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Notropis megalepis fossil scan

A series of studies in evolutionary biology during the 1970’s and early 1980’s focused on the North American fish families Centrarchidae (sunfishes and basses) and Cyprinidae (minnows and shiners) as exemplars of the extremes of the punctuated and gradual “modes” of differentiation with regard to both anagenesis (evolution within a species lineage) and cladogenesis (speciation).  The family Centrarchidae includes roughly 30 species and is thought to be endemic to North America and, based on fossil evidence, dates to the Pliocene period (Cavender 1986).  Cyprinidae is the most diverse group of freshwater fishes in North America with over 300 species.  Within this family the genus Notropis is the most diverse with upwards to 120 species depending upon the classification, and the oldest fossils are from the Middle Pliocene. The contrast in diversity of these two, presumably natural groups of equal age made them particularly interesting for addressing questions related to rates of anagenesis and cladogenesis.

Image of fossil Notropis megalepis

Historically, the stark contrast in diversity between North America's sunfish and minnow and shiner species served as a fertile research area for studying differential rates of evolution in two groups of equal age (based on fossil record).  Cladistic analyses of osteological characters by Mayden (1986, 1989) revealed fundamental flaws with previous studies of this nature and classifications of these fishes, thus questioning the accuracy of prior evolutionary studies.  Today, phylogenetic relationships of species of Notropis (shiners) and relatives are relatively well known due to recent morphological and molecular analyses.  Relationships of species of Centrarchidae, also now available based on morphological and molecular data, will provide the necessary information to begin comparative studies between the sunfish and shiner groups as envisioned by earlier researchers – rate heterogeneity in evolution.  However, a notable problem remaining in advancing this research has been the phylogenetic placement of the species Notropis megalepis Smith 1962, the oldest known member of the genus and the species used to date the Notropis lineage.  This “largescale shiner” dates to the Middle Pliocene and is represented by one complete specimen and three partial specimens.  It is described from the Ogallala Formation, a diatomaceous marl deposit dating to the Middle Pliocene found in present day western, Kansas.  This species has never been examined for its phylogenetic placement among other cyprinids.  We have examined this species using the high-resolution CT scans using the recent technology developed at The University of Texas NSF Multi-user facility maintained by Dr. Tim Rowe.  This fossil is fundamental in dating the age of the North American shiners and other minnow groups to which it belongs, and in evaluating rate molecular and morphological heterogeneity in evolution for these two groups of fishes. The one interesting aspects of this study is that we have, for the first time, experimented with the specimens of Notropis megalepis in reconstructing part and counterpart “specimens” in the same visualization.  While done by artists this is the first time that it has been accomplished using this technology.

The link to see animations of the fossil scans is located at:
http://digimorph.org/specimens/Notropis_megalepis/KU_849a/

This research was supported by a grant from the Division of Environmental Biology, National Science Foundation.

Questions or comments? Contact Director, Dr. Richard Mayden

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