition from water to land, but the water-to-land and fin-to-limb transitions are no longer assumed to be synonymous [15,16,17]. Soft tissue reconstruction and biomechanics have become common methods used to infer which early tetrapod was the earliest candidate capable of walking on land [1,3,6,20,21,22]. Using osteological correlates (in conjunction with the EPB), may not lead to a complete inference of soft tissue location or orientation in fossil material and removes some constraint from the biomechanical model thereby derived [3,20,21]. In 2012, the limb joint mobility of Ichthyostega was modelled [1] and the authors concluded the animal had potential terrestrial capabilities, based on the possible range of motion. We ask, do the fossils show evidence of responding to gravity and ground reaction forces, directly demonstrating terrestrial behaviour, rather than inferring its possibility?
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