Braincase simplification and the origin of lissamphibians

Abstract

Dissorophoidea, a group of temnospondyl tetrapods that first appear in the Late Carboniferous, is made up of two clades ⎼ Olsoniformes and Amphibamiformes (Branchiosauridae and Amphibamidae) ⎼ the latter of which is widely thought to have given rise to living amphibians (i.e., Lissamphibia). The lissamphibian braincase has a highly derived morphology with several secondarily lost elements; however, these losses have never been incorporated into phylogenetic analyses and thus the timing and nature of these evolutionary events remain unknown. Hindering research into this problem has been the lack of phylogenetic analyses of Dissorophoidea that includes both taxonomically dense sampling and specific characters to document changes in the braincase in the lineage leading to Lissamphibia. Here we build on a recent, broadly sampled dissorophoid phylogenetic analysis to visualize key events in the evolution of the lissamphibian braincase. Our ancestral character state reconstructions show a clear, step-wise trend towards reduction of braincase ossification leading to lissamphibians, including reduction of the sphenethmoid, loss of the basioccipital at the Amphibamiformes node, and further loss of both the basisphenoid and the hypoglossal nerve foramina at the Lissamphibia node. Our analysis confirms that the highly derived condition of the lissamphibian braincase is characterized by overall simplification in terms of the number and extent of chondrocranial ossifications.

Fig 1

A comparison between the braincases of the extinct tetrapod Eryops (A, B) and the extant salamander Hynobius (C, D), in lateral (A, C) and occipital (B, D) views, summarizing the main differences between lissamphibian braincase composition and morphology and that of other tetrapods. The sphenethmoid is purple, the basioccipital is blue, the basisphenoid is red, the exoccipitals are green, the parasphenoid is yellow, the opisthotic is orange and the prootic is brown.

Fig 2. The results of the phylogenetic analyses of Dissorophoidea.

The Bayesian analysis All Compatible Clades tree (left) obtained from MrBayes and the strict consensus tree (right) of 81 922 most parsimonious trees obtained from PAUP.

Fig 3. Summary of the ancestral character state reconstructions of the braincase characters on slightly pruned trees derived from both parsimony and Bayesian analyses.

A) Parsimony and maximum likelihood ancestral character state reconstructions of the braincase characters on the strict consensus tree derived from the parsimony analysis. Coloured lines represent zones retrieved as ambiguous for the optimization of each character state under the parsimony criterion, and dots represent the locations retrieved as most probable location for each character state. B) Maximum Likelihood ancestral character state reconstructions of the braincase characters on the All Compatible clades consensus tree derived from the Bayesian analysis. The discrete states of the four characters used in this analysis (93, basioccipital loss in blue; 109, basisphenoid loss in red; 110 hypoglossal nerve, n. XII, foramina lost in green; 111, sphenethmoid reduced to paired structures in purple) are described in the locations where states were optimized. Other reductions that are described in the text, but unable to be confirmed in fossil taxa (certain potential reductions) are indicated with asterisks.