Today Paul Barrett generously gives up his time to write about the newly named igunaodonitan dinosaur Kukufeldia and the problems of providing definitions and diagnoses of new taxa in the fossil record:
What are the criteria that palaeontologists use when proposing a new species? Historically, many different categories of information have been used to justify naming new taxa, most often the recognition of unique morphology. However, new names have also been erected on the basis of material that is not anatomically distinctive, but which hails from previously unexplored or undocumented geographical regions or time periods. Currently, the ‘ideal’ basis for identifying a new taxon would be to find material that possesses clear unambiguous features that are unique to that species (known technically as ‘autapomorphies’), although where such features are not present specimens with unique combinations of characters also have currency. However, it needs to be remembered that even autapomorphies can be shown to have wider distributions within groups of organisms as more species are described, so that features once considered unique and diagnostic can become commonplace. A good example of this phenomenon, which has been termed ‘obsolescence’, comes from the sauropod dinosaur genus ‘Titanosaurus’ (Wilson & Upchurch 2003). When named in 1877, on the basis of isolated vertebrae from the latest Cretaceous of India, ‘Titanosaurus’ could be distinguished on the basis of its strongly procoelous caudal vertebrae (in which the articular ball of the vertebra is on the rear surface), a feature that was absent in all other sauropods named at that time. However, numerous discoveries have since demonstrated that procoely is widespread among sauropods (in titanosaurs in general and some ‘mamenchisaurs’, for example) and as a result, could not be regarded as a reliable diagnostic feature for ‘Titanosaurus’.
As the type material of this genus can no longer be shown to possess either autapomorphies or a unique character combination, it has since been sunk into the taxonomic limbo of nomen dubium (i.e. an invalid species). These kinds of historical problems have led to an aspiration (or desire) to name new species on better specimens that have the potential to offer as much anatomical information as possible. Nevertheless, it should be remembered that it is not the quantity of material that is important per se (for example, many beautiful, complete hadrosaur skulls were given their own names in the past and are now considered as junior synonyms of other taxa), but its quality. In this sense quality does not refer to the completeness or preservational state of the specimen, but its informativeness. A poorly preserved single element can still fulfil all of the criteria to serve as a robust, name-bearing type specimen if it can be demonstrated to be unique beyond reasonable doubt, whereas an almost complete specimen would be useless if missing those features that might allow it to be recognised as either something new or as a referred specimen of an existing species.
All of these points have a strong bearing on a series of current controversies surrounding a plexus of iguanodontian dinosaur species from the Lower Cretaceous Wealden Group of southern England and its equivalents in western Europe. Historically, all of the large ornithopod dinosaurs from these deposits have been attributed to the genus Iguanodon, although a large number of species were recognised (I. anglicus, I. atherfieldensis, I. bernissartensis, I. dawsoni, I. fittoni, I. hollingtonensis, I. mantelli, I. seeleyi and several other iguanodontian genera such as Sphenospondylus and Vectisaurus). Detailed work by David Norman recognised that several of these species were probably the same taxon (e.g. I. fittoni and I. hollingtonensis). However, more recently many of these species have been shown to be distinct from Iguanodon and removed to genera of their own (Paul 2007, 2008; Norman 2010). These include the new genera Barilium (Norman 2010: for ‘I.’ dawsoni), Dollodon (Paul 2008: for a specimen previously referred to ‘I.’ atherfieldensis), Hypselospinus (Norman 2010: for ‘I.’ fittoni and ‘I.’ hollingtonensis) and Mantellisaurus (Paul 2007: for ‘I.’ atherfieldensis). As several authors were working independently, but simultaneously, on these specimens, a recent paper by Carpenter & Ishida (2010) added to this list with several other new names: Proplanicoxa, Sellacoxa, Torilion and Wadhurstia. Wadhurstia refers to the specimen formerly called ‘I.’ fittoni and Torilion to ‘I.’ dawsoni – however, as Norman (2010) had already provided replacement names for these species, the new names proposed by Carpenter & Ishida (2010) instantly become junior objective synonyms of Norman’s new names (Hypselospinus and Barilium, respectively). Sellacoxa was erected for a specimen that Norman (2010) regards as an individual of Barilium and Proplanicoxa for a specimen previously regarded as ‘I.’ atherfieldensis (or Vectisaurus). As a result of this work, the taxonomy of these animals has become considerably more complex, with little agreement among authors on the criteria used to pull specimens apart into different species. Nevertheless, it does seem that the diversity of these faunas was higher than has generally been appreciated.
Adding to these discussions was a paper by Andrew McDonald, myself and Sandra Chapman, naming a new species of iguanodontian on the basis of a single right dentary from the classic Wealden locality of Cuckfield (McDonald et al. 2010). This dentary, housed in the collections of the Natural History Museum, was acquired by Captain Lambart Brickenden and presented to Gideon Mantell, who subsequently described it as a jaw of Iguanodon in 1848. Since that time it has gone largely unnoticed in the museum’s collection and was only mentioned in passing in a handful of nineteenth century publications, that is until Andrew spent time examining the iguanodontians in our basement as part of his PhD research. Andrew initially noted some unusual features of the jaw and our further discussions convinced us that it possessed a combination of features that allowed it to be distinguished from all other iguanodontians. In particular, the dentary possesses one clear autapomorphy: a secondary row of small holes for blood vessels and nerves that extends behind and sub-parallel to the usual, and more conspicuous, row of such holes that is commonly present in other ornithischian dinosaurs. As we compared the ‘Brickenden jaw’ with other iguanodontians, our conviction that this specimen represented something different from existing Wealden taxa grew. Eventually this led to us to submit a paper on the specimen to the journal Zootaxa, where we described it as Kukufeldia tilgatensis, honouring the Cuckfield locality. Although the material described is fragmentary, and not the ideal specimen that one might wish for as a holotype, it is taxonomically informative and currently bears a feature that is otherwise unknown elsewhere among iguanodontians. It may be that Kukufeldia ultimately proves to be referable to one of the other named Wealden iguanodontians, but until overlapping material of sufficient quality is present to test that possibility, the unique nature of its anatomy suggests that it is useful to regard it as a new species, representing a morphology that was previously unrecognised.
Given this recent flurry of activity, a period of consolidation is now required to assess the validity of all of these taxa and to find consensus upon which specimens can be confidently referred to each species. The differing opinions of these authors, as well as the features that their analyses share in common (there are some!), illustrate well the need to assess objectively the information offered by a specimen when deciding on whether it is sufficiently distinctive to deserve its type status.
Carpenter, K. & Ishida, Y. 2010. Early and “Middle” Cretaceous iguanodonts in time and space. Iberian Journal of Geology 36: 145–164.
McDonald, A. T., Barrett, P. M. & Chapman, S. D. 2010. A new basal iguanodont (Dinosauria: Ornithischia) from the Wealden (Lower Cretaceous) of England. Zootaxa 2569: 1–43.
Norman, D. B. 2010. A taxonomy of iguanodontians (Dinosauria: Ornithopoda) from the lower Wealden Group (Cretaceous: Valanginian) of southern England. Zootaxa 2489: 47–66.
Paul, G. S. 2007. Turning the old into the new: a separate genus for the gracile iguanodont from the Wealden of England. 69–77. In Carpenter, K. (ed.) Horns and beaks: ceratopsian and ornithopod dinosaurs. Indiana University Press, Bloomington and Indianapolis, 369 pp.
–––– 2008. A revised taxonomy of the iguanodont dinosaur genera and species. Cretaceous Research 29: 192–216.
Wilson, J. A. & Upchurch, P. 2003. A revision of Titanosaurus (Dinosauria – Sauropoda), the first ‘Gondwanan’ dinosaur genus. Journal of Systematic Palaeontology 1: 125–60.