An investigation of the genus Mesacanthus (Chordata: Acanthodii) from the Orcadian Basin and Midland Valley areas of Northern and Central Scotland using traditional morphometrics.

Matthew G. Baron

doi:10.7717/peerj.1331

Abstract

Mesacanthus is a common and speciose genus of acanthodian fish from Lower Old Red Sandstone and Middle Old Red Sandstone assemblages (representing the Lower Devonian and Middle Devonian respectively) and is well represented in many palaeoichthyology collections in the UK. Based upon descriptions given during the 19th century, specimens of the genus Mesacanthus from the Orcadian Basin and Midland Valley areas of Northern and Central Scotland have historically been referred to a number of different species; of these, the most frequently discussed in the literature are M. mitchelli, M. peachi and M. pusillus. In order to test the validity of these three species, traditional morphometric analyses were carried out on over 100 specimens of Mesacanthus, from both the Lower Devonian and the Middle Devonian, that cover the full range of known localities for these taxa in Northern and Central Scotland. Based upon morphological and morphometric comparisons, this investigation has found that at least two species of Mesacanthus are valid (M. mitchelli and M. pusillus) as specimens from the Lower Devonian and Middle Devonian have been shown to differ significantly in a number of important ways. However, no evidence has been found for the validity of the second and distinct Middle Devonian species, M. peachi.

Highlights

Acanthodians (‘spiny sharks’) are important early representatives of Gnathostomata (Gegenbaur, 1874) and have been central in the debate on the origin of modern gnathostomes (Davis, Finarelli & Coates, 2012; Zhu et al, 2013)
Combining all of the Middle Old Red Sandstone specimens into one category and comparing their length to width ratios with those of M. mitchelli recovered a significant difference between these two groups (p-value = 8.07E−09)
It is worth noting that, of the other combinations of measurements that were subjected to t-tests, the sum of the distance between the pectoral and pelvic fin spines divided by the overall length of the specimen ((PP + PA)/L) was found to be significantly different between M. mitchelli and M peachi (p-value = 0.0204) and M. mitchelli and M. pusillus (p-value = 0.0436), but, importantly, not significant between M. peachi and M. pusillus (p-value = 0.755)

Summary

Introduction

Acanthodians (‘spiny sharks’) are important early representatives of Gnathostomata (Gegenbaur, 1874) and have been central in the debate on the origin of modern gnathostomes (Davis, Finarelli & Coates, 2012; Zhu et al, 2013). The genus Mesacanthus (Traquair, How to cite this article Baron (2015), An investigation of the genus Mesacanthus (Chordata: Acanthodii) from the Orcadian Basin and Midland Valley areas of Northern and Central Scotland using traditional morphometrics. 1888) is found in the sedimentary rocks of the Orcadian Basin and Midland Valley areas of Northern and Central Scotland, in both the Lower Old Red Sandstone and the Middle Old Red Sandstone, which correspond to the Lower Devonian and Middle Devonian respectively. The faunal composition markedly changed between the Lower and Middle Devonian, but the genus Mesacanthus is present in both of these stages and is quite common, representing up to 48 percent of the total number of specimens found in early Devonian formations (Trewin & Davidson, 1995)

Methods

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Discussion

Conclusion