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

MARTA S. FERNÁNDEZ; EVANGELOS VLACHOS; MÓNICA. R. BUONO; LUCIA ALZUGARAY; LISANDRO CAMPOS; JULIANA STERLI; YANINA HERRERA; FLORENCIA PAOLUCCI

Virtual Meeting

Otro; 9th International Meeting on the Secondary Adaptation of Tetrapods to Life in Water Virtual Meeting; 2021

The application of network methodology in anatomical structures offers new insights on theconnectivity pattern of skull bones, skeletal elements, and their muscles. Anatomicalnetworks helped understanding better the water-to-land transition and how the pectoral finswere transformed into limbs via their modular disintegration. Here, we apply the samemethodology to the forefins of 19 tetrapods that have been secondarily adapted to the marineenvironment, including turtles, ichthyosaurs, mosasaurs, plesiosaurs, metriorhynchidcrocodylomorphs, and mammals (whales, dolphins, sea lions, seals, and sea cows). We findthat these animals achieved their return to the sea with four types of morphological changes,which can be grouped into two different main strategies. In all marine mammals and themajority of the reptiles the fin is formed by the persistence of superficial and interdigitalconnective tissues, like a “baby mitten”, whereas the underlying connectivity pattern of thebones does not influence the formation of the forefin. These tetrapods managed to exploreregions outside the known morphospace, attempting higher disintegration of the limb or somemoderate reintegration — but without losing their digits. On the contrary, ichthyosaurs“zipped up” their fingers and transformed their digits into carpal-like elements, forming ahomogeneous and better-integrated forefin, showing a costly reintegration of their limb to amodular pattern that is analogous to fishes, with the addition of interdigital bony elementsand lateral connections. These strategies led these vertebrates into three differentmacroevolutionary paths exploring the possible spectrum of morphological adaptations.Mosasaurs and plesiosaurs placed new limits in the disintegration of the limb, by addingnumerous new phalanges on their digits, increasing its modularity, while reducing its densityand integration. Marine crocodiles, and possibly basilosaurids, lost elements and increasedconnections of the metapodials, resulting in forefins that were more complex and betterintegrated. The most impressive changes are noted in the forefins of ichthyosaurs, whoreintegrated their digits into the mesopodium with the addition of anterior and posteriorcontacts and articulations. Their metacarpals and phalanges radically adopted theconnectivity pattern of carpal bones (increased clustering, betweenness centrality, anddegree), forming forefins that were highly integrated and homogeneous. However, thisstrategy allowed ichthyosaurs to have forefins that did not lose much of their modularity.Anatomical networks help understanding that all these secondary adaptations to the marine97environment are not the same, and to speculate that they are the result of differentdevelopmental mechanisms, but also physical, phylogenetic, and morphological constraints.