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Trilobite
Origins
The
story of trilobite evolution must begin with that of the
arthropods. Trilobites were a member of what is far and
away the largest phylum of animals, Arthropoda, with well
more than a million described species of out of a total
of perhaps 5 to 10
million extant, comprising around 80% of all animal species (Ødegaard,
2000). Prior to modern sequencing, scientists can only conjecture
as to the characteristics of the first common ancestor of arthropods,
but
it surely lived
in the Precambrian, a period for which the fossil record is
exceedingly sparse and enigmatic. The evolutionary origins of
trilobites is unsettled science, with the distinct possibility
it always will be. Consensus exists that their origins were in
the Precambrian
time among the diverse bilaterians, probably in the Ediacaran
Period (Lieberman,
2002). Their ancestors (or a common ancestor are likely among
from the early Ediacaran arthropods, that are often labeled arachnomorphs
or
trilobitamorphs
in the early Cambrian. A place to begin is the ancestory
and sequencing-based phylogeny of Ecdysozoa, the moulting animals,
among which is trilobita.
Ecdysis unites trilobites with arthropods
From
phylogenetic analyses constructed using 18S
ribosomal RNA genes, Aguinaldo et
al., 1997 defined Superphylum Ecdysozoa
as a monophyletic clade
uniting eight phyla, arthropoda, tardigrades
and
onychophorans
that
share
segmentation
and appendages as well as nematodes, nematomorphs, priapulids,
kinorhynchs and loriciferans, that are worms with an anterior proboscis.
Telford, et.
al. (2008) reviews subsequent sequencing work replicating Aguinaldo's
results, or approaches using Hox gene signature peptides. The
whole area of arthropod (and therefore trilobite origins) continues
unabated, as illustrated by recent wotk by Janssen,
et. al (2014) supporting Onychophora as a sister group of Arthropoda
based embryonic expression patterns og Hox genes. Body
plans of numerous members of Arthropoda have evolved significantly
differences since the Cambrian,
lobopodians
now represented by still extant onychophorans exhibit but minor
change (Whittington,
1978). Many of the major morphological traits
of the arthropods such as jointed limbs and distinct adult body
segmentation are absent
in onychophorans, and, considering onychophorans as an
arthropod stem-group, these absences are best interpreted as
being primitive. Taken together, evisence grows that the early
onychophorans, such as lobopodia, are ancestral to arthropods,
and therefore trilobites.
Through
modern sequencing, there
is strong support combining the phyla Arthropoda, Tardigrada,
and Onychophora into the unranked taxon
Panarthropoda within superphylum Ecdysozoa. The phyla are
united by common characteristics of ventral
nervous system, a
segmented body plan, and legs and claws.
Ediacaran animals Parvancorina and Spriggina,
García-Bellido,
D. C.; Collins, D. H. (2004) Marrella
from Burgess Shale earliest unequivocal evidence of moulting.
[ (May 2004),
Moulting arthropod caught
in the act", Nature 429 (6987): 4]
Superphylum:
Ecdysozoa
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Examples of Trilobite Evolution |
Evolutionary
Secondary Blindness Among Trilobites
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The
old saying goes, "use it or lose it". Charles Darwin spoke
about seconadary blindness (progressive loss of vision through
natural selection) in Origin
of Species:
"The
eyes of moles and of some burrowing rodents are rudimentary
in size, and in some cases are quite covered by skin and
fur. This state of the eyes is probably due to gradual reduction
from
disuse, but aided perhaps by natural selection. In South
America, a burrowing rodent, the tuco-tuco, or Ctenomys,
is even more
subterranean in its habits than the mole; and I was assured
by a Spaniard, who had often caught them, that they were
frequently
blind. One which I kept alive was certainly in this condition,
the cause, as appeared on dissection, having been inflammation
of the nictitating membrane. As frequent inflammation of
the eyes must be injurious to any animal, and as eyes are
certainly
not necessary to animals having subterranean habits, a reduction
in their size, with the adhesion of the eyelids and growth
of fur over them, might in such case be an advantage; and
if so,
natural selection would aid the effects of disuse."
Often,
the visual organs become vistigal, that is, while the organ/structure
remains, the function that existed in ancestors is lost,
in part or in whole. A function no longer needed in the fight
for survival is no longer subject to positive natural selection,
and function can be lost (e.g., ostrich wings). Further,
if the function (or organ) in any way hinders survival,
it might be selected against, and the organ further atrophy
or disappear. Nature
is replete with examples among extant animals (blind cave
animals) of secondary blindness, and it occured among trilobites
too. Secondary
blindness is exhibited in evolutionary series among numerous
species of trilobite. Examples are particularly notable among
Order Agnostida, Superfamily Trinucleioidea of Asaphada,
Order Proetida and Superfamily Phacopina of Asaphida (Clarkson,
1997). (Also see Trilobite
Eyes and Secondarly Blindness)
Secondary
Eye Loss in Trilobite Family Proetidae
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The
first three trilobites below are closely related and
from Order Proetida, Family Proetidae. The left most
has typical
holochroal eyes,
the next a Gerastos with reduced eye size, and the next
is entirely eyeless Gerastos. The last trilobite, Eocryphops,
is a phacopid trilobite with but a few facets in its
schizochroal Eyes, way less than is typical for the Phacopids.
Interestingly, like the reduced-eyes and blind Gerastos,
it is also from Jorf, Morocco. Mysteriously, trilobites
with reduced eyes are found in association with species with typical
eyes. While the environment is believed to
have been in deep water, it was still one
in the photic zone, where good eye sight would seem to have been a
survival benefit and selected for. Was there a particular niche in
the environment
where good sight was not selected for, or even selected against, leading
to secondary
eye loss? Was the Eocryphops trilobite we see below actually blind,
and its reduced eyes just vistigal organs? |
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Size
matters. Size matters a lot in evolution. But, it’s
the species that can best adapt to its environment, particularly
a changing environment that survives. In survival, the benefits
accrued by larger size do not always endow the animal with
a survival advantage – size matters in both providing
advantages and disadvantages – there are tradeoffs. Famed
American paleontologist Edward Drinker Cope (Hone,
et al., 2005) postulates (in what some call Cope’s rule)
that population members tend to increase in body size over
evolutionary time scales. While larger body size provides higher
fitness for many reasons, there are also many disadvantages
for both individual organisms and entire populations. Moreover,
the fossil record suggests that clades of larger animals are
more susceptible to extinction as generational time period
is longer and fecundity is reduced, clearly the worst outcome
for the bullies and brutes. Here are some pros and cons to size
to consider:
Evolutionary fitness advantages:
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Better
defense from big bullies, unless hiding is harder
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Better
able to be the bully, and overcome prey
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More
types of food can be overcome and eaten
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Can
beat out competition for mating
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Increased
longevity and opportunities to mate
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Bigger
brain to outsmart predators and prey
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Reduced
heat loss per unit mass
Evolutionary fitness disadvantages:
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Longer
development time, and juvenile vulnerability
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Need
more food
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Inter-generational
time lengthened, with associated reduction in ability
to adapt
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Lowered
fecundity since parents must invest more in offspring.
It’s
difficult to know which of the above affected
trilobite size changes, and which were more important
and when, but trilobites
clearly exhibited
changes in size
over geologic time. Trilobites have a more
than 700 fold adult size range, enormous by any standard.
The minimum is on the order of one mm, and the maximum
more than 700 mm. The largest trilobite found in the fossil
record
is Isotelus rex, an Ordovician Asaphid trilobite
discovered in 1998 (Rudkin,
2003) near the Hudson Bay in Canada. Bell,
et al (2012) studied trilobite size variation over geologic
time (click for larger image):
That
is, while trilobites exhibit a fast size increase from
the lower Cambrian to middle Cambrian, as well as into
the Ordovician, size decreases thereafter. There is a gradual
size decline through the Silurian. An initial size increase
in the early Devonian is attributed to evolution of spiny
exoskeletons in some species (Chatterton,
2006), noting also that the Frasnian/Famennian
mass extinction removed all predatory species among
orders Phacopida and Lichida, leaving generally more diminutive trilobites
through the Carboniferous and into the Permian, when trilobites
all went extinct.
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Russian
Ordovician Asaphidae Trilobite Eye Stalk Evolutionary Adaptation
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The
trilobite fossil record is among the best of all the animal
groups. It is not surprising then that it
sometimes reveals sequences of evolutionary transition, recording
the adaptation, or descent with modification, to new selective
pressures as a matter
of survival,
or perhaps record failure to adapt,
leading to extinction.
One great
example is the radiation of Baltic asaphid trilobites of family
Asaphidae from the region that is now Saint Petersburg,
Russia from late in the
lower
Ordovician and into the middle Ordovician. A hundred years
of collecting of these
strata has provided tremendous data set for cladistics analysis.
The region that has long yielded a tremendous diversity of
trilobites is
believed to have been an inland sea during the earlier Ordovician
that was cut off from the ocean to the west. At some point toward
the late Ordovician the inland sea again became connected to
the ocean. The resulting flow of sediment (turbidity) clouded
the water
and settled on the seafloor. Through some 50 foot of limestone
encompassing some two million years, some
Asaphidae lines of descent developed stalked eyes of ever increasing height, ostensibly
to
enable them to better spot either prey or predators, or both.
The figure shows selected species and lines of descent. Not
all lines
produced elevated eyes. The figure below depicts the two best
known: 1) Asaphus lepidurus to Asaphus expansus, which branches
into
Asaphus cornutus with moderate stalked eyes, and then to Asaphus
kowalewskii,
with very high eye stalks that almost seem unnatural. The other
branch from Asaphus expanses leads to three species with successively
higher eyes, Asaphus kotlukovi, Asaphus
punctatus and Asaphus
intermedius.
It also warrants mention that the period beginning in the early Ordovician
is at times called "The
Great Ordovician Radiation", or even the
"Ordovician Explosion", denoting an enormous diversification of marine
animals, that had been decimated in the Cambrian-Ordovician
Extinction Event. |
Figure
above: Turbidity flows are selective pressure for trilobite in
family Asaphidae
to evolves eyes on ever higher stalks. Not all species in evolutionary
lines are shown - click image to open new page with larger image.
(Reference,
Adamek, 2014) |
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