Conodonts are an extinct group of organisms, known from the Upper Cambrian to the Triassic. They have no extant representatives, and tooth-like buccal elements are usually the only remains of the animal found in the sediments. Therefore, most of their taxonomy rests on these elements that are good stratigraphic tools for these ancient periods, due to their rapid morphological evolution. Conodont species are usually described species that are based on either clusters of elements corresponding to an entire apparatus (natural assemblages), or on the most frequently preserved element. These described species are acceptable stratigraphic tools, but hardly consider the dimension of the variation that a biological species can encompass through time and space. In order to tackle temporal, environmental and biogeographical changes, recent studies have shown that morphological variation should be taken into account by quantitative analyses, aiming at getting at the closest of what the former species might have been.
Les conodontes sont des organismes éteints, connus du Cambrien supérieur au Trias. Ils n’ont pas de représentants actuels, et seules leurs pièces buccales sont en général préservées dans les sédiments. Ainsi, l’essentiel de la taxonomie repose sur ces pièces qui sont de bons outils stratigraphiques pour ces périodes anciennes, en raison de leur évolution morphologique rapide. Les espèces de conodontes sont généralement des espèces typologiques basées sur des regroupements d’éléments correspondant à un appareil complet (assemblage naturel) ou bien sur l’élément le mieux préservé. Ces espèces typologiques constituent des outils stratigraphiques appropriés, mais elles ne prennent pas en compte la variation qui caractérise une espèce biologique à travers le temps et l’espace. Pour suivre les changements temporels, environnementaux et géographiques, des études récentes ont montré que la variation morphologique devait être prise en compte par l’intermédiaire d’analyses quantitatives, permettant d’appréhender au mieux ce que les espèces de l’animal conodonte ont pu être.
The species concept is a key issue in a wide range of topics dealing with organisms, either extant or extinct, because the “species” is currently regarded as a basic unit at the heart of the evolutionary theory. Yet, it is a multifacetted concept and a wide range of definitions hide behind this single term. A diversity of meanings for the species was already recognised by Darwin, who claimed in his
In order to give a testable basis to this concept, a biological definition of a species has been proposed; that is, a species is a group of organisms that is capable of interbreeding and producing fertile offspring of both genders. A species should be distinct from others with which interbreeding does not (normally) happen (
A further problem arises when dealing with fossil organisms. Beyond a certain age and depending on the taphonomical setting, ancient DNA is much too degraded and cannot bring any light about the genetic similarity between extinct and living taxa. Morphological similarities remain thus as the only basis for a “species” concept in most of the fossil record. When close extant relatives still exist, confrontation between genetic and morphological data can provide a robust basis to the interpretation for morphological clusters as “species” even in the fossil record (
The case of the conodonts, which are extinct organisms known only from the ancient fossil record, from the Cambrian to the Triassic, presents a further challenge to the definition of the palaeontological species. Since conodonts have no known close extant relatives, any identification of conodont species necessarily relies on the sole morphological similarity. Yet, depending on the conservation of the fossils and their abundance and the purpose of scientific investigation, it appears that a wide range of underlying assumptions influence the definition of conodont “species”.
Conodonts were first discovered in the form of small denticles made of apatite, a kind of calcium phosphate similar to that of vertebrate teeth. They have been described first by
Although fundamental for a better understanding of what the animal may have looked like, these exceptional remains were nevertheless insufficient to estimate the morphological variation encompassed by a conodont species. Despite the exceptional preservation of soft tissues, these fossils were often incomplete, and this hindered advancement beyond a generic determination for most of these ten well-preserved specimens (
Within the conodont-bearing animal, only the elements made of apatite that compose its feeding apparatus have a good potential for fossilisation. A classification of the elements into proto-, para-, and euconodonts has been proposed, based on the structure of the apatite, and suggests differences in the growth pattern of the elements (
Defining species in conodonts should ideally be close to what a “biological” species would have been regarding the former entire animal, but should empirically rely on the morphology of the most frequent fossil remains, namely the elements possibly found as articulated apparatuses. By integrating more dimensions of the variation of the former animal, considering the entire apparatus seems the most seducing approach. Yet practical consideration aiming at gathering the largest possible number of conodonts for stratigraphic purposes led to the more and more frequent use of acid to dissolve the matrix of the sediments, from the 1840 s onwards. Consequently, conodont elements were more frequently found in isolation and conodont taxonomy turned into a parataxonomy, each kind of elements (needle-like-, comb-like-, or platform elements) receiving its own genus and species name, although potentially belonging to the same animal.
A taxonomy based on the apparatuses appears a good basis for proposing species notion integrating a multifacetted variation of the different elements. Finding really complete apparatuses is however quite rare. Approximately a hundred of such complete apparatuses are known from bedding-plane assemblages (
These exceptional cases of preservation provided important information about the composition of apparatuses, the relative position of the 7 to 8 types of morphologically distinct elements (needle-, comb-, and platform-like elements) in this apparatus (
Such complete apparatuses provided a basis for further interpretation of incomplete or disjoint clusters of elements that are occasionally found at the surface of bedding-planes (
Such an approach, which bases the description of a species on the entire apparatus (
Although widely applied for many periods of time, this approach based on the description of apparatuses sometimes meets with practical limitations, especially during the Devonian. First, such articulated apparatuses are rare during this time interval, possibly because of the dominant occurrence of carbonate deposits. In such sediments, dissolving the matrix for clearing fossils is a common practice that is highly efficient in delivering conodont elements even when rare, but these are then found in isolation. This contrasts with shale deposits that offer a good preservation potential for fragile clusters. A second problem that arises is that, despite attempts to reconstruct apparatuses based on the relative frequency of each type of element (
Devonian conodont assemblages are characterised by an overrepresentation of platform elements. These elements were both well preserved and highly variable across taxa of the same period and also through time. Therefore, focusing on platform elements only appeared as an acceptable alternative for such time periods and, accordingly, most taxonomic reviews bear on these elements that are extensively used for stratigraphic purposes (
Once it is accepted that considering a single element is a valid proxy for the entire animal, the problem remains to identify species within the variety of P1 elements of all size and shape in a given sample. Conodont species were traditionally identified on the basis of characters mostly related to the shape of the platform, but also including ornamental features such as nodes and costulations (
This practice has led to the description of numerous Late Devonian genera and species. This approach was proven very efficient for recognizing stratigraphic zones and allowed the definition of a well-resolved time-scale for the Late Devonian, during which the duration of conodont zones is estimated at less than one million year, an exceptionally good resolution for such ancient period (
The proliferation of “species”, and the effort to bring some statistical support to their demarcation, can be exemplified by the Late Devonian genus
However, a closer consideration of such morphometric analyses raises some major issues about their true significance. The specimens included in such analyses corresponded to a subset of the total variation, and only documented “typical” elements of the considered species. Of course, if only groups of specimens chosen to be different are included in a discriminant analysis, differences between these groups will hardly fail to be significant (
Indeed, the lack of well-defined clusters that would support the described species has been recognised by many authors, who mentioned transitional forms between species (
Such facts challenge the significance of the described species that have been widely used mostly for stratigraphic purposes. Indeed, some authors suggest that the described species may represent end-members within a range of morphological variation of a single species at different stages of its evolution (
Such results suggest that studies focused on evolutionary patterns in time and space should better rest on a definition of species that still relies on morphology, but explicitly includes the dimension of the variation that might have occurred within the former biological species. Morphological species in this sense still correspond to “a unit or a group of units that differs morphologically from other units”, but their definition should heavily rest on recognising the range of variation encompassed by the unit. This definition has been challenged in the modern biota by evidence that genetically distinct populations may look very similar (
The species concept used in conodont taxonomy varies depending on several empirical factors. Any definition of species relies on morphology only, since no further biologically relevant information, such as genetic data, are available to support a given definition of species in these long-extinct fossil animals that lack modern close relatives. Practically, several definitions of conodont species nevertheless lie behind this common notion of morphological similarities. Depending on the abundance of more or less complete apparatuses, species were defined either on the basis of clusters of elements corresponding to an articulated apparatus (multi-element approach), or on a particular type of elements (usually the platform elements, best preserved and showing many diagnostic features), considered as diagnostic for the entire animal (mono-element approach).
In both cases, the most widely used definition of conodont species insists on close similarities between the members of a species, exemplified by the holotype; this definition is close to the traditional typological definition of a species. This notion does not properly address the variation that might have occurred through time and space within the actual biological entity, and might lead to artificially split a large and continuous variation that occurred within the actual species. This problem does not prevent the relevance of the described species as valuable stratigraphic markers: being considered as practical tools, these stratigraphic markers only reach their limit when, in an effort to refine the accuracy of the stratigraphic zones, index forms are chosen that are too rare in the assemblages. This is the case of
Whatever the definition of species, one should admit that any attempt to define a conodont species is doomed to remain hypothetical for such ancient fossils devoid of any unambiguous modern relatives. The exceptionally preserved but extremely rare remains of the whole animal, despite their importance for a better understanding of the animal's biology and phylogenetic position, are too few to provide a further basis to definition of species through time and space. Confronted with these limits, any attempt to define species in conodonts should remain critical and oriented towards the aimed purpose, either stratigraphic or evolutionary problems.
We thank Didier Néraudeau for his invitation to participate to this special volume. An early version of the manuscript greatly benefited from the constructive comments of Pascale Chevret, Daniela Schmidt and Carlo Corradini. We are very grateful to P. Bultynck for his comments and to an anonymous reviewer for his constructive comments and a careful reading of the manuscript which greatly improved the English language. This is publication ISEM 2010-086.
Schematic representation of a
Représentation schématique de l’appareil buccal d’un
Schematic map of the relative position of elements in the typological scheme of notation proposed by
Représentation schématique de la position relative des éléments dans le schéma typologique de notation proposé par
Different specimens of the genus
Différents spécimens du genre
Morphological differentiation between four species of Late Frasnian
Différenciation morphologique de quatre espèces du genre
Morphological variation of
Variabilité de forme des éléments P1 du genre