Trilobite Taxonomy and Phylogeny

Phylum Arthropoda
Subphylum Chelicerata
Class Trilobita

    Order Agnostida
    Order Redlichiida
    Order Corynexochida
    Order Phacopida
    Order Lichida
    Order Proetida
    Order Harpetida
    Order Ptychopariida
    Order Asaphida

Trilobite Relatives & Ancestors
   

o Arthropoda
`--o Trilobita
   |?-Agnostida
   |?-Eodiscina
   `--+--o Redlichiida
      |  |--Olenellina
      |  `--Redlichiina
      |--Corynexochida
      |--Phacopida
      |--Lichida
      `--o Librostoma
         |--Proetida
         |--Harpetida
         |--Ptychopariida
         `--Asaphida

Trilobites comprise a complex and huge class of arthropods with estimates of number of species ranging from 10 to 15 thousand among the nine distinct Orders. Despite their extensive fossil record, the extinct trilobites remain problematic in terms of both phylogeny within the trilobita, and trilobite placement within Phylum Arthropoda. More stratigraphical and cladistics work is needed, since the problems are unlikely to be resolved with modern genomic science.

Darwin was confident in his conjecture that trilobites descended from one Pre-Cambrian crustacean ancestor. But, the trilobite's position in the universal tree of life remains confounded, with debate remaining whether their closest extant cousins are, for example, a crustacean, the horseshoe crab, or among the spiders or scorpions. Classification requires following the tree of life back to points of branching. This we cannot do for the trilobite whose first appearance in the fossil record is in the lower Cambrian. When they appear, they are already diverse in form, and dispersed in geography, clearly indicative of the paucity of the fossil record in the Precambrian, where the roots of trilobite ancestory extend. By the time trilobites appear in the fossil record they are already highly diverse and possess two characteristics that likely served them well to survive in the Paleozoic seas, a well- mineralized exoskeleton, and a highly advanced visual system. Both of these attributes likely resulted from the selective pressures of the ageless evolutionary war between predator and prey, an arms race in which trilobites were surely bold participants. Despite setbacks during numerous Paleozoic extinction events, the trilobite lineage persisted for some 300 million years before finally becoming extinct at the end of the Permian Period.

The Nine Orders of Trilobita

The nine orders of class trilobita are briefly summarized below, with links provided to more information and fossil image galleries for each order.
Agnostida	Redlichiida	Ptychopariida	Corynexochida	Lichida
Lower Cambrian to Upper Ordovician	Lower Cambrian to Middle Cambrian	Early Cambrian to Upper Ordovician	Middle Cambrian to Upper Devonian	Middle Cambrian to Upper Devonian
Among the most primitive of trilobites, often lacking eyes Length of a few mm and smaller Similar cephalon and pygidium (isopygous)	Among most primitive of trilobites Many thoracic segments Spinocity usually limited to pleurae tips Small pygidium	Appeared early and persisted long, yielding much variability in form Formerly included in what is now Order Harpetida	Hypostomal attachment in common Normally spinous, but Suborder Illaenina is typically effaced	Likely descended from either early Corynexochida or Redlichiida Often elaborate and highly spinous exoskeleton
Asaphida	Proetida	Harpetida	Phacopida	Trilobites Relatives
Middle Cambrian to Upper Silurian	Upper Cambrian to Upper Permian	Upper Cambrian to Upper Devonian	Lower Ordovician to Upper Devonian	Precambrian to Present Day
Ubiquitous trilobites sharing distinct suture structure Effacement of features common with typically large pygidium	The last survivoring trilobites lasting to the Permian extinction Typically small with small spineless pygidium	Presence of the broad semicircular to ovate brim Lacks rostral plate Suborder Harpina was placed in Order Ptychopariida until 2002	Particularly noted for detailed preservation of compound eyes Typical deep furrows between thoracic segments Typically not spinous	Closest living relative may be the horseshoe crab

Despite a rich fossil record dispersed both stratographically and across thousands of genera, taxonomy and phylogeny of the Class Trilobita remains problematic. A huge literature describes the systematic division of trilobites into nine distinct orders. Earlier work was largely based on stratigraphical approaches. Recent decades has benefited by cladistics approaches and parsimony analysis. Something close to a consensus has emerged with respect to placement of genera within families and superfamilies. Additionally, with the possible exception of Phacopida, all trilobite orders appeared prior to the end of the Cambrian. Higher level systematics among the orders and suborders is more problematic, that is, when and how did the seven orders, excluding Agnostida, descent from the Redlichiids or their progeny, mostly before the end of the Cambrian period?
An important continuing debate is whether or not Order Redlichiida is paraphyletic. Redlichiida has two suborders, Olenellina and Redlichiina that have an unresolved relationship. The Olenellids are differentiated by the lack of facial sutures, a distinction that in the past has led to arguments to exclude them from Class Trilobita. Stratigraphical data and cladistic analysis both support Fallotaspidoidea within Suborder Olenellina as the earliest trilobites, and that the many trilobite orders have a lineage tracing back to the Suborder Redlichiina, which must then be considered paraphyletic. Most phylogenies have Suborder Redlichiina giving rise to Orders Corynexochida and Ptychopariida during the Lower Cambrian. The Lichida are variously shown as having arisen from either the Redlichiida or Corynexochida in the Middle Cambrian. Order Ptychopariida remains, as it has always been, the most problematic order for trilobite classification. In the 1959 Treatise on Invertebrate Paleontology, what is now Orders Ptychopariida, Asaphida, Proetida, and Harpetida were grouped together as Order Ptychopariida, a huge and paraphyletic group; subclass Librostoma was erected in 1990 by Fortey (1990) to encompass these orders that are united at least in earlier forms by a natant hypostomal condition. The final reorganization of trilobita occurred when Ebach & McNamara (2002) raised Harpetida to order status because all members lack a rostral plate and have a marginal facial suture, and therefore can not be defined as Ptychopariida. Consequently, they raised Harpetida to ordinal status within the trilobite subclass Librostoma. Asaphida, Proetida, and Harpetida arose from Ptychopariida in the Cambrian. The origin of Order Phacopida is unclear. The three Phacopid suborders, Phacopina, Calymenina, and Cheirurina, are united by a unique protaspis characteristic. The Calymenina are likely the earliest Phacopids with characteristics that would ally them with the Ptychopariida, whereas other characteristics would ally Phacopida with Order Lichida.

Trilobite Relatives

The currently most accepted theory is that the Trilobita is a Class within the Superclass Arachnomorpha, one of two Superclasses within the Subphylum Schizoramia of the Phylum Arthropoda, and as such are more closely related to Chelicerata, than to Myriapoda, Crustacea or Hexapoda, including insects (see chart above). Subphylum Schizomoria also contains Crustaceomorpha among whose members are primitive arthropods common to the Burgess shale and Chengjiang.

Perhaps unraveling the complex ancestry of the trilobite from among the diverse arthropod cousins of the Cambrian Explosion and into the Precambrian will never be satisfactorily accomplished – different consensus opinions are likely to hold sway until giving way to a new one. The disappearance of the last trilobite gene some 260 million years ago precludes forever a molecular determination of lineage based on conserved sequences as is possible among animals that have left descendants into modern time. The taxonomy and phylogeny of Trilobita shown in the table above right is but one plausible arrangement, placing trilobites within Subphylum Chelicerata. Much more information is given at the links.

Trilobites Through Geologic Time

The Paleozoic is often called the age of the trilobite. Trilobites radiated repeatedly, expanding in diversity and distribution beginning with and after the Cambrian Explosion, but also suffered periodic declines in major extinction events. Nine Orders of Arthropod Class Trilobita are recognized. Trilobites particularly flourished in the oceans of the Cambrian and Ordovician periods, beginning around 540 million years ago, with a diminishing number of families persisting until the Permian. The number of families actually peaked in the Late Cambrian when an extinction event removed many. The morphological diversity actually peaked in the Ordovician. Many more families were removed at the end of the Ordovician 440 million years ago during a great ice age where ice sheets advanced to the equator. The diminished number of trilobite families that survived to the Silurian radiated into new and exotic forms, and still more exotic spiny and pustulose forms in the Devonian. The Devonian was punctuated by periods of rising seas that disrupted the reef systems where the trilobites flourished forcing selective adaptation. The end of the Devonian saw the Frasnian-Famennian event where only Proteus survived into the Carboniferous. Despite reduced ancestry, with decent with modification ruling, these trilobites filled the same ecological niches such that adaptation led to a repeating of many of the forms of their extinct cousins. While the genetic path was assuredly different, the newly evolved forms had recognizable morphological similitude with those long extinct. Regrettably, trilobites never truly recovered in the Carboniferous, with but a handful of genera extant by the Permian. Failing to adapt to deep-water habitats, their vulnerability to climatic change remained and led to their disappearance prior to yet another great mass extinction at the end of the Permian. The age of the trilobite yielded to the age of the insect.

References:

Ebach, M.C. & K.J. McNamara. 2002. A systematic revision of the family Harpetidae (Trilobita). Records of the Western Australian Museum 21:135-67.
Fortey RA 1990. Ontogeny, hypostome attachment and trilobite classification. Palaeontology 33:529-576.
Fortey RA. 2000 Trilobite! Eyewitness to Evolution. HarperCollins, London.
Fortey RA. 2001. Trilobite systematics: The last 75 years. Journal of Paleontology 75:1141–1151.
Kaesler RL, ed. 1997. Treatise on Invertebrate Paleontology, Part O, Volume 1, revised, Trilobita. Geological Society of America and University of Kansas Press, Lawrence, Kansas.
Levi-Setti R 1993. Trilobites. University of Chicago Press, Chicago.
A Guide to the Orders of Trilobites - website by Dr. Sam Gon III
Zhang Xiguang & Pratt, B.R. 1999. Early Cambrian trilobite larvae and ontogeny of Ichangia ichangensis Chang, 1957 (Protolenidae) from Henan, China. J. Paleontol. 73: 117-128.