A new hynobiid-like salamander (Amphibia, Urodela) from Inner Mongolia, China, provides a rare case study of developmental features in an Early Cretaceous fossil urodele

Abstract

A new fossil salamander, Nuominerpeton aquilonaris (gen. et sp. nov.), is named and described based on specimens from the Lower Cretaceous Guanghua Formation of Inner Mongolia, China. The new discovery documents a far northern occurrence of Early Cretaceous salamanders in China, extending the geographic distribution for the Mesozoic fossil record of the group from the Jehol area (40th-45th parallel north) to near the 49th parallel north. The new salamander is characterized by having the orbitosphenoid semicircular in shape; coracoid plate of the scapulocoracoid greatly expanded with a convex ventral and posterior border; ossification of two centralia in carpus and tarsus; and first digit being about half the length of the second digit in both manus and pes. The new salamander appears to be closely related to hynobiids, although this inferred relationship awaits confirmation by research in progress by us on a morphological and molecular combined analysis of cryptobranchoid relationships. Comparison of adult with larval and postmetamorphic juvenile specimens provides insights into developmental patterns of cranial and postcranial skeletons in this fossil species, especially resorption of the palatine and anterior portions of the palatopterygoid in the palate and the coronoid in the mandible during metamorphosis, and postmetamorphic ossification of the mesopodium in both manus and pes. Thus, this study provides a rare case study of developmental features in a Mesozoic salamander.

Keywords: Developmental features; Early Cretaceous; Hynobiid-like salamander; Larval–juvenile–adult forms; New fossil taxon.

Figure 1. Area map showing geographic distribution of salamander fossil localities in the Lower Cretaceous of northeastern China.

The shaded area indicates the former “Jehol Province,” after which the “Jehol Biota” was named.

Figure 2. Holotype of Nuominerpeton aquilonaris gen. et sp. nov. (part slab of PKUP V0414): photograph (A) and line drawing (B) of the upper body.

Note the skull has the roofing elements exposed in ventral view as the consequence of how the shale slabs split through the skeleton. Dark shades denote palatal, braincase and hyobranchial elements.

Figure 3. Holotype of Nuominerpeton aquilonaris gen. et sp. nov. (counter-part slab of PKUP V0414): photograph (A) and line drawing (B), displaying articulated skeleton with part of the tail missing.

Note the skull has the palatal elements exposed in dorsal view resulting from split of shale slabs. Dark shades denote palatal, braincase and hyobranchial elements.

Figure 4. Holotype skull of Nuominerpeton aquilonaris gen. et sp. nov. (PKUP V0414): photograph and line drawing of the skull roof (A, B) and palatal (C, D) structures.

Note that skull roof elements are exposed in ventral view and the palatal elements in dorsal view as consequence of how the shale slabs split. Dark shades denote palatal, braincase and hyobranchial elements.

Figure 5. Line drawing of a nearly complete skeleton of Nuominerpeton aquilonaris gen. et sp. nov. (PKUP V0421): line drawing of part slab (A) with skeleton exposed in ventral view; line drawing of the counter-part slab (B) with skeleton exposed in dorsal view.

Note that the upper left inset is the magnified photograph and line drawing of an isolated maxillary tooth with bicuspid crown. Dark shades denote palatal, braincase and hyobranchial elements.

Figure 6. Incomplete skeleton of the only known postmetamorphic juvenile specimen of Nuominerpeton aquilonaris gen. et sp. nov. (PKUP V0416): photograph (A) and line drawing (B) of an articulated skeleton exposed in dorsal view.

Note that the pterygoid has been re-shaped to the postmetamorphic configuration, but the carpus and tarsus remain unossified. Dark shades denote palatal, braincase and hyobranchial elements.

Figure 7. Photographs and line drawings of skull and mandibles in larval and juvenile specimens of Nuominerpeton aquilonaris gen. et sp. nov., showing ontogenetic resorption of palatine and anterior pterygoid portions of the palatopterygoid.

(A) PKUP V0419 in palatal view, displaying a slightly expanded, dentate palatine portion as part of the palatopterygoid; (B) PKUP V0418 in dorsal view, displaying slightly resorbed palatine as a narrow strip; (C) PKUP V0417 in palatal view, displaying further resorption of the palatopterygoid as metamorphosis approaches; (D) PKUP V0416, a postmetamorphic juvenile specimen in dorsal view, displaying ossified ceratobranchial II and re-shaped pterygoid with palatal process oriented anterolaterally. Dark shades denote palatal, braincase and hyobranchial elements.

Figure 8. Referred specimen of Nuominerpeton aquilonaris gen. et sp. nov. (PKUP V0415): photograph (A) and line drawing (B) of the incomplete skeleton in ventral view.

Figure 9. A larval specimen of Nuominerpeton aquilonaris gen. et sp. nov. (PKUP V0420): photograph (A) and line drawing (B) of the articulated skeleton in ventral view.

Note the premetamorphic shape of the palatopterygoid, and longer radius than ulna in the arm and longer tibia than fibula in the leg (magnified insets in line drawing) indicating preaxial polarity in limb development. Dark shades denote palatal and braincase elements.

Figure 10. Right forelimb of the new salamander (Nuominerpeton aquilonaris gen. et sp. nov.) in comparison with patterns in extant hynobiids.

(A) Nuominerpeton, reconstruction based on PKUP V0415; (B) Ranodon sibiricus; (C) Hynobius leechii; (D) Batrachuperus pinchonii; (E) Salamandrella keyserlingii; (F) Liua shihi; (G) Pachyhynobius shangchengensis; (H) Onychodactylus sp. Dotted area indicates cartilage. Not to scale.

Figure 11. Right hind limb of the new salamander (Nuominerpeton aquilonaris gen. et sp. nov.) in comparison with patterns in extant hynobiids.

(A) Nuominerpeton aquilonaris, reconstruction based on PKUP V0415; (B) Ranodon sibiricus; (C) Hynobius leechii; (D) Batrachuperus pinchonii; (E) Salamandrella keyserlingii; (F) Liua shihi; (G) Pachyhynobius shangchengensis; (H) Onychodactylus sp. Dotted area indicates cartilage. All in dorsal view and not to scale.

Figure 12. Ossification sequence of mesopodial elements (color coded) in the new salamander (Nuominerpeton aquilonaris gen. et sp. nov.).

(A–C) Right fore limb; (D–F) right hindlimb. Note that element y is ossified before radiale in manus and before tibiale in pes. Color codification: blue-centralia; purple-element y; red-basale commune; orange-distal carpal/tarsal 3; yellow-distal carpal/tarsal 4; green-ulnare/fibulare; maroon-intermedium; lime-radiale/tibiale.