Fingers zipped up or baby mittens?: Two main tetrapod strategies to return to the sea

Abstract

The application of network methodology in anatomical structures offers new insights on the connectivity pattern of skull bones, skeletal elements, and their muscles. Anatomical networks helped understanding better the water-to-land transition and how the pectoral fins were transformed into limbs via their modular disintegration. Here, we apply the same methodology to the forefins of 19 tetrapods that have been secondarily adapted to the marine environment, including turtles, ichthyosaurs, mosasaurs, plesiosaurs, metriorhynchidcrocodylomorphs, and mammals (whales, dolphins, sea lions, seals, and sea cows). We find that these animals achieved their return to the sea with four types of morphological changes, which can be grouped into two different main strategies.