Evolutionary trends in trilobites

Evolutionary Trends in Trilobites
This page last revised 20 July 2008 by S.M. Gon III

Through the 300 million years that trilobites existed, prior to their extinction in the Permian, there were many opportunities for diversification of form, starting from the presumed primitive morphology exemplified by a species such as Redlichia (left). This typical primitive morphotype had a small pygidium, well developed eye ridges, a simple, lobed glabella, several thoracic segments, and a rather flattened body form. The first trilobites were characterized by this primitive form. Among the over 20,000 species of described trilobites there are species in which aspects of morphology have diverged greatly from the primitive state. These are discussed in Fortey & Owens 1997 (see citations below). Thoracic segments were reduced to as few as two or increased to over 100, overall body shape was greatly elongated in some, or rendered transverse (widened) in others. Examples are shown below:

	elongate form	transverse form	increased segments	reduced segments
Related topics: Trilobite Origins Trilobite Extinction
	Crotalocephalina Cheirurina Phacopida	Harpillaenus Illaenina Corynexochida	Balcoracania Emuelloidea Redlichiida	Thoracocare Corynexochina Corynexochida

Shapes and furrow patterns of the glabella, and the shape and placement of eyes and eye ridges of course also ranged widely. An analysis of morphological diversity of trilobite forms showed that increasing from the Cambrian, there was a peak in morphological diversity in the Ordovician (which parallels a peak in overall diversity of trilobites families) that decreased only as overall trilobite diversity decreased toward their extinction in the late Permian.

Within this diversification, there were a number of evolutionary trends in morphology that developed in unrelated clades, creating homeomorphy (attainment of similar forms in unrelated groups). These homeomorphic trends, such as effacement, increased spinosity, reduction in body size, streamline shape, and loss of eyes, can not be reliably or consistently used to assess higher systematic relationships. Instead, these features can tell us about selective pressures on trilobites and how similar solutions were derived in parallel by different evolutionary lineages. Each of these is discussed below, and examples are given from different orders.

Illaenus atavus

Effacement
In several trilobite orders, but perhaps most notably among the Agnostida, Corynexochida (Suborder Illaenina), and Asaphida, effacement of cephalic, pygidial, and even thoracic furrows is not uncommon. This loss of surface detail can be confounding to systematics, since effaced features (for example loss of glabellar details) can mask evidence of relationships. Some workers suggest that effacement is an adaptation related to a burrowing lifestyle, especially in Illaenina, but such effacement might also play a role in streamlining of pelagic Asaphida, and is also seen in many Agnostida (which may have been planktonic). No single selection pressure seems to have been responsible for the effaced morphotype.

Asaphida Cyclopyge	Corynexochida Bumastus	Asaphida Nileus	Agnostida Lejopyge

Effaced trilobites, such as those above, can be difficult to classify, since they lack key characters.

Leonaspis is a spiny trilobite from Morocco

Leonaspis sp (Morocco)

Spinosity
The development of spines is commonly thought of as primarily a defensive adaptation, and increased spinosity is seen in a wide variety of trilobite species. Alternate hypotheses for the adaptive significance of spines include stabilization structures on a loose silty substrate (e.g. genal spines of trinucleid Asaphida, such as Ampyx, below), and flotation/stabilization structures for slow-swimming taxa (e.g., odontopleuroid Lichida such as Leonaspis at left). Spines may originate from just about any part of the exoskeleton, especially the margins. Sometimes these patterns provide consistent and diagnostic characters for higher classification (for example, the pattern of pygidial spines among odontopleuroid Lichida). However, as seen in the spinose species below, development of spines occurs in many orders of trilobites.

Phacopida Comura	Asaphida Ampyx	Corynexochida Oryctocephalus	Proetida Phaetonellus

Spinose trilobites are a clear indication that it was dangerous world in the Paleozoic, in which an unarmed trilobite might likely be swallowed whole.

Miniaturization
Reduction in size is seen in several trilobites, such as Acanthopleurella (just about 1 millimeter at maturity). The general argument for evolution of small size typically evokes the numerous microhabitats of complex marine systems, coupled with a correlation of size to rapid maturity (early maturation means smaller size at adulthood). When this reduction is due to progenesis (arrested development) the trilobites may also display a reduction in the number of thoracic segments (see section on Ontogeny). Thoracocare, for example, has only two thoracic segments at maturity, yet is not a member of the order Agnostida (formerly thought to be the only order bearing three or fewer thoracic segments), but of Corynexochida. The entire order Agnostida is composed of small species that may have originated as a miniaturized and specialized early offbranch of the Redlichiida or Ptychopariida.

Ptychopariida Acanthopleurella	Corynexochida Thoracocare	Ptychopariida Shumardia	Agnostida Pagetia	Ptychopariida Schmalenseeia
	.

In a complex marine environment, many niches (physical and ecological) are available for small species.

Ellipsocephalus hoffi

Atheloptic Morphology
Reduction and loss of eyes in trilobites has been discussed elsewhere, and secondary reduction and loss of eyes is thought to be a trend among benthic species living in deep, poorly-lit or aphotic habitats. In these deep water biotopes, blind or nearly-blind trilobites are the dominant element. Typically, these atheloptic species have close relatives in which eyes are of normal size and function. It is interesting to note that another trend of deep bottom habitat adaptation is an increase in the number and width of thoracic segments, which might be related to specialized feeding adaptations (see Olenimorph section below).

Ptychopariida Lermontovia	Ptychopariida Conocoryphe	Phacopida Trimerus

Atheloptic trilobites had little use for eyes on the dark bottoms of ancient seas.

Reconstruction of Carolinites genacinaca

Pelagic Morphology
There are a number of trilobites that have developed extremely large eyes and elongate, streamlined body shape associated with swimming in the photic water column. The paleogeography of some of these pelagic species (for example, Carolinites, shown at left), suggests that their swimming abilities were good enough them to spread into a global oceanic distribution.

Proetida Telephinus	Asaphida Remopleurides	Proetida Carolinites	Asaphida Novakella	Proetida Opipeuterella

Pelagic (oceanic free swimming) trilobites typically had large eyes and an elongate body form.

Wujiajiania sutherlandii

Olenimorph
Thin exoskeleton, increased numbers of thoracic segments, and a widened, flat body form is associated with benthic habitats marked by low oxygen and high sulphur compound concentrations. Fortey suggests that olenimorphs (so named because many of the Ptychopariida Suborder Olenina have this form) may represent the first symbiotic relationships with sulphur-eating bacteria as a feeding strategy. The numerous transverse thoracic pleurae presumeably overlaid a series of laterally extended gill exites, maximizing oxygen absorption and providing a large surface area upon (or within) which symbiotic bacteria could live.

Proetida Aulacopleura	Ptychopariida Olenus	Ptychopariida Balnibarbi	Asaphida Seleneceme	Ptychopariida Conocoryphe
		.		..

Numerous segments with wide pleurae covered enhanced gills for better oxygen absorption and perhaps for symbiotic bacteria to live.

Cryptolithus sp.

Pitted Fringe
An expansion of the cephalon into a concave chamber perforated by numerous fenestrations is thought to be an adaptation related to filter-feeding. This morph has arisen in at least two unrelated groups: Trinucleioid Asaphida and members of the Order Harpetida . In both cases the cephalon is the dominant part, with transverse thoracic segments and an unremarkable pygidium. Long genal spines or genal prolongations are also notable in this morphotype. They are thought to help stabilize the trilobite during filter-feeding.

Harpetida Harpes	Asaphida Cryptolithus

The pitted fringe trilobites are among the most distinctive in appearence.

References for this page

Fortey, R. A., and R. M. Owens. 1999. Feeding habits in trilobites. Palaeontology 42(3):429-65.

Fortey, R. A., and R. M. Owens. 1997. Evolutionary History. In Kaesler, R. L., ed.Treatise on Invertebrate Paleontology, Part O, Arthropoda 1, Trilobita, revised. Volume 1: Introduction, Order Agnostida, Order Redlichiida. xxiv + 530 pp., 309 figs. The Geological Society of America, Inc. & The University of Kansas. Boulder Colorado & Lawrence, Kansas.

Paterson, J.R. & G.D., Edgecombe. 2006. The Early Cambrian trilobite family Emuellidae Pocock, 1970: Systematic position and revision of Australian species. Journal of Paleontology, 80(3): 496-513.

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