Are we naming to many species?

After studying hundreds of specimens of Ichthyosaurus, Dean Lomax believes many 'new' fossil species could be variants of existing ones

The Biologist 65(3) p6

From deep under the sea to the treetops of unexplored forests, numerous new species of living animals and plants are found and described each year. Similarly, hundreds to thousands of new fossil species, ranging from insects to dinosaurs, are also discovered each year.

With less than 1% of prehistoric life estimated to be represented in the fossil record, each new fossil discovery is important and contributes to our understanding of past life. However, in one of my recent studies with professor Judy Massare (SUNY College at Brockport, New York, US), we question the practice of naming new fossil species on the basis of fragmentary material.

Traditionally, a species is defined as a group of organisms that can interbreed and produce fertile offspring. Since Carl Linnaeus (1707–1778) introduced a system to document, describe and name living and fossil organisms, the concept of a species has provided scientists with a way of classifying and recording each organism, which in turn helps scientists uncover how organisms relate to each other and where they sit in the evolutionary tree of life.

Describing new species of living animals is much easier than describing new fossil species. Fossil organisms cannot be observed in their natural habitat and specimens differ substantially in completeness and preservation. Other factors that are difficult to assess in fossils include sexual dimorphism and ontogenetic variation (from a juvenile to an adult).

Palaeontologists fall into one of two camps when it comes to naming species: 'lumpers', who lump groups of similar specimens together; and 'splitters', who split up specimens and distinguish new species. One feature that might look 'different' or 'unique' to one researcher might appear 'variable' to another.

There are several implications of overestimating and underestimating species. One of the most important is this can provide a false picture of species diversity.

Our study[1] examined hundreds of specimens of the common early Jurassic ichthyosaur, Ichthyosaurus, and focused on one part of the skeleton, the hindfin. We wanted to evaluate the variation among the six known species of Ichthyosaurus and identified 99 specimens that provided useful information.

Initially, we found different types of hindfin that appeared to represent distinct species. However, the more specimens we examined, the more 'variation' we uncovered, such as the relative differences in the size and number of bones. In isolation, a single hindfin could not be used to distinguish among species of Ichthyosaurus, but a particular variation was representative of certain species.

Our findings show that with only a few specimens, features can be found that differ substantially from one specimen to the next, and so appear as if there are multiple species. Thus, gaps in hindfin variation are larger when fewer specimens are available.

In reality, with a much larger sample size, the gaps in the 'unique' variations are filled in, showing that differences are simply the result of individual variation and a lack of the full picture. This means that it is therefore easier to identify a new species on the basis of a single specimen than with multiple specimens.

We also found several very odd variations that, again, in isolation could easily be interpreted as representing a new species. But on examination of more specimens, we also found that these were simply the result of a continuum of variation, whereas others were likely the result of pathology.
If we split up all of these specimens, based on individual variations, it would mean we would potentially have 30 or more species. This is effectively what happened for much of the 19th century, when any new fossil find, from a new location or horizon, was named as a new species if it differed slightly from previous specimens.

DL OP IchthyosaurusA depiction of Ichthyosaurus anningae by James McKay

Various issues may arise when new species are named on this basis. One of the most extreme examples is the Cambrian arthropod Anomalocaris: prior to the discovery of a complete specimen, individual parts had been identified as new individual animals. The appendages were misidentified as the body of a crustacean-like creature, while the mouth was mistaken for a jellyfish.

Considering the fragmentary nature of the fossil record, it can be extremely difficult to decipher whether a specimen does genuinely represent something new. A more complete specimen or collection of specimens can demonstrate the 'new taxon' is simply variation of a known species.

To avoid making such errors, which lead to taxonomic complications, fossils should not be given new species names on the basis of just a couple of minor differences from known taxa, or defined on very fragmentary remains.

Dean Lomax is a palaeontologist and visiting scientist at the University of Manchester. He can be found tweeting @Dean_R_Lomax

1) Massare, J. A. & Lomax, D. R. Hindfins of Ichthyosaurus: effects of large sample size on 'distinct' morphological characters. Geological Magazine, 1–20 (2018). doi:10.1017/S0016756818000146

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