 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.
Trilobites
occupy a special place in the hearts of collectors and professionals
alike. To these cohorts: "trilobites are very pretty".
Riccardo Levi- Setti in his book "Trilobites"
describes them as the butterflies of seas. Richard Fortey's more
recent book "Trilobite! Eyewitness to Evolution"
wonderfully projects this professional paleontologist's love of
trilobites. There are also many wonderful websites that reflect
a labor of love of the webmaster; particularly among these is
Sam Gon's site, "A Guide to the Orders of Trilobites"
that is, at once, artistic and a distilation a huge amount of
science into concise, non-recondite language and illustrations.
I'm sure Sam's site has and will continue to increase the number
of people who consider that trilobites are very pretty. The popularity
of trilobites is due in large part to the magnificent preservation
of their calcified exoskelton that is seen in a myriad of forms,
many very exotic; the many trilobite forms evolved due to natural
selection for features of armament
and stealth, in the ageless arms race between predator and
prey.
 Trilobites
comprise a complex and huge clade of arthropods with estimates
of number of species ranging from 10 to 15 thousand among the
nine distinct Orders. The extinct trilobite represents a problem
for classification, a problem unlikely to benefit from 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 a mystery
today, with debate remaining whether their closest extant cousins
are, for example, the horseshoe crabs, the spiders or the 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 Cambrian. When they
appear, they are already diverse in form, and dispersed in geography,
clearly indicative ofthe incomplete nature of the fossil record.
The highly diverse soft-bodied and jointed-legged animals that
were ostensibly produced in the Cambrian explosion, but must also
have Pre-Cambrian ancestry intensify the mystery. Among the Cambrian
fauna, the crystal eyes of trilobites are unique. In the eyes
is a strong clue, since the fossil record indicates pre-sight
neural tissue existed in forms of worms that  also
contain segmented morphology also retained in the trilobite forms.
Mystery is allure for those with a scientific propensity, and
the trilobite does not disappoint. Reading Fortey's book, you
can almost feel the author's sadness that these magnificent animals
that once dominated life on Earth, declined near the end of the
Paleozoic with but a single order surviving to the Carboniferous,
before it too faded away.
The
scientific bible for Subphylum Trilobita are books within the prodigious Treatise
on Invertebrate Paleontology, Volume O, published and sold by the Geological Society
of America through the University of Kansas (usually just called "the Treatise"
by trilobite aficionados). Trilobites are divided into 9 Orders. The original
treatise, first published in 1959, covered 8 Orders. In a major effort, Trilobita
is under revision. Harpetida has been recently split from the Ptychopariida. The
first volume of the revised Treatise covering two Orders, Agnostida and Redlichiida,
was published in 1997. We await this effort's completion for the final volume
to incorporate the other seven orders.
References:
Fortey RA 1990. Ontogeny, hypostome attachment and trilobite classification.
Palaeontology 33:529-576.
Fortey RA. 2000. Trilobite! Eyewitness to Evolution. HarperCollins,
London.
Fortey RA A. 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.
Sam
Gon's site, "A
Guide to the Orders of Trilobites"
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