Presentation Information
[O12-P73]Relationship between changes in limb skeletons and habitat in early tetrapods
*Rika Tastumi1 (1. Tokyo Metropolitan Mitaka Secondary School)
Keywords:
limb skeletons,expansion into land,number of fingers
Understanding the transition from aquatic to terrestrial environments in early tetrapods requires examining changes in both limb skeletal structure and digit number. In this study, six Late Devonian taxa—Panderichthys, Tiktaalik, Elpistostege, Acanthostega, Ichthyostega, and Tulerpeton—were selected based on their phylogenetic positions in the framework of Cloutier et al. (2020), representing key stages in the fin-to-limb transition. The aim of this research is to compare limb morphology and digit number among these taxa and clarify the evolutionary trends associated with terrestrial adaptation.
Information on forelimb and hindlimb skeletal structures, digit counts, and inferred habitats was compiled from primary and secondary literature. These six taxa were compared along a gradient from fully aquatic to more terrestrial environments to identify patterns in limb strengthening and digit reduction.
The results show a clear sequence of morphological changes. Panderichthys (Frasnian, ~380 Ma) lacks distinct digits, possessing only distal radials at the fin tip. Tiktaalik (Frasnian, ~375 Ma) exhibits a nearly complete set of limb bones, including humerus, radius, ulna, and carpals, along with digit-like distal radials, indicating an early stage of weight-bearing capability in shallow-water habitats. Elpistostege (Frasnian, ~380–375 Ma) shows even more clearly arranged digit-like radials, although its hindlimb remains poorly known. In contrast, Acanthostega (Fammenian, ~365 Ma) possesses eight digits on both forelimbs and hindlimbs and is interpreted as fully aquatic. Ichthyostega (Fammenian, ~365 Ma) has seven digits on the hindlimb and shows strong aquatic adaptations, with forelimbs suited for paddling. Tulerpeton (Fammenian, ~365 Ma) has six digits on both limbs and is thought to have inhabited tidal, nearshore environments, representing a more terrestrial-adapted morphology.
These comparisons indicate that limb skeletal strengthening and digit reduction progressed in parallel with increasing terrestriality. In aquatic environments, buoyancy reduces the need for strong limb support, and multiple digits may have aided maneuverability. On land, however, body weight must be supported directly, favoring fewer, more robust digits capable of efficiently transmitting force. The six-digit condition of Tulerpeton suggests an intermediate stage in the reduction from polydactyly toward the pentadactyl condition characteristic of extant tetrapods.
Future work should incorporate functional and biomechanical analyses, including assessments of joint mobility, muscle attachment patterns, and load distribution. Comparative studies using extant amphibians and 3D reconstructions of fossil limbs may further clarify the mechanical constraints that drove digit reduction and limb strengthening during the water-to-land transition.
Information on forelimb and hindlimb skeletal structures, digit counts, and inferred habitats was compiled from primary and secondary literature. These six taxa were compared along a gradient from fully aquatic to more terrestrial environments to identify patterns in limb strengthening and digit reduction.
The results show a clear sequence of morphological changes. Panderichthys (Frasnian, ~380 Ma) lacks distinct digits, possessing only distal radials at the fin tip. Tiktaalik (Frasnian, ~375 Ma) exhibits a nearly complete set of limb bones, including humerus, radius, ulna, and carpals, along with digit-like distal radials, indicating an early stage of weight-bearing capability in shallow-water habitats. Elpistostege (Frasnian, ~380–375 Ma) shows even more clearly arranged digit-like radials, although its hindlimb remains poorly known. In contrast, Acanthostega (Fammenian, ~365 Ma) possesses eight digits on both forelimbs and hindlimbs and is interpreted as fully aquatic. Ichthyostega (Fammenian, ~365 Ma) has seven digits on the hindlimb and shows strong aquatic adaptations, with forelimbs suited for paddling. Tulerpeton (Fammenian, ~365 Ma) has six digits on both limbs and is thought to have inhabited tidal, nearshore environments, representing a more terrestrial-adapted morphology.
These comparisons indicate that limb skeletal strengthening and digit reduction progressed in parallel with increasing terrestriality. In aquatic environments, buoyancy reduces the need for strong limb support, and multiple digits may have aided maneuverability. On land, however, body weight must be supported directly, favoring fewer, more robust digits capable of efficiently transmitting force. The six-digit condition of Tulerpeton suggests an intermediate stage in the reduction from polydactyly toward the pentadactyl condition characteristic of extant tetrapods.
Future work should incorporate functional and biomechanical analyses, including assessments of joint mobility, muscle attachment patterns, and load distribution. Comparative studies using extant amphibians and 3D reconstructions of fossil limbs may further clarify the mechanical constraints that drove digit reduction and limb strengthening during the water-to-land transition.