EVOLUTION - THE TRANSITIONAL FOSSILS
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salamander stem group

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The caudates (order Caudata, class Amphibia) comprise the salamanders (and newts, which comprise a subfamily, Pleurodelinae, within the salamander family Salamandridae). The Caudata are also known as the Urodela, although some researchers (e.g. Marjanović and Laurin, 2013) confine the term Urodela to the crown-salamanders and use the name Caudata for the total group. There are more than 700 species of caudates, distributed between 10 families.

Note that this website does not include a page on the stem group of the clade to which the salamanders belong (the Batrachia). Although one species, Gerobatrachus hottoni, has been assigned to the stem-Batrachia in some analyses (e.g. Anderson et al, 2008; Atkins et al, 2019), others (e.g. Schoch, 2019) have placed it in the stem-Amphibia (as shown in Figure 1 of the stem-Amphibia page). In view of this uncertainty, we do not consider the batrachian stem group to be sufficiently well documented to warrant further discussion here.

There are many articles in the literature that discuss the synapomorphies of salamanders, but there is very little consistency between them. The following synapomorphies were recently proposed by Rong et al (2021) for the crown-group salamanders:
  1. Anterior process of the maxilla (upper jawbone) elongated and extensively overlapped by the premaxilla (small cranial bones at the very tip of the upper jaw)
  2. Arrow-shaped basibranchial II (a bone at the base of one of the gill arches)
These evolutionary novelties that developed along the salamander stem line all relate to features of the skeleton that will be difficult to observe without detailed inspection of fossils.

A recent interpretation of the phylogeny of the stem-Urodela, based on a few of the known fossils, is illustrated below:
Picture
​Figure 1.    Time tree of the  stem-Urodela
​The following fossils can be considered as the oldest known members of the stem-Urodela:
  1. Kokartus honorarius, described from the “Balabansai Suite” of Middle Jurassic (Bathonian) age at three localities in the Kugart River Basin of Kyrgyzstan (Skutschas and Martin, 2011; Jia et al, 2022).
  2. Two species of the genus Marmorerpeton, first described from the Middle Jurassic (Late Bathonian) sediments at Kirtlington, Oxfordshire, England (Evans et al, 1988; Skutschas and Gubin, 2012).
No public-domain images are available for either of these genera, but others of the stem-group fossils shown in Figure 1 are illustrated below (click on image for a larger view):
Names in   red indicate   that the fossil is younger  than the oldest known crown-group fossil.
Figure 2. Images of stem-Urodela
​Comparison of the above images, placed in order from the most basal forms to species that are closest to the crown group, indicates a trend from a wider to a narrower head, such that one of the more crownward species, Chunerpeton tianyiensis, has at least a superficial resemblance to modern salamanders.

Two fossils that are very close in age represent the oldest known representatives of the salamander crown group:
  1. Kiyatriton krasnolutskii, described from the Middle Jurassic (Bathonian) Itat Formation at the Berezovsk Quarry locality in Western Siberia, Russia (Skutschas, 2016). No image is available in the public domain.
  2. Neimengtriton daohugouensis, found in the Haifanggou Formation of Middle Jurassic (Bathonian) age at the Daohugou locality, Ningcheng County, Inner Mongolia, China (Jia et al, 2021). A fossil and a life restoration are shown below:
Figure 3. Images of crown-group salamander Neimengtriton daohugouensis
​As illustrated above in Figure 1, there is virtually no difference in age between the fossils of the salamander stem group and the oldest crown-group fossil. This contemporaneity corresponds to the big difference in age (around 81 million years) between the salamander stem group and the oldest fossils of the stem-Anura (the frogs and toads). This time gap is represented by the ghost lineage shown in blue in Figure 1. Thus the stem-to-crown transition of the salamanders must also have lasted around 81 million years, from Early Triassic to Middle Jurassic times.
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References

Evans, S. E., Milner, A. R., & Mussett, F. (1988). The earliest known salamanders (Amphibia, Caudata): a record from the Middle Jurassic of England. Geobios, 21(5), 539-552.

Jia, J., Li, G., & Gao, K. Q. (2022). Palatal morphology predicts the paleobiology of early salamanders. Elife, 11, e76864.

Marjanović, D., & Laurin, M. (2013). The origin (s) of extant amphibians: a review with emphasis on the “lepospondyl hypothesis”. Geodiversitas, 35(1), 207-272.

Rong, Y. F., Vasilyan, D., Dong, L. P., & Wang, Y. (2021). Revision of Chunerpeton tianyiense (Lissamphibia, Caudata): Is it a cryptobranchid salamander?. Palaeoworld, 30(4), 708-723.

Skutschas, P. P. (2016). A new crown-group salamander from the Middle Jurassic of Western Siberia, Russia. Palaeobiodiversity and Palaeoenvironments, 96(1), 41-48.

Skutschas, P. P., & Gubin, Y. M. (2012). A new salamander from the late Paleocene—early Eocene of Ukraine. Acta Palaeontologica Polonica, 57(1), 135-148.

Skutschas, P., & Martin, T. (2011). Cranial anatomy of the stem salamander Kokartus honorarius (Amphibia: Caudata) from the Middle Jurassic of Kyrgyzstan. Zoological Journal of the Linnean Society, 161(4), 816-838.

Image credits – Stem-Urodela
  • Header (A spotted salamander (Ambystoma maculatum) in Searsmont, Maine - September 30, 2014):  Fyn Kynd Photography from Searsmont, Maine, United States, CC BY 2.0 <https://creativecommons.org/licenses/by/2.0>, via Wikimedia Commons
  • Figure 2 (Karaurus sharovi, fossil):  Ghedoghedo, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons
  • Figure 2 (Karaurus sharovi, life restoration):  Nobu Tamura under a Creative Commons 3.0 Unported (CC BY-NC-ND 3.0) license
  • Figure 2 (Beiyanerpeton jianpingensis, 1 and 2):  Open Access article Gao, K. Q., & Shubin, N. H. (2012). Late jurassic salamandroid from western liaoning, China. Proceedings of the National Academy of Sciences, 109(15), 5767-5772.
  • Figure 2 (Chunerpeton tianyiensis, fossil):  Open Access article Rong, Y. F., Vasilyan, D., Dong, L. P., & Wang, Y. (2021). Revision of Chunerpeton tianyiense (Lissamphibia, Caudata): Is it a cryptobranchid salamander?. Palaeoworld, 30(4), 708-723.
  • Figure 2 (Chunerpeton tianyiensis, life restoration):  Nobu Tamura under a Creative Commons 3.0 Unported (CC BY-NC-ND 3.0) license
  • Figure 2 (Pangerpeton sinensis):  Wang, Y., Evans, S. E., CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons
  • Figure 3 (Neimengtriton daohugouensis, fossil):  Open Access article Jia, J., Anderson, J. S., & Gao, K. Q. (2021). Middle Jurassic stem hynobiids from China shed light on the evolution of basal salamanders. iScience, 24(7), 102744.
  • Figure 3 (Neimengtriton daohugouensis, life restoration):  Open Access article Jia, J., Anderson, J. S., & Gao, K. Q. (2021). Middle Jurassic stem hynobiids from China shed light on the evolution of basal salamanders. iScience, 24(7), 102744.
  • Home
  • Introduction
  • Evolution of life
    • Overview
    • Origin of the Eukaryotes
    • Animals >
      • Vertebrate stem group >
        • Cyclostome stem group
        • Hagfish stem group
        • Lamprey stem group
        • Gnathostome stem group
        • Chondrichthyan stem group
        • Chimaera stem group
        • Shark stem group
        • Osteichthyan stem group
        • Actinopterygian stem group
        • Bichir and reedfish stem group
        • Sturgeon and paddlefish stem group
        • Neopterygian stem group
        • Teleostean stem group
        • Holostean stem group
        • Sarcopterygian stem group
        • Coelacanth stem group
        • Lungfish stem group
        • Tetrapod stem group >
          • Mammalian stem group
          • Monotreme stem group
          • Therian stem group
          • Marsupial stem group
          • Eutherian stem group
          • Bat stem group
          • Pangolin stem group
          • Carnivoran stem group
          • Odd-toed ungulate stem group
          • Rodent stem group
          • Lagomorph stem group
          • Paenungulate stem group
          • Hyrax stem group
    • Land plants
    • Glossary
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