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The trilobite orders

Diversity in size and shape

On this page, we present the nine trilobite orders as widely accepted today, with a short description of the major peculiarities to each group. I would like to thank Dr. Sam Gon III for his generosity in approving our using his fantastic line drawings to illustrate this subpage. Sam’s web site has played and continues to play a major role in attracting more attention to these fascinating creatures of the Palaeozoic. We also would like to thank PaleoDirect for allowing us to use some of their photographs.

Note (Sept. 2007): There appears to be a growing tendency among trilobite workers to split the order Lichida as errected by MOORE in 1959, thereby creating a new order under the name of Odontopleurida which would contain the superfamilies Odontopleuroidea and Dameselloidea, leaving the Lichoidea as the only remaining superfamily within the order Lichida. This idea is based on several arguments we are not entirely comfortable with, but it would lead too far to discuss our reservations here in more detail. In literature, some workers seem to have already fully adopted the new order and it remains to be seen whether what Jellinek called the “normative force of the factual” will keep the upper hand on this. Until then, we will stick to the old classification as being used in the 1997 revised edition of the Treatise on Invertebrate Paleontology, Part O.


Order AGNOSTIDA SALTER, 1864

AgnostusAgnostus General: Small to very small trilobites, mostly just a few millimeters in length. Cephalon and pygidium of very similar size and shape, often found „enrolled“.
Cephalon: Head shield almost circular in shape, corresponding in size to the tail shield which makes it difficult at times to tell both shields apart in an instant. Facial sutures usually not visible, mostly blind / eyeless.
Thorax: Either two (suborder Agnostina) or three (Eodiscina) thoracic segments.
Pygidium: Corresponding to head shield in size and shape.
Appearance: Lower Cambrian to Upper Ordovician
Suborders: Agnostina and Eodiscina
Line drawing: Agnostus, Photograph: Phalagnostus
All line drawings on this page ©1999, 2000 by S. M. Gon III


Order REDLICHIIDA RICHTER, 1932

ParadoxidesParadoxides General: Early „primitive“ trilobites, usually showing a large amount of thoracic segments ending in distinct pleural spines.
Cephalon: Large and semicircular in shape, usually bearing genal spines, glabella typically elongated and distinctly segmented. Mostly large and crescent-shaped eyes.
Thorax: Numerous (up to 60!) thoracic segments of varying length, usually ending in pleural spines.
Pygidium: Usually very small pygidium, in some cases consisting of only a few down to a single segment.
Appearance: Lower Cambrian to Middle Cambrian
Suborders: Olenellina and Redlichiina.
Line drawing: Paradoxides, Photograph: Paradoxides
All line drawings on this page ©1999, 2000 by S. M. Gon III


Order CORYNEXOCHIDA KOBAYASHI, 1935

PolypleuraspisBumastus Cephalon: Elongated glabella, often showing concave flanks (glabella resembling a flower pot), usually with well-developed eyes.
Thorax: Regularly consisting of seven or eight segments, although in rare instances variations between two and more than twelve segments possible (restricted to certain taxa). Thoracic segments often ending in pleural spines.
Pygidium: Usually large to medium-sized, with great variation in shape, sometimes showing pygidial spines..
Appearance: Lower Cambrian to Middle Devonian
Suborders: Corynexochina, Illaenina, Leisotegiina
Line drawing: Polypleuraspis, Photograph: Bumastus
All line drawings on this page ©1999, 2000 by S. M. Gon III
© All images on this page courtesy of PaleoDirect


Order LICHIDA MOORE, 1959

DicranopeltisDicranurus General: Usually spiny trilobites with densely granulated carapaces (tubercles).
Cephalon: Large and broad glabella, longitudinally reaching as far as the cephalic border. Glabella showing simple to complex segmentation. Regularly with holochroal eyes of medium to small size.
Thorax
: Varying between eight and thirteen segments, regularly ending in short pleural spines, though in some species extremely developed (e. g. Odontopleuroidea).
Pygidium: Equal to or larger in size compared to head shield, often showing bilateral pleural ribs which usually end in pygidial spines.
Appearance: Cambrian to Devonian
Suborders: None, but superfamilies Lichoidea, Odontopleuroidea, Dameselloidea.
Line drawing: Dicranopeltis, Photograph: Dicranurus
All line drawings on this page ©1999, 2000 by S. M. Gon III
© All images on this page courtesy of PaleoDirect


Order PHACOPIDA SALTER, 1864

PhacopsDrotops General: Large and very diverse group of related suborders.
Cephalon
: Preglabellar field often very short or missing; four or less glabellar furrows; eyes, when present, schizochroal (Phacopina) or holochroal (Cheirurina and Calymenina).
Thorax: Ranging between eight and nineteen segments, sometimes distinctly furrowed and with very broad axis.
Pygidium: Typically smaller than head shield (in most Calymenina and Phacopina) but varying.
Note: All suborders show a very similar development from larval stages to the adult animal. This form of ontogenesis is one reason why the Calymenina are thought to be in close relationship to the other Phacopids.
Appearance: Lower Ordovician to Upper Devonian
Suborders: Calymenina, Phacopina, Cheirurina.
Line drawing: Phacops, Photograph: Drotops megalomanicus
All line drawings on this page ©1999, 2000 by S. M. Gon III


Ordnung ASAPHIDA FORTEY & CHATTERTON, 1988

AsaphusAsaphus General: Large and morphologically very divergent order within the trilobita, covering roughly 20% of all trilobite species.
Cephalon: Mostly corresponding in size to the pygidial shield, usually with a smooth surface, glabellar furrows hardly visible, usually with large eyes. Some taxa, however, showing a secondary blindness.
Thorax: Typically between five and twelve segments, with some exceptional taxa having less or more than the typical number.
Pygidium: Identical, similar or larger than head shield..
Appearance: Middle/Upper Cambrian boundary to Upper Ordovician/Lower Silurian boundary.
Suborders: None, but superfamilies Anomocaroidea, Asaphoidea, Dikelokephaloidea, Remopleuridoidea, Cyclopygoidea, Trinucleioidea.
Line drawing: Asaphus, Photograph: Asaphus lepidurus
All line drawings on this page ©1999, 2000 by S. M. Gon III


Order PROETIDA FORTEY & OWENS, 1975

ProetidaProetus General: Usually small trilobite taxa which have been split from the order Ptychopariida some time ago.
Cephalon: Regularly large, tapering glabella showing four glabellar furrows. Eyes, when present, holochroal and of considerable and convex size. Genal spines usually present.
Thorax: Between eight and twentytwo, with eight being the typical number.
Pygidium: Showing a wide range in size, from very small to almost equalling the size of the cephalic shield, in most cases without spines. Tail shield usually shows four to ten or more pleural furrows.
Appearance: Ordovician to Permian
Suborders: None, but superfamilies Proetoidea, Aulacopleuroidea, Bathyuroidea.
Line drawing: Cyphoproetus, Photograph: Proetus
All line drawings on this page ©1999, 2000 by S. M. Gon III
© All images on this page courtesy of PaleoDirect


Order HARPETIDA EBACH & McNAMARA, 2002

HarpetidaAristoharpes General: Easily identifiable group of trilobites showing huge cephalic fringe. Recently split from the Ptychopariida.
Cephalon: Semicircular to oval in shape, extending far backwards and ending in broad genal spines; convex glabella, tapering forward, large preglabellar field. Very small holochroal eyes, at times with but a single lens, positioned in the center of the cheek.
Thorax: Usually twelve segments, sometimes more, axis showing broad furrows.
Pygidium: Usually short, tapering backwards.
Families : Entomaspididae, Harpetidae, Harpididae.
Appearance : Upper Cambrian to late Devonian
Line drawing: Harpes, Photograph: Aristoharpes
All line drawings on this page ©1999, 2000 by S. M. Gon III
© All images on this page courtesy of PaleoDirect


Order PTYCHOPARIIDA SWINNERTON, 1915

ElrathiaElrathia General: Large, heterogenous order with sometimes problematic classification, in particular because of specialized „outliers“ proving difficult to classify.
Cephalon
: Slightly tapering forward, with a broad and circular frontal area, glabella usually showing three pairs of parallel glabellar furrwos.
Thorax: Regularly large thorax showing eight or more segments.
Pygidium
: Very variable in size and shape, but regularly small with pygidial border during the Cambrian and lacking the pygidial border after the Cambrian.
Appearance: Lower Cambrian to Upper Ordovician.
Suborders: Ptychopariina, Olenina
Line drawing: Elrathia, Photograph: Elrathia kingii
All line drawings on this page ©1999, 2000 by S. M. Gon III

Don’t let yourself get fooled by the line drawings and photographs shown on this page. Looks are deceiving and suborders, superfamilies, families and species within a single order often look completely different than the associated exemplary images we use on this page.

Would you believe, at first glance, that this trilobite belongs to the order Lichida? ;-)

Trilobite Ceratarges
Ceratarges aus Marokko
© All images on this page courtesy of PaleoDirect

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Last Update : 01/30/2010 5:06 PM