SURFICIAL TRACKS PRODUCED BY MUDSKIPPERS (PERIOPHTHALMUS & PERIOPHTHALMODON) IN FINE-GRAINED COASTAL SUCCESSIONS: IMPLICATIONS FOR THE VERTEBRATE COLONIZATION OF LAND

The transition from sea to land presented some of the greatest physiological & ethological challenges in the evolution of life on Earth. Despite these challenges, the transition occurred multiple times & independently within different groups of vertebrates & invertebrates. The first evidence of amphibious arthropods are trace fossils, such as Protichnites & Siskemia. Vertebrates first made the transition during the Devonian. Terrestrial tetrapods evolved from finned organisms that occupied shallow marginal marine swamps & lagoons.

The first vertebrates to venture on to land were likely more mobile in water than on land but exploitation of new habitats afforded them the opportunity to access new food resources & avoid predation. Curiously, however, the earliest evidence for vertebrate life on land consists of trackways with clearly differentiated manus & pes impressions. Arguably, an older transitional, amphibious form must exist but has either not yet been discovered or has not been recognized as a vertebrate trace. Vertebrates made the transition from aquatic to terrestrial environments several times & some extant groups (such as dipnoan sarcopterygians) appear to represent transitional forms. Several actinopterygian groups are also capable of breathing air & transporting themselves through subaerially exposed environments by wriggling, jumping, or ‘crutching’.

Mudskippers differ from other amphibious fishes in that they use their pectoral limbs for locomotion, rather than relying their axial skeleton. They have developed unique behavioral & physiological adaptations that allow them to move effectively in both settings. Similar to Devonian tetrapods, mudskippers have derived fins, with adaptations that allow mobility on land. Traces emplaced by mudskippers are unique in that they record the movements of a finned organism in its exploration of a new environment & provide a modern example of a vertebrate that is equally at home in aquatic & terrestrial settings. Devonian traces, similar in all important aspects to mudskipper traces, are invariably interpreted as arthropod traces. Analysis of traces produced by modern amphibious fishes clearly indicates the need for scientists to refine their expectations when searching for evidence of the oldest forays of Paleozoic fishes into the terrestrial realm.