Wednesday, November 24, 2021

[Herpetology • 2021] Gastrotheca guentheri • Re-evaluating the Morphological Evidence for the Re-evolution of Lost Mandibular Teeth in Frogs

Gastrotheca guentheri (Boulenger, 1882)

in Paluh, Dillard, ... et Blackburn, 2021.

Dollo's law of irreversibility states that once a complex structure is lost, it cannot be regained in the same form. Several putative exceptions to Dollo's law have been identified using phylogenetic comparative methods, but the anatomy and development of these traits are often poorly understood. Gastrotheca guentheri is renowned as the only frog with teeth on the lower jaw. Mandibular teeth were lost in the ancestor of frogs more than 200 million years ago and subsequently regained in G. guentheri. Little is known about the teeth in this species despite being a frequent example of trait “re-evolution,” leaving open the possibility that it may have mandibular pseudoteeth. We assessed the dental anatomy of G. guentheri using micro-computed tomography and histology and confirmed the longstanding assumption that true mandibular teeth are present. Remarkably, the mandibular teeth of G. guentheri are nearly identical in gross morphology and development to upper jaw teeth in closely related species. The developmental genetics of tooth formation are unknown in this possibly extinct species. Our results suggest that an ancestral odontogenic pathway has been conserved but suppressed in the lower jaw since the origin of frogs, providing a possible mechanism underlying the re-evolution of lost mandibular teeth.

Keywords: Anura, dentition, Dollo's law, Hemiphractidae, trait reversal

Dental ontogeny of Gastrotheca guentheri.
(A) 17.1 mm snout-vent length (SVL) neonate, KU:KUH:200260; (B) 30.9 mm SVL juvenile, KU:KUH:178464;
 (C) 47.6 mm SVL juvenile, KU:KUH:221634; (D) 62.4 mm SVL adult, KU:KUH:195628;
(E) 71.5 mm SVL adult, KU:KUH:164226; and (F) 78.9 mm SVL adult, KU:KUH:221635.

Skulls in lateral view: dentigerous cranial elements are colored. Isolated premaxilla (yellow), maxilla (green), dentary (blue), and vomer (purple) in lingual views. Scale bars = 1 mm.

Daniel J. Paluh, Wesley A. Dillard, Edward L. Stanley, Gareth J. Fraser and David C. Blackburn. 2021. Re-evaluating the Morphological Evidence for the Re-evolution of Lost Mandibular Teeth in Frogs. Evolution. DOI: 10.1111/evo.14379

Underbite regained: Species feared extinct is the only frog with true teeth on its lower jaw

Frogs have lacked teeth on their lower jaw since their first appearance in the fossil record more than 200 million years ago. But scientists were finally able to verify one species defies the standard -- a large marsupial frog named Gastrotheca guentheri.

For decades, no one was sure whether the structures on G. guentheri’s lower jaw were bones masquerading as teeth or the genuine article.

“They’re incredibly small, each about the size of a grain of sand,” said lead author Daniel Paluh, a doctoral candidate in the University of Florida’s department of biology. “There’s no way to confirm the presence of dentin and enamel in frog teeth without using high-resolution techniques.”