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 ,
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. |
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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 |
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Among
the most primitive of trilobites, often
lacking eyes
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Length
of a few mm and smaller
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Similar
cephalon and pygidium (isopygous)
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Appeared
early and persisted long, yielding much
variability in form
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Formerly
included in what is now Order Harpetida
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- Hypostomal
attachment in common
- Normally
spinous, but Suborder Illaenina is typically
effaced
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Middle
Cambrian to Upper Silurian |
Upper
Cambrian to Upper Permian |
Upper
Cambrian to Upper Devonian |
Lower
Ordovician to Upper Devonian |
Precambrian
to Present Day |
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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.
