Middle Jurassic stem hynobiids from China shed light on the evolution of basal salamanders

Abstract

Highlights The Middle Jurassic ''Liaoxitriton'' daohugouensis is not congenic with Liaoxitriton zhongjiani Neimengtriton daohugouensis comb. nov. is a semiaquatic stem hynobiid salamander Sequential morphological evolutionary patterns were revealed for Cryptobranchoidea Variations in caudosacral numbers are associated with fertilization mode evolution

Palatal morphology predicts the paleobiology of early salamanders

“…However, the evolutionary history of paleoecology in modern amphibians and particularly in early salamanders has received insufficient attention. 4 Cryptobranchoidea is the most primitive clade of the crown group salamanders (Urodela) and contains two subclades: Pancryptobrancha (crown + stem cryptobranchids; Vasilyan et al, 2013) and Panhynobia (crown + stem hynobiids; Jia et al, 2021a). The two subclades are united by a set of synapomorphies as sister-group taxa (Dunn, 1922;Estes, 1981;Jia et al, 2021a), but are different from each other in life history strategies and ecological preferences at their respective adult stage: most pancryptobranchans are neotenic or partially metamorphosed and live in water permanently by retaining larval features (e.g., gill slits), albeit the pancryptobranchan Aviturus from the Paleocene was interpreted as semiaquatic with an unknown life history strategy (Vasilyan and Böhme, 2012).…”

“…4 Cryptobranchoidea is the most primitive clade of the crown group salamanders (Urodela) and contains two subclades: Pancryptobrancha (crown + stem cryptobranchids; Vasilyan et al, 2013) and Panhynobia (crown + stem hynobiids; Jia et al, 2021a). The two subclades are united by a set of synapomorphies as sister-group taxa (Dunn, 1922;Estes, 1981;Jia et al, 2021a), but are different from each other in life history strategies and ecological preferences at their respective adult stage: most pancryptobranchans are neotenic or partially metamorphosed and live in water permanently by retaining larval features (e.g., gill slits), albeit the pancryptobranchan Aviturus from the Paleocene was interpreted as semiaquatic with an unknown life history strategy (Vasilyan and Böhme, 2012). Alternatively, panhynobians are predominantly metamorphosed, except that the stem hynobiid Regalerpeton from Early Cretaceous (Rong, 2018) and some populations of the living hynobiid Batrachuperus londongensis are neotenic (Jiang et al, 2018).…”

“…Cryptobranchoidea are critical in understanding the paleoecology of early salamanders because the earliest fossil cryptobranchoids represent the oldest known crown urodeles; and disparities in both life history strategies and ecological preferences have been underway among these earliest crown urodeles from the Middle Jurassic Bathonian, including 'Kirtlington salamander B' from the UK, Kiyatriton krasnolutskii from Russia, and Chunerpeton, Neimengtriton, and Jeholotriton from China (Evans and Milner, 1994;Gao et al, 2013;Skutschas, 2016;Jia et al, 2021a). Both Chunerpeton and Jeholotriton are neotenic as confirmed by the presence of external gills and high caudal dorsal fin in adult specimens (Gao and Shubin, 2003;Wang and Rose, 2005), whereas Neimengtriton is the oldest metamorphosed and semiaquatic cryptobranchoid (Jia et al, 2021a; see below). The 'Kirtlington salamander B' and K. krasnolutskii are both represented by fragmentary materials and their paleoecology unfortunately remains unknown.…”

“…To date, seven other basal cryptobranchoids have been reported from the Upper Jurassic to Lower Cretaceous of northern China: Laccotriton, Liaoxitriton, Linglongtriton, Nuominerpeton, Pangerpeton, Regalerpeton and Sinerpeton, most of which are represented by articulated specimens and have been recently recovered as stem hynobiids or hynobiid-like taxa (see Gao et al, 2013;Jia and Gao, 2016;Jia et al, 2021a). Besides the neotenic Regalerpeton as aforementioned, living habitat preferences of these metamorphosed taxa and paleoecological disparity patterns of Cryptobranchoidea remain largely unexplored mainly due to yet established osteological indicators for ecology (see Discussion).…”

“…Our series of studies on living and fossil cryptobranchoids noticed that besides vomerine teeth, the palate varies in shape and proportion, and could potentially serve as an indicator for paleoecological reconstruction (Jiang et al, 2018;2021a;2021b; Figure 1 and Figure 1-figure supplements 1-23). To test these hypotheses and to address the constraints underlying the morphological disparity of the palate, here we conducted a 2D landmark-based geometric morphometric analysis on the palate of all living and most aforementioned basal genera of cryptobranchoids, stem and other basal crown urodeles based primarily on micro-CT scanned specimens.…”