A new pelomedusoid turtle,Sahonachelys mailakavava, from the Late Cretaceous of Madagascar provides evidence for convergent evolution of specialized suction feeding among pleurodires

Abstract

The Maevarano Formation in northwestern Madagascar has yielded a series of exceptional fossils over the course of the last three decades that provide important insights into the evolution of insular ecosystems during the latest Cretaceous (Maastrichtian). We here describe a new genus and species of pelomedusoid turtle from this formation, Sahonachelys mailakavava , based on a nearly complete skeleton. A phylogenetic analysis suggests close affinities of Sahonachelys mailakavava with the coeval Madagascan Sokatra antitra . These two taxa are the only known representatives of the newly recognized clade Sahonachelyidae , which is sister to the speciose clade formed by Bothremydidae and Podocnemidoidae . A close relationship with coeval Indian turtles of the clade Kurmademydini is notably absent. A functional assessment suggests that Sahonachelys mailakavava was a specialized suction feeder that preyed upon small-bodied invertebrates and vertebrates. This is a unique feeding strategy among crown pelomedusoids that is convergent upon that documented in numerous other clades of turtles and that highlights the distinct evolutionary pathways taken by Madagascan vertebrates.

Cranial ecomorphology of turtles and neck retraction as a possible trigger of ecological diversification

“…Predictions for fossil pelomedusoids indicate high feeding disparity, with varied indices ranging from very low (0.1, Phosphatochelys tedfordi) to very high in fossil species that were proposed to be suction-feeders (0.84, Sah. mailakavava; Joyce et al 2021b).…”

“…However, when using the smaller "partial landmark dataset" (N = 65), the effect of relative neck length is nonsignificant if included together with the evasiveness index as an explanatory variable (Table S7), and the best model remains the same as that with greater sample size (i.e., N = 76; see above). This suggests relative neck length to have importance in the feeding ecology of turtles, as turtles that specialize on more elusive prey tend to have relatively longer necks (e.g., Van Damme and Aerts 1997; Joyce et al 2021b).…”

“…We recorded two variables ("durophagy," "suction feeding") regarding specific feeding modes of some turtles. These are binary variables to test if these feeding strategies indeed have a strong effect on skull shape, as commonly suggested (e.g., Pritchard 1984;Joyce et al 2021b). The use of "suction feeding" in the turtle literature is somewhat ambiguous, and sometimes used for all aquatic feeders (due to compensatory suction employed during forward head movements; Van Damme and Aerts 1997), or limited to chelids (Lemell et al 2002).…”

“…Paleoecologists have long shown interest in reconstructing ecology from morphology, and the diets or feeding strategies of fossil turtles have often been inferred from qualitative comparisons with extant species (e.g., Archibald and Hutchison 1979;Meylan et al 2000;Parham and Pyenson 2010;Cadena et al 2020;Joyce et al 2021b). Previous quantitative approaches have documented specific feeding adaptations (i.e., durophagy) in specific parts of the skull or for specific turtle clades (e.g., Claude et al 2004;Ferreira et al 2015), and also questioned the reliability of skull shape to estimate paleoecology across turtles (Foth et al 2017).…”

Independent origin of large labyrinth size in turtles

“…Forelimb webbing thus may reflect ecology more accurately than observational field data that underlies the other categorization approaches. Turtle necks facilitate head movements against the rigid shelled body and are related to head motion and feeding behavior, with long-necked species performing more rapid head movements 87 , 88 . Different anatomical configurations of the neck in stem turtles, pleurodires, and cryptodires additionally affect the ability for neck retraction as well as the primary plane of neck movements 46 , 63 .…”