Comparative Postembryonic Skeletal Ontogeny in Two Sister Lineages of Old World Tree Frogs (Rhacophoridae: Taruga, Polypedates)

Tadpole of the Vietnamese jelly-nest tree frog, Feihyla palpebralis (Anura: Rhacophoridae): external morphology and chondrocranium

The genus Feihyla Frost, Grant, Faivovich, Bain, Haas, Haddad, de S, Channing, Wilkinson, Donnellan, Raxworthy, Campbell, Blotto, Moler, Drewes, Nussbaum, Lynch, Green & Wheeler was established relatively recently (Frost et al. 2006), but to date its composition has changed several times due to the extremely confusing and labile systematics of small rhacophorids (e.g., Grosjean et al. 2008; Hertwig et al. 2013; Biju et al. 2020). It embeds rather miniature, delicate tree frogs and currently comprises six species (Frost 2023). The type species, Feihyla palpebralis (Smith 1924), was described from Mount Langbian in the southern part of the Truong Son Range (Annamite Mountains) and was long considered endemic to Vietnam; recent reports that it is also distributed in China can be based on misidentification and need reliable confirmation (Biju et al. 2020).

Anatomical features of the phytotelma dwelling, egg-eating, fanged tadpoles of Rhacophorus vampyrus (Anura: Rhacophoridae)

Tadpoles of the Vampire tree frog Rhacophorus vampyrus differ substantially from other rhacophorid tadpoles, by having profound modifications in external morphology. The morphological peculiarities of this species likely correlate with their arboreal microhabitat and strict oophagous diet. In this work, we examine buccal and musculoskeletal anatomy and compare them to other rhacophorid and egg-eating larvae. The shape and arrangement of cartilages of the lower jaw are unique among tadpoles, and the lack of a palatoquadrate suspensorium is only known in the distantly related macrophagous tadpoles of the dicroglossid Occidozyga baluensis. The cranial musculature is massive, and the morphology of several mandibular, hyoid, and abdominal muscles could be related to the ingestion and transit of large eggs. In the buccal cavity, conspicuous aspects are the absence of ridges and papillae, and the development of a unique glandular zone in the buccal floor. Finally, observations of the skeletal support of keratinized mouthparts allow us to present a topography-based hypothesis of homology of the conspicuous fangs of these tadpoles.

Amphibian Hormones, Calcium Physiology, Bone Weight, and Lung Use Call for a More Inclusive Approach to Understanding Ossification Sequence Evolution

Skeleton plays a huge role in understanding how vertebrate animals have diversified in phylogeny, ecology and behavior. Recent evo-devo research has used ossification sequences to compare skeletal development among major groups, to identify conserved and labile aspects of a sequence within a group, to derive ancestral and modal sequences, and to look for modularity based on embryonic origin and type of bone. However, questions remain about how to detect and order bone appearances, the adaptive significance of ossification sequences and their relationship to adult function, and the utility of categorizing bones by embryonic origin and type. Also, the singular focus on bone appearances and the omission of other tissues and behavioral, ecological and life history events limit the relevance of such analyses. Amphibians accentuate these concerns because of their highly specialized biphasic life histories and the exceptionally late timing, and high variability of their ossification sequences. Amphibians demonstrate a need for a whole-animal, whole-ontogeny approach that integrates the entire ossification process with physiology, behavior and ecology. I discuss evidence and hypotheses for how hormone mediation and calcium physiology might elicit non-adaptive variability in ossification sequence, and for adaptive strategies to partition larval habitats using bone to offset the buoyancy created by lung use. I also argue that understanding plasticity in ossification requires shifting focus away from embryonic development and adult function, and toward postembryonic mechanisms of regulating skeletal growth, especially ones that respond directly to midlife environments and behaviors.