Rapid Ossification Helps Explain Small Body Size in Urspelerpes brucei (Caudata: Plethodontidae)
Urspelerpes brucei (Patch-nosed Salamander) is a miniaturized lungless salamander (family Plethodontidae) native to select headwater tributaries of the Tugaloo River along the border of Georgia and South Carolina. Due to the rarity and relatively recent discovery of this species, many aspects of its biology are unstudied. In particular, there has not yet been any description of its larval skeleton. We provide the first description of larval osteology for U. brucei, and we compared cleared and stained specimens of U. brucei close to the minimum size at metamorphosis to similarly sized specimens of Eurycea cirrigera (Southern Two-lined Salamander). Our results showed that the skull and long bones of U. brucei are more heavily ossified at smaller sizes than in E. cirrigera. In addition, U. brucei possesses more robust vomers and pterygoids, coronoids that are already incorporated into the dentary, and an ossified os thyroideum. These discrepancies in ossification suggest that the skeletons of U. brucei complete development at smaller sizes than E. cirrigera, a result likely tied to heterochronic accelerations of mineralization and sexual maturation in U. brucei. Taken together, these factors appear to help explain how U. brucei has developed and maintained its small body size.
Examples of skull measurements taken in ImageJ as well as bones of the skull represented as colored polygons. (A) Yellow lines used to measure the length and width of the skull roof. (B) Yellow polygon outlining combined perimeter of premaxilla, frontals, and parietals used to obtain the ossified area of the skull roof. Red line past rear of parietals marks outer boundary of second polygon used to obtain unossified area of skull roof. (C) Dorsal view of the skull of larval Urspelerpes brucei. (D) Ventral view of the skull of larval U. brucei. Mandibles and cartilage omitted. cl, columella; fp, footplate; fr, frontal; oc, occipito-otic; os, orbitosphenoid; pa, parietal; pm, premaxilla; ps, parasphenoid; pt, pterygoid; qu, quadrate; sq, squamosal; vo, vomer.
Dorsal comparison of the skull of larval Urspelerpes brucei (A) to Eurycea cirrigera (B). The premaxilla, frontals, and parietals (outlined in yellow) are more heavily ossified in U. brucei than in E. cirrigera at this size. This results in a noticeably smaller frontoparietal foramen (filled in red) in U. brucei. Scale bars represent 1 mm.
Ventral comparison of the skull of larval Urspelerpes brucei (A) to Eurycea cirrigera (B). At this size the vomers and pterygoids (outlined in yellow) are larger and further along in development in U. brucei, the coronoids (yellow arrows) have been incorporated into the dentary, and the os thyroideum (yellow circle) is ossified, unlike E. cirrigera. Scale bars represent 1 mm.
Comparison of long bone ossification in larvae of Urspelerpes brucei (A) and Eurycea cirrigera (B). All long bones in U. brucei are completely ossified at this size, whereas the level of ossification is variable (though never complete) in E. cirrigera.
Contributor Notes
Associate Editor: J. Lamb.