Trilobita - Trilobites

Tree of Life
 

 

Also see:
Paleozoic Paleobiology
Trilobite Evolutionary Arms Race
Museum Trilobites


Trilobites Introduction

Trilobites comprise a complex and huge clade of arthropods with estimates of number of species ranging up to 20,000 thousand among some 5000 genera, 150 families, and nine distinct orders. There are 10 orders if Odontopleurida is erected from Order Lichida, and 11 if the Nektaspida (commonly called soft-bodied trilobites are classified as a trilobite order). Trilobites burst into amazingly diversity in the fossil record within some five million years of the base of the Cambrian. Such diversity is evidence that trilobites had a past dating long before this famous period and its apparent explosion of life known as the Cambrian Explosion. What made them seem new was that they had acquired a readily preserved skeleton, one they had to be shed to grow. Trilobite progenitors with soft bodied had died and decomposed leaving no trace. Trilobites radiated into the most diverse class of extinct creatures during the Paleozoic, but the Cambrian marked their middle age, not their beginnings. Their size ranged from less than a mm to over one meter in length. The made their living in diverse ways, some crawling, some swimming ,
Trilobite Taxonomy
Trilobite Phylogeny
Phylum Arthropoda
Subphylum Chelicerata
or Subphylum Schizoramia *
Superclass Arachnomorpha *
Class Trilobita

    Order Agnostida
    Order Redlichiida
    Order Corynexochida
    Order Phacopida
    Order Lichida
    Order Proetida
    Order Harpetida
    Order Ptychopariida
    Order Asaphida
    Order Odontopleurida
    Order Nectaspida
Trilobite Relatives
o Arthropoda
`--o Trilobita
   |?-Agnostida
   |?-Eodiscina
   `--+--o Redlichiida
      |  |--Olenellina
      |  `--Redlichiina
      |--Corynexochida
      |--Phacopida
      |--Lichida
      |--Odontopleurida
      `--o Librostoma
         |--Proetida
         |--Harpetida
         |--Ptychopariida
         `--Asaphida
* Different taxonomies are found, most recently with trilobites contained in Superclass Arachnomorpha in Subphylum Schizoramia.
and some living a planktonic existence. They were detritivores, predators, and scavengers, and evolved stealthy and defensive phenotypic characteristics in order to survive in an evolutionary arms race between predator and prey. When they appearin the Cambrian fossil record they are not only diverse in form, and dispersed in geography across all continents. Among the Cambrian fauna, the crystal eyes of trilobites are unique. In the eyes is a strong clue of their ancestry in deep time of the Precambrian, since the fossil record indicates pre-sight neural tissue existed in forms of worms that also contain segmented morphology retained in the trilobite body plan.

Trilobite Taxonomy and Phylogeny

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.
Trilobite 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, descend 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. Another 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. The 1997 Treatise defines Order Lichida as comprising Superfamilies Lichoidea, Odontopleuroidea, and Dameselloidea. While the science remains unsettled, another taxonomy is gaining favor that erects Order Odontopleurida from Superfamilies Odontopleuroidea and Dameselloidea.


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 as closely related to 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.
Trilobites Family Album
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.