Welcome to the blog of the Mesozoic vertebrates research group of the Bayerische Staatssammlung für Paläontologie und Geologie

Wednesday, 11 July 2012

What, if anything, is Lepidotes?

There are certain taxa of Mesozoic vertebrates that every palaeontologist knows. One of these is certainly Lepidotes. Originally named by the great Louis Agassiz in 1832 based on a fish from the Early Jurassic of south-western Germany, the genus soon became a "wastebasket" for any remain of a Mesozoic fish with thick bony scales. Thus, there is a plethora of Mesozoic fish species referred to the genus Lepidotes, spanning the time from the Early Jurassic to the latest Cretaceous, or a time span of some 130 million years. Many of the fishes that were not referred to Lepidotes became species of Semionotus, another wastebasket taxon. Disentangling this mess seemed a task that no palaeontologist was willing to tackle.

Yes, this is a Lepidotes! Lepidotes gigas from the type horizon of the genus, the Posidonia Shale, at the BSPG.

Lepidotes and the other bony-scaled fishes represent a number of primitive lineages that were common in the Mesozoic, but dwindled to a mere nine or ten species today, which seem insignificant in comparison to the more than 25,000 species of the modern ray-finned fishes (teleosts). Nevertheless, these few species are the only available evidence for the evolutionary history that led to the origin of the modern fishes - with the exception of the fossils. However, interpreting the evolution of fishes in the Mesozoic is severely hampered by the lack of hypotheses on the interrelationships of many of the important lineages. Referring every second fish with thick bony scales from Mesozoic deposits to one of two genera certainly doesn't help in this respect. Lepidotes and several other Mesozoic fishes - most notably the almost as inflated Semionotus - are usually included in a group called Semionotiformes - which is widely used, despite the fact that most experts in fossil fishes recognized that semionotiforms are not a natural group.

Not a Lepidotes: Macrosemimimus fegerti under UV light. Foto H. Tischlinger.
This was the situation when Adriana López-Arbarello started with a project on semionotiform fishes in 2006. The German Research Foundation (DFG) financed this impossible seeming task, first for two years, but with extensions of anther two years. And thus Adriana went to work, logically first with revising materials referred to Lepidotes and Semionotus, starting with the original species of these two genera. Thus, over the years, she and her co-workers published new descriptions of the type species of Semionotus, a redescription of Neosemionouts, and coined new names for two other groups of species that were originally referred to Lepidotes, Scheenstia and Macrosemimimus. She further studied new semionotiforms, three of which are already published, Tlayuamichin from the Cretaceous of Mexico,  Sangiorgioichthys sui from the Triassic of China and Lepidotes pankowski from the Cretaceous of Morocco. But, apart from this taxonomic work, she mainly used the detailed studies of many specimens and the visits to collections to collect data for an analysis of the interrelationships of these fishes.

This analysis was now published in the journal PLoS One. It represents the largest and most explicit phylogenetic analysis of semionotiform fishes. Adriana found many interesting results, most importantly the division of semionotiforms into two separate lineages, one of them including Semionotus and its relatives and the other Lepidotes and the fishes more closely related to this genus. This might still not be too surprising - but the latter lineage was found to lead to the modern gars, and the former includes a group that was so far considered to be an own lineage of Mesozoic fishes, the macrosemiids. Based on these relationships, Adriana decided to use the name Lepisosteiformes for the Lepidotes-gar lineage (Lepisosteiformes being the group that modern gars belong to) and restrict Semionotiformes for the Semionotus-macrosemiid lineage. Both lineages were united in the group Ginglymodii.

The interrelationships of ginglymoidian fishes according to López-Arbarello (2012).

These results have far-reaching implications. For one thing, ginglymodians are a more diverse group than previously recognized, being represented by many different lineages with often long ghost lineages (extensions of the lineage for which fossil evidence is still missing), indicating that these fishes were also taxonomically more divers. Furthermore, ginglymodian fishes were ecologically divers, and the recognition of Lepidotes and its kin as fossil relatives of modern gars helps to understand the evolutionary history of these fishes.

So, Adriana has made an important step in her studies of the interrelationships of Mesozoic fishes that stand at the base of modern groups. Does that mean that everything is known now about the evolution of the Ginglymodii, and she can turn to other things? Certainly not, Adriana's work represents only a beginning. There are many more species of Lepidotes and Semionotus to be revised and to be included in the analysis of the interrelationships, and detailed studies of other groups of Mesozoic fishes are necessary to further elucidate the origin of modern clades of bony fishes. However, her paper presents a first reference frame for such studies. It includes a wealth of anatomical and character data that are of importance for the interpretation of the interrelationships of Mesozoic fishes and presents hypotheses of interrelationships that are sure to spark discussion and renewed interest in these groups in ichthyologists. Thus, it will provide fertile ground for future studies of the role that Mesozoic fishes play in our understanding of the origin of our modern vertebrate diversity.


  1. Any chance you can provide the references to the papers you cite in your post?

  2. The references are usually linked in the actual blog text. Try using the links on the taxon names or journal names in the text.