Trilobite Names

Trilobites are named using the rules of zoological nomenclature published in English and French in the InternationaJ Code of ZooJogicaJ NomencJature (ICZN). This code is used worldwide by all scientists irrespective of the language of publication. In dealing with the names of trilobites (or any other kind of organism), it helps to understand the basic rules, how names come about, and how they can change over time.

For example, and as mentioned already, in 1824 J. E. Dekay first reported IsoteJus gigas Dekay, 1824, a name that remains unchanged to this day. The first name, with a capital first letter, IsoteJus, is the genus name and refers to a group of animals in which similar characteristics indicate a close evolutionary relationship. Genus names must be original and not used for any other grouping of fossil or living animal. The second mme, gigas, which is not capitalized, is the species name and ideally should refer only to a coherent group of interbreeding populations. Species names do not have to be unique, except within the same genus. There can be only one gigas within the genus IsoteJus, but species names can and do reoccur in different genera (plural of genus). Accordingly there are seven different New York trilobite genera with the species name trentonensis. The proper name, genus and species, is italicized in print. In descriptive literature the name of the describing author (Dekay in our example) and sometimes the date of publication (1824 in our example) follow the name. Changes to the genus name are not uncommon, and in these cases the name of original author and date are given in parentheses.

The etymology of trilobite names often refers to a morphological feature. IsoteJus means "similar" (iso-) "end" or "tail" (-teJus), referring to the similarity between the head and tail of this species, and gigas means "large" or "giant," referring to the large size of this species compared to most trilobites. Names are usually derived from Latin, ancient Greek, latinized ancient Greek, or Indo-European and are selected by the describing author. Names are often latinized, and the gender of the species follows that of the genus. In other words, if the genus name is changed, the ending on the species name must sometimes change to agree with it in gender.

Genera, that are considered to be closely related on the basis of shared characteristics, are grouped into families. Because families are larger and more distinctive, it is often easier to determine the family to which a new trilobite belongs than to determine its genus. New genera are constantly being erected, and trilobite species are frequently moved around as descriptive methodology becomes more sophisticated and new classification standards are adopted.

Families are further collected into orders, again based on inferred evolutionary relationships. There are currently eight orders in the class Trilobita of the phylum Arthropoda (Kaesler, 1997). There are additional taxonomic relationships rarely used herein, such as superfamily and suborder. Family names always have the suffix -idae; superfamily names, -acae; suborder, -ina; and order, -ida.*

The common New York trilobite EJdredgeops rana is taxo-nomically described as follows:

phylum Arthropoda class Trilobita order Phacopida family Phacopidae genus EJdredgeops species rana

To take another, somewhat more complex example, EJdredgeops rana (Green, 1832) was first named CaJymcne bufo var. (short for variety) rana by J. Green in 1832. (Green in the same publication had described CaJymene bufo from a poorly preserved phacopid specimen in a float boulder. The condition of the fossil was too poor to use for determining clear relationships and the name subsequently was abandoned.) In 1860 Emmons changed the species to Phacops bufo because of the greater similarity of the genus to the European genus Phacops than to CaJy-mene. Hall (1861) first called the common trilobite from the Hamilton shales and limestones, Phacops rana. Green's name and publication date now appear in parentheses because of the change in his original genus designation. This story is further complicated by the assignment of some new phacopids to subspecies of Phacops rana such as Phacops rana miJJeri Stewart, 1927

and Phacops rana rana (Green, 1832). (The designation of subspecies is not often used with trilobites because strictly speaking the term implies geographically separated populations that could interbreed given the opportunity. This is nearly impossible to determine from fossils, and most authors prefer the single species names Mich as Phacops rami ami Phacops milleri.) Struve ( 1990) re-examined the New World "Phacops" species and found them significantly different from the type species Phacops latifrons that was originally described from the Devonian of Germany. (The type species is the single species used to describe and define the genus.) He erected a new genus, Eldredgeops, with Eldredgeops milleri as the type species. The very familiar former Phacops rana is thus now properly referred to as Eldredgeops rana.

Species names are never changed by later authors, not even to correct spelling errors. The exceptions to this are if the name has already been used for a closely related animal in the same genus or if the same animal has been named by another person in an earlier publication. In almost all cases, priority is with the name given by the first author.

The number of trilobite species and genera is constantly increasing due to both new finds and redescriptions of previously collected material. There are also differing approaches to the concept of species. Some authors view speciation on the basis of small external changes and tend to propose new species based on these differences. Others view many small external differences as within the normal intraspecies variation and include a wider variety of specimens within a single species. The concept of species is not unambiguous in extant animals, and in the case of fossils morphological features are usually all there is available for evaluation. Statistical evaluation of fossils using measurements of key features is often currently used to define intraspecies variability, and the comparison of derived (uniquely shared) characteristics between closely related species is used to determine their evolutionary relationship. Systematics is the study of the similarities and differences in organisms and their related species. A. B. Smith (1994) presented an excellent in-depth review of systematics for the fossil record.

The rules of zoological nomenclature now require that an author, when describing a new species, must designate a specimen that clearly exemplifies this new species. This specimen should then be deposited in an appropriate public collection, such as a museum. This one specimen is called the holotype. Other specimens of a reference group (the "type series" or hypodigm) from which the holotype was chosen are called paratypes. In the 1800s and early 1900s authors often illustrated their new specimens but failed to designate a single type specimen and its repository. Later authors in referring to these specimens, provided they could be found, consider the members of the type series to be syntypes. The term cotype is also seen in collections; it is a synonym for syntype or paratype and its use is discouraged by the ICZN. Should an author need to choose a single specimen from the designated syntypes to be the single species representative, then this specimen becomes a lectotype. Other specimens in the original series become paralectotypes, and the term syntype can no longer be applied. If the species has no original type specimens that can be found and a sufficient taxonomic purpose is present, an author may designate a specimen to represent the type for the species. This specimen is called a neotype. If at all possible, the neotype should be from the same location and horizon as the originally described material. The description and name must be publicly issued as a permanent scientific record and available in multiple, identical copies. All these rules are thoroughly spelled out in the ICZN (1985).

Other type designations are commonly used in collections but do not bear ICZN recommendation. Hypotype is a specimen that was referred to, usually in publications, to extend or correct the knowledge of a species. Topotype refers to a specimen from the type locality, and plesiotype refers to specimens very close to the type. Plastotype is an artificial cast of the original type.

A difficulty one often encounters with trilobites, as well as other fossils, is that species were originally named when only a partial specimen was available. There are a number of instances where, for example, the pygidium of an uncommon trilobite bore one name and the cephalon a different one. Also in the 1800s trilobites were often identified with the same names as species from other locations but of the same geological age. A number of New York trilobites, for example, were given the same names as species from the Midwest, particularly Ohio, as well as some from Europe. Some of these names are only now being corrected as careful studies are made.

Another interesting situation is that early in the twentieth century, scientists going through museum collections saw differences in specimens and gave them new names, or listed them as subspecies, by noting the name on a label and leaving it with the specimen. These "museum label" names are commonly encountered with the Ordovician trilobites at the National Museum of Natural History (Smithsonian or USNM). Museum label names are not recognized by the ICZN and have no priority or recognition, except when subsequent authors choose to use them to name specimens. Later authors sometimes recognized these unpublished names with the designation "MS" (for "manuscript"). The official date for the name is when it is published, however, not when the museum label was made. An example of such a name is Isotelus walcotti Ulrich in C. D. Walcott, 1918. In this case E. O. Ulrich saw differences between the specimens from the New York Trenton Limestone, which were being called Isotelus iowensis, and the authentic trilobite from Iowa. He named the New York species /. walcotti on a museum label, and C. D. Walcott recognized the name in a subsequent publication in 1918. There is no specific rule or protocol for the recognition of museum label names, and it is solely to the discretion of the publishing author whether the name is recognized or not. Walcott chose to recognize Ulrich; thus it is appropriate, but not required, to add his name to the final formal trilobite name.

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Responses

  • Berylla
    Why was phacops renamed to eldredgeops?
    8 years ago
  • amerigo costa
    What are the living conditions of phacops?
    7 years ago
  • paladin
    How do name a triobite?
    5 months ago
  • belba
    What name to give a trilobite?
    3 months ago

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