Fossils Reveal Long-Term Continuous and Parallel Innovation in the Sacro-Caudo-Pelvic Complex of the Highly Aquatic Pipid Frogs

Abstract

Within the already peculiar Bauplan of anurans, pipid frogs have evolved an array of bizarre features that are commonly linked to their highly aquatic lifestyle. Among the latter, there are several distinctive sacro-caudo-pelvic features shared by extant pipids, which have been regarded as evolutionary novelties taking part of a specialized foreaft-sliding ilio-sacral joint. Pipids and their kin (pipimorphs) have a rich fossil record documenting 130 million years of uninterrupted evolution in aquatic environments, which, along with our current understanding of their phylogeny and recently available musculoskeletal data, allows us to inquire on the patterns and processes that have led to their distinctive sacro-caudo-pelvic system with a deep-time perspective. Here, we take a phylomorphospace approach based on discrete character matrices and a scaffold tree derived from recent studies, making comparisons of morphospace occupation between pipids and other anurans, and morphospace occupation, disparity, levels of homoplasy, and shared evolving characters between different groups and/or over time across pipimorphs. In doing so, we focus on trends of morphological diversification and discuss the potential role that ecological and developmental constraints might have had in driving the evolution of the sacro-caudo-pelvic complex of pipid frogs. Our main findings reveal a pattern of continuous and parallel innovation early in the history of pipids, followed by arrested evolution of novel morphologies toward the Recent. The latter, in turn, is mirrored by rampant homoplasy in the ilio-sacral sliding joint among extant pipid frogs. This study highlights the importance of fossils in revealing macroevolutionary patterns that will be otherwise veiled based on neontological data alone.

High ecomorphological diversity among Early Cretaceous frogs from a large subtropical wetland of Iberia

“…Some classifications do not depart from expectation, as in the cases of the pipimorph taxa or Wealdenbatrachus. The pipimorphs Neusibatrachus and Gracilibatrachus, which are part of the stem of the highly aquatic pipids (Báez, 2013;Báez and Sanchiz, 2007;Gómez, 2016), are consistently inferred as swimmers, in agreement with the aquatic lifestyle inferred on the basis of several skeletal features other than limb proportions (Báez, 2013;Gómez and Pérez-Ben, 2019). Also concurring with previous suggestions mainly based on limb proportions and ilial morphology (Báez and Gómez, 2019), specimens of Wealdenbatrachus are invariably classified as jumpers.…”

Frog limbs in deep time: is jumping locomotion at the roots of the anuran Bauplan?

“…In particular, Wealdenbatrachus , classified herein as a jumper with a high posterior probability ( p J = 0.98; Supplementary Table S4) consistent with the results of Gómez and Lires (2019), has previously been proposed to be an efficient long-distance jumper based not only on its long hindlimbs, but also on its iliac anatomy (Báez and Gómez 2019). Likewise, some early Cretaceous taxa were almost undoubtedly swimmers: pipimorphs such as Cratopipa and Cordicephalus are classified as swimmers ( p Sw = 0.96 and 0.84, respectively; Supplementary Table S4) and present postcranial features of extant pipids that have been classically linked to their aquatic lifestyle (e.g., robust hindlimbs, proportionally long fingers in hands and feet, expanded sacral diapophyses; Cannatella and Trueb 1988; Trueb 1996; Báez et al 2012; Cannatella 2015; Gómez and Pérez-Ben 2019; Turazzini and Gómez 2023).…”

A new Late Cretaceous snake from Patagonia: Phylogeny and trends in body size evolution of madtsoiid snakes

“…The stratocladogram was derived from one of the optimal trees obtained in the parsimony analysis, including madtsoiids plus a few outgroup taxa. Numerical ages of each extinct taxon were derived from the youngest possible age of the oldest fossil-bearing unit following the current chronostratigraphic framework (Cohen et al, 2013, updated) and internodes were set to a minimum time interval (1 Myr herein), following previous approaches (Gómez and Pérez-Ben, 2019;Laurin, 2008, 2014). Stratocladogram assemblage and character optimization under squared change parsimony were performed in Mesquite Version 3.31 (Maddison and Maddison, 2011).…”