High Level Trilobite Systematics

Abstract

This page reviews the Trilobita at the ordinal level.

Keywords: Trilobite orders, Agnostida, Redlichiida, Lichida, Corynexochida, Phacopida, Ptychopariida, Harpetida, Asaphida, Proetida

Introduction

"Trilobite classification systems have been claimed as 'natural' at least since the time of Salter (1864). Different authors have pursued their individual meanings of 'natural', although most have claimed that their classifications also reflect phylogeny. Two approaches to phylogenetics are representative: the tree-based, or stratigraphic approach, and the key-character approach respectively. The former is exemplified particularly by Cambrian specialists such as Franco Rasetti; the intention is to reproduce the actual tree of descent by means of stratigraphic and evolutionary series of species - the classification is imposed after the reconstruction of the tree. The key character approach identifies characters which are considered to be reliable indicators of descent - synapomorphies in modern understanding" (Fortey 1997, Abstract.)

Today there are nine generally recognised orders of trilobites (some authorities still recognise the Naraoiidae or Nectaspida as a tenth). The following table lists them, their main diagnostic characteristics, plus some representative taxa from each one.

The last four orders listed – Ptychopariida, Harpetida, Asaphida and Proetida – share (at least in primitive forms) a natant hypostomal condition, leading Fortey 1990 to propose a phylogenetic relationship between these forms and a new subclass, Libristoma, to group them. A subclass-level organisation for the remaining orders has not emerged.

"Both approaches seek to identify confusing examples of convergence (multiple homoplasies). These convergences were probably a response to repeatedly adopted modes of life, although there are few morphotypes corresponding with plausible life scenarios. As an example, I shall consider the effectiveness of the stratigraphic approach versus the character-based approach in resolving the classification and relationships of those trilobites with highly secondarily specialised pelagic morphology. Although it is certainly parsimonious to assume that unusual characters have appeared only once, it will be shown that past assumptions about uniquely derived characters as a basis for classification have frequently been overturned by subsequent critical studies. It is possible that some of the characters currently employed as the basis for high level taxa will also be shown to be capable of multiple derivation. For example, ventral skeletal morphology shows that the mode of attachment of the hypostome may have been directly related to feeding habits - and hence a possible target for convergent evolution." (Fortey 1997, Abstract.)

Systematics

Phylum Arthropoda von Siebold & Stannius 1845

Tactopoda Budd 2000

Supersubphylum Euarthropoda Lankester 1904

Subphylum Arachnomorpha Heider 1913

Superclass Trilobitomorpha Stĝrmer 1944

(= "Trilobite clade" Cotton & Braddy 2000)

Class Trilobita Walch 1771

Description: Marine arthropods characterised by a generally subelliptical, dorsal, chitinous exoskeleton divided longitudinally into three distinct lobes, and having a distinct, ralatively large head shield (cephalon) articulating axially with a thorax comprising articulated transverse segments, the hindmost of which are almost invariably fused to form a tail shield (pygidium).

Order Agnostida Kobayashi 1935

Stratigraphic Range: Lower Cambrian to Upper Ordovician

Discussion: The Agnostida mostly lack eyes – however, the oldest well-preserved trilobite eyes are in fact found in eodiscids from the Lower Cambrian of China (Clarkson 1993, p. 352).

The Agnostida includes two suborders, the Agnostina and Eodiscina, comprising (to a first order approximation) members having two and three thoracic segments, respectively. However, the order may not be a natural clade; it may be paraphyletic. No one doubts that Eodiscina are trilobites, but the systematic position of the Agnostina has been much debated. Some authors maintain that they are not trilobites at all, placing emphasis on their unique characteristics (primitive blindness, unusual limbs, cephalothoracic aperture) and lack of any morphologically intermediate taxa connecting them to other trilobites. Others – probably the majority – view them as 'good' trilobites, albeit highly specialised, sharing a common ancestry with the more primitive Eodiscina and possibly derived from them.

Representative Genera: Peronopsis (fig. 1), Condylopyge (suborder Agnostina), Eodiscus (suborder Eodiscina)

Order Redlichiida Richter 1932

Stratigraphic Range: Lower Cambrian to Middle Cambrian

Discussion: These trilobites, particularly Fallotaspis, Olenellus and allied genera are of special interest in being the first trilobites to appear in the fossil record. They possess a number of features which are regarded as being primitive; particularly the lack of facial sutures (cf. Naraoiidae). It is believed these forms are the most likely to shed light on the obscure origin of trilobites and their phylogenetic relationship with other arthropods.

Representative Genera: Redlichia (fig. 2B), Emuella, Paradoxides (suborder Redlichiina); Olenellus, Holmia (suborder Olenellina); Fallotaspis, Nevadella (conventionally placed in Olenellina but there is evidence that the superfamily Fallotaspidoidea belongs in suborder Redlichiina)

Order Corynexochida Kobayashi 1935

Stratigraphic Range: Lower Cambrian to Middle Devonian

Discussion: Three suborders comprise the Corynexochida, the   Corynexochina, Leiostegiina and the widely-known and collected Illaenina.

In 1975, several member groups of the Illaenina were reassigned to a newly erected order, Proetida, by Richard Fortey and R. M. Owens (Fortey and Owens 1975). Earlier references (e.g. in Moore 1959) depict a larger suborder Illaenina, including such taxa as Bathyurus and Proetus.

Representative Genera: Corynexochus (suborder Corynexochina, also Olenoides, fig. 3A); Illaenus (fig. 3B, suborder Illaenina); Lloydia (suborder Leiostegiina)

Order Lichida Moore 1959

Stratigraphic Range: Lower Ordovician to Upper Devonian

Discussion: The first edition of the Treatise (Moore 1959, p. O504) established the Odontopleurida as an order in its own right. Later, Thomas and Holloway 1988 argued that the Lichida and Moore's Odontopleurida comprised a monophyletic group. This view has been supported by subsequent authors (e.g. Ramsköld 1991) and is the view adopted in the revised Treatise (Fortey in Whittington et al. 1997, p. 299). However, opinion is divided and the Odontopleuroidea is still considered by some (e.g. Rich et al. 1996) to be an order in its own right.

Representative Genera: Arctinurus (fig. 4A, superfamily Lichoidea); Leonaspis (fig. 4B, superfamily Odontopleuroidea); Damesella (superfamily Dameselloidea)

Order Phacopida Salter 1864

Stratigraphic Range: Lower Ordovician to Upper Devonian

Discussion: The Phacopida comprises three suborders which share a distinctive protaspis type. The Cheirurina and Calymenina retain a rostral plate (an apomorphy) but in virtually all Phacopina the free cheeks are yoked as a single piece. The Phacopina further have schizochroal eyes and a distinctive glabellar structure, typically highly inflated and protruding forward over the anterior edge of the cephalon.

The Calymenina exhibit the most primitive facial suture morphology of the group. Their circumocular suture is also primitive. Eldredge 1977 considered that the Calymenina might in fact be more closely related to the Ptychopariida; however, this view was not adopted in the revise Treatise (Whittington et al. 1997).

Representative Genera: Cheirurus (fig. 5A, suborder Cheirurina); Calymene (suborder Calymenina); Acaste, Dalmanites, Phacops (suborder Phacopina)

Subclass Libristoma Fortey 1990, p. 557

Original Diagnosis: Trilobites having natant hypostomal condition, or with secondarily conterminant or impendent hypostomal condition.
 

Order Ptychopariida Swinnerton 1915

Stratigraphic Range: Lower Cambrian to Upper Devonian

Discussion: The Ptychopariida comprises three suborders: the Ptychopariina, Olenina and Harpina. Early (Lower Cambrian to Ordovician) Ptychopariina (fig. 7A) are characterised by "a tapering glabella, opisthoparian sutures, rim-like cepahlic borders extending into genal spines, natant hypostome, usually more than 12 thoracic segments, and a small, transverse, well-furrowed pygidium without remarkable features. Such a morphology is often referred to as generalized, but it is quite far removed from the primitive Redlichiida design, and it might be better to refer to it as exceptionally durable by virtue of a successful specialization" (Fortey in Whittington et al. 1997, p. 296). Partly because this obviously successful design was so persistent, the ptychopariid morphology usually varying only slightly within these basic parameters, it has been difficult to construct useful families and even genera within this group. The Ptychopariina represent the primitive morphology of the libristomate trilobites, though the suborder itself may yet prove to be paraphyletic.

"[A]ll except the very earliest members of the Olenina (which have rostral plates) have yoked free cheeks, connected by a very narrow strip of cephalic doublure. Extremely thin cuticles are typical" (Fortey in Whittington et al. 1997, p. 302).

The Harpina were formerly grouped alongside the Trineucleoidea (e.g. Moore 1959, pp. O415-430) but the resemblances between them, such as the pitted cephalic fringe, are probably convergences.

Representative Genera: Ellipsocephalus, Elrathia (fig. 6, suborder Ptychopariina); Olenus (suborder Olenina)

Order Harpetida Ebach & McNamara 2002

Stratigraphic Range: Upper Cambrian to Upper Devonian (Frasnian).

Discussion: Formerly considered a suborder of the Ptychopariida, distinguished from other members by marginal sutures, lack of a rostral plate, and presence of the "Harpid fringe."

Representative Genera: Entomaspis (family Entomaspididae); Harpes, Scotoharpes (formerly Aristoharpes, fig. 7, family Harpetidae); Harpides, Loganopeltis (family Harpididae, = Loganopeltidae)

Order Asaphida Salter 1864 emend. Fortey & Chatterton 1988

Sometimes considered (e.g. by Rich et al. 1996) to be a suborder of the Ptychopariida, the diagnostic feature is a ventral median suture. Only primitive forms retain the natant hypostomal condition, the majority being secondarily conterminant or impendent. Higher Asaphida also have a distinctive inflated protaspis (Fortey 1990, p. 560).

Stratigraphic Range: Upper Cambrian to Silurian

Discussion: The superfamily Anomocaroidea represents the primitive morphology of the Asaphida, which includes a number of families retaining the natant hypostomal condition.

The Trineucleoidea is now included within the Asaphida; older classifications (e.g. Moore 1959) regarded the Trineucleoidea as members of the Ptychopariida.

Representative Genera: Anomocaris (superfamily Anomocaroidea); Asaphus (fig. 8, superfamily Asaphoidea); Ptychaspis (superfamily Dikelocephaloidea); Remopleurides (superfamily Remopleuridoidea); Cyclopyge (superfamily Cyclopygoidea); Trinucleus (superfamily Trineucleoidea)

Order Proetida Fortey and Owens 1975

Stratigraphic Range: Upper Cambrian to Middle Permian

Discussion: This order was introduced to accommodate some families formerly placed in the Illaenina, but thought to be unrelated to the type family. The realisation that the reassigned families further differed in being libristomate was made later.

Representative Genera: Proetus (fig. 9, superfamily Proetoidea); Aulacopleura (superfamily Aulacopleuroidea); Bathyurus (superfamily Bathyuroidea)

References

Clarkson, E.N.K. 1993: Invertebrate Palaeontology and Evolution (3rd ed.) Chapman & Hall.

Ebach, M.C.; McNamara, K.J. 2002: A Systematic Revision of the Family Harpetidae (Trilobita). Records of the Western Australian Museum 21: 135-67.

Eldredge, Niles 1977: Trilobites and Evolutionary Patterns. In Hallam, Anthony 1977: Patterns of Evolution. Elsevier, p. 305-332.

Fortey, R.A. 1997: Phylogenetic Concepts in the History of Trilobite Classification. Second International Trilobite Conference, Brock University, St. Catharines, Ontario.

Fortey, R.A. 1990: Ontogeny, Hypostome Attachment and Trilobite Classification. Palaeontology, 33(3): 529-576.

Fortey, R.A.; Owens, R.M. 1975: Proetida: A New Order of Trilobites. Fossils and Strata 4: 227-239.

Moore, R.C. 1959: Treatise on Invertebrate Paleontology Part O Arthropoda 1. Geological Society of America and University of Kansas Press.

Ramsköld, Lars 1991: Pattern and Process in the Evolution of the Odontopleuridae (Trilobita): The Selenopeltinae and Ceratocephalinae. Transactions of the Royal Society of Edinburgh, Earth Sciences 82: 143-181.

Rich, Patricia Vickers; Rich, Thomas Hewitt; Fenton, Mildred Adams; Fenton, Carroll Lane 1996: The Fossil Book. Dover.

Thomas, A.T.; Holloway, D.J. 1988: Classification and Phylogeny of the Trilobite Order Lichida. Philosophical Transactions of the Royal Society of London, B. Biological Sciences 321: 179-262.

Whittington, H.B. et al. 1997: Treatise on Invertebrate Paleontology Part O Arthropoda 1 Trilobita, Revised, Volume 1: Introduction, Order Agnostida, Order Redlichiida. The Geological Society of America and The University of Kansas.