The
Paleozoic is often called the age of the trilobite.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.
Trilobites
comprise a complex and huge clade of arthropods with estimates
of number of species ranging from 10 to 15 thousand. 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.
|
The
9 Orders of Trilobita
|
| Trilobite
Order |
Salient
Distinguishing Characteristics of the Order |
|
Representative
Trilobite
& Gallery Like |
| Agnostida |
- Among
the most primitive of trilobites
- Length
of a few mm and smaller
- Similar
cephalon and pygidium (isopygous)
|
Lower
Cambrian to Upper Ordovician |
|
| Redlichiida |
- Among
the most primitive of trilobites
- Many
thoracic segments
- Spinocity
usually limited to pleurae tips
- Small
pygidium
|
Lower
to Middle Cambrian |
|
| Corynexochida |
- Hypostomal
attachment in common
- Normally
spinous
|
Lower
to Middle Cambrian |
|
| Lichida |
- Often
elaborate and often highy spinous (making them highly
sought)
|
Ordovician
to Devonian |
 |
| Phacopida |
- Particularly
noted for detailed preservation of compound eyes
- Typical
deep furrows between thoracic segments
- Typically
not spinous
|
Lower
Ordovician to Upper Devonian |
|
| Proetida |
- Among
the last survivors before Trilobita faded away, and
disappeared in the Permian extinction
-
Typically small with small spineless pygidium
|
Ordovician
to Permian |
|
| Asaphida |
- Ubiquitous
trilobite sharing distinct suture structure
- Effacement
of features common with typically large pygidium
|
Middle
Cambrian to Lower Silurian
|
|
| Harpetida |
- Presence
of the broad semicircular to ovate brim
- Lack
of rostral plate
|
Upper
Cambrian to Upper Devonian
|
|
| Ptychopariida |
- Appeared
early and persisted long, yielding much variability
in form
- Formerly
included what is now Order Harpina
|
Lower
Cambrian to Devonian |
|
