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Transcript
Michelle Larkin
Creation Revisited
Professor Kitchen
March 18, 2008
Brachiopods
What is a brachiopod? Basically, a brachiopod is an ancient animal that could be
mistaken by some for a clam, or some other form of bivalve. However, while they may
bear an outward resemblance to our mollusk friends, there are indeed many differences
when the two are closely compared. According to Wikipedia, brachiopods are a small
phylum of benthic invertebrates, also known as “Brachiopoda” and “lamp-shells”, which
belong to the kingdom animalia, phylum brachiopoda, and also are the Kentucky state
fossil (1). The brachiopod is small marine organism that lives in extreme depths, and as
one of the world’s longest surviving life forms, it has the potential to provide mankind
with many valuable clues about our origins.
Brachiopods first emerged on the geological scale during the early Cambrian
period, which would place them at nearly 550 million years old. What would be
considered a fairly simple organism by today’s standards, these animals were once one of
the dominant life forms on the ocean’s floors, and have survived for a vastly greater
period of time than has man thus far. There have even been fossils resembling
brachiopods in older rocks dating back to the Neoproterozoic, but they have not been
absolutely assigned by scientists to the brachiopod family. Some of the earliest varieties
of brachiopoda are even thought to have displayed sophisticated flanges and spines.
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One potential candidate that has been suggested as a possible ancestor of the
brachiopod is a type of sluggish creature with shell-like armor at either end, called
Halkieria. It is possible this that this organism was given an “evolutionary push” to
reduce the gap between the shells, allowing it to better protect itself from the growing
number of predators on the ocean’s floor. “During this marine revolution, the basic food
supply of the oceans - phytoplankton - increased significantly, organisms up the food
chain became more meaty, and predators - bony fish, snails, and crustaceans - increased
in frequency and diversified. As predators became specialized in attacking their shellprotected victims, prey groups also diversified and became better armored” (“Brachiopod
Shells Record Arms Race”). Increases in predation are commonly linked with the demise
or decline of species who do not figure out a way to keep up with the evolutionary
competition.
Brachiopods are generally divided into two distinct classes, the articulate and the
inarticulate. The earliest brachiopods are the inarticulate hingeless variety, which are
connected by musculation between the shells. “The inarticulate brachiopod genus Lingula
is the oldest, relatively unchanged, animal known” (Wikipedia 3). These forms evolved
into a slightly more efficient brachiopod, the articulate, which was joined with a hingelike joint. Brachiopods, although not nearly as numerous today as they were during the
Cambrian, are not extinct and can still be found by those who choose to look hard
enough. Although there are still many classes of extant brachiopod, Wikipedia contends
that “the most abundant modern brachiopods are the class Terebratulida” (3).
While there are only to main types, several thousand separate species of
brachiopods have been identified. These various species have been known to range in
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size from 1mm to 15cm, but the average size of those found today is generally about 58mm. All brachiopods have two shells, each one symmetrical independently, but they
will not match up when compared to each other. They use muscle power to open and
close their shells, which are commonly calcitic, can be phosphatic, and they seldom
produce argonitic shells, made from less stable form of calcium-carbonate.
The clam-shaped structure of a brachiopod is anchored to a substrate by a fleshy
stalk called a pedicle. They are suspension feeders whose characteristic feeding organ,
known as a lophophore, is a ring of ciliated tentacles surrounding a small opening,
basically a mouth, through which the brachiopod can capture and filter nutrients. This
organ is not visible when simply observing a brachiopod, instead you must pry open the
poor little creature to see it.
Their basic morphology, see Charts A, B, and C, is simple in nature both inside
and out, and has not made any drastic changes over the course of a few million years,
although there is some diversity among the many varieties of classes of brachiopods
(Brachiopod External Morphology, Brachiopod Internal Shell Morphology, and
Brachiopod Internal Morphology). These charts show the relatively uncomplicated
anatomy of a brachiopod, and the internal processes at work behind their shells. On the
surface, they do appear to have many similarities to bivalves, but the construction of the
shell is completely different in the two groups: in brachiopods, the two valves are on the
upper and lower surfaces of the body, while in bivalves, they are on the left and right
sides.
Given that they have such a propensity to become fossils, brachiopods have long
been the subject of scientific study. According to Pennington et al., the “Brachiopoda
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compose a major part of the fossil record ([approximately] 30,000 described species), yet
the biology of extant brachiopods ([approximately] 280 species) is understood poorly
relative to that of other macrofaunal, in part because brachiopods often inhabit cryptic or
inaccessible habitats and are rarely conspicuous members of communities that attract the
attention of zoologists and ecologists” (2). While brachiopod fossils may be in
abundance, study of modern brachiopods is difficult because of the conditions in which
they thrive, and there is limited knowledge on the subject.
The odds that you will find a live brachiopod while at the beach on your family
vacation are nil. Unlike mollusks, these ancient marine animals have moved away from
the shallow pools they inhabited long ago, and now prefer to live, bundled in groups, in
the much colder water of the poles or the deeper parts of the ocean. Perhaps it was the
appearance of predators that forced the brachiopods to seek out more solitary depths, for
once they appeared, life began to take on a new shape. The exact reasons for their
changes in habitat, numbers, and diversity are still under debate, and several possible
explanations have been proposed.
. Animals developing the ability to crush or “drill” holes into the shells of
bivalves and brachiopods could have forced them to find more secluded areas to settle in.
However, this alone does not explain the dramatic decline in the quantity and variety, or
the habitat shift of the brachiopda. The bivalves and the mollusks had their own way of
dealing with the new predation occurring in the ocean, and they were largely successful
in adapting as burrowing or free-floating feeders, giving them what would appear to be
an advantage over the brachiopods. Unfortunately for them, the majority of the predators
seem to prefer them as a meal or tasty snack when in comparison, but the disturbance of
5
the substrate that the bivalves create when they burrow, and their rise in numbers and
diversity could be enough on its own to expel the brachiopods from their habitat.
During the Silurian and Ordovician periods they were particularly numerous in
more shallow habitats, sometimes creating whole banks with the remnants of their shells.
In some areas sizeable portions of limestone strata and reef deposits have been found that
are composed chiefly of their shells. Their system of grouping, along with the
construction of their shells, makes brachiopods excellent sources of fossils for
paleontologists and geologists. “Brachiopod shells are among the most common marine
macroscopic fossils found in the Phanerozoic fossil record. From a paleontological
perspective, spionid-infested brachiopod shells may be a prime target for studying
parasite-host interactions over evolutionary time scales” (Rodrigues 1).
While some classes of brachiopod do still exist today, they have suffered from
several major extinctions during the long period of time since they first appeared on the
earth. The resilience of these small organisms is evidenced by the mere fact that they,
unlike ninety-six percent of all other marine life at the time, survived the PermianTriassic Extinction. This extinction event is known as the “mother” of all extinctions as it
terminated the majority of life that existed on the planet. Still, the abundance, diversity,
and rapid evolution of brachiopods during the Paleozoic make them useful as index
fossils when identifying geologic periods.
The brachiopods remained to be found in the Mesozoic, thought the number and
diversity of their classes had suffered greatly. Many thousands of the original classes of
brachiopods had become extinct by this time period, and the classes that remained had
thinned dramatically. The attached, Chart D, is a spindle diagram. These kinds of
6
diagrams are often used by the paleontologist to gain an understanding of how varied a
group of organisms has been through geologic time. On one axis of the chart represents
time, from the Cambrian, (at the bottom), to today, (at the top). The bars indicate the
different types of brachiopod fossils that have been found by paleontologists during each
time period. “Brachiopods were most diverse during the Devonian period, with the widest
bar representing just over 200 different genera that have been found for that time. The
chart also shows you that the brachiopods were much more diverse and numerous during
the Paleozoic era, which corresponds to the periods Cambrian, Ordovician, Silurian,
Devonian, Carboniferous and Permian, and that they have never been as diverse since the
Permo-Triassic mass extinction” (Collins and Waggoner 1).
Brachiopods have become a source of study, not only as fossils, but as live marine
animals as well. Pennington et al. asserts that the “present-day Brachiopoda are typically
regarded as a relic phylum, with extant species living in relic or marginal habitats, with a
striking exception to this pattern occurring in Monterey Bay, California, where
populations of the articulate brachiopod Laqueus californianus (Koch 1848,
Terebratellacea) are found as dense epifaunal 'beds' at the outer margin of the continental
shelf (1). Continued research on the brachiopods in this region could yield evidence not
yet discovered by science in the matter of evolution, and is certainly worth the effort.
There is no doubt that further study will be conducted on brachiopoda by scientists in
many highly-specialized fields, and, perhaps, it will someday provide us with significant
clues as to the beginnings of life.
7
WORKS CITED
"Brachiopod." Wikipedia, The Free Encyclopedia. 22 Feb 2008, 21:37 UTC. Wikimedia
Foundation, Inc. 1 Mar 2008
<http://en.wikipedia.org/w/index.php?title=Brachiopod&oldid=193355868>.
"Brachiopod Shells Record Shadow Of Arms Race In Ancient Oceans." Space
Daily (June 17, 2005): NA. General OneFile. Gale. University of Richmond. 29
Feb. 2008
<http://newman.richmond.edu:3089/itx/start.do?prodId=ITOF>.321 3
Collins, Allen, and Waggoner, Ben. “Brachiopoda: Fossil Record.” University of
California Museum of Paleontology. 29 Feb 2008. UCMP Web and Educational
Outreach Team. July 1995
Chart D: Brachiopoda: Fossil Record. Spindle Diagram. California. 1995.
http://www.ucmp.berkeley.edu/brachiopoda/brachiopodafr.html
Pennington, J. Timothy, Mario N. Tamburri, and James P. Barry. "Development,
temperature tolerance, and settlement preference of embryos and larvae of the
articulate brachiopod Laqueus californianus." The Biological Bulletin 196.3 (June
1999): 245(1). Academic OneFile. Gale. University of Richmond. 29 Feb. 2008
<http://newman.richmond.edu:3089/itx/start.do?prodId=AONE>.
Rodrigues, Sabrina Coelho. "Biotic interactions recorded in shells of recent
rhynchonelliform brachiopods from San Juan Island, USA.(SPECIAL THEME
SECTION)." Journal of Shellfish Research 26.1 (April 2007): 241(12). Academic
OneFile. Gale. University of Richmond. 29 Feb. 2008
<http://newman.richmond.edu:3089/itx/start.do?prodId=AONE>.
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Wells, Jr., Roger M. “Brachiopoda: Morphology and Ecology.” University of SUNY
Cortland. 29 Feb. 2008
Chart A: Brachiopod External Morphology. Diagram. New Jersey. 1998.
Chart B: Brachiopod Internal Shell Morphology. Diagram. New Jersey. 1998.
Chart C: Brachiopod Internal Morphology. Diagram. New Jersey. 1998.
http://paleo.cortland.edu/tutorial/Brachiopods/brachiopoda.htm
CHARTS AND DIAGRAMS
CHART A: BRACHIOPOD EXTERNAL MORPHOLOGY
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CHART B: BRACHIOPOD INTERNAL SHELL MORPHOLOGY
CHART C: BRACHIOPOD INTERNAL MORPHOLOGY
10
CHART D: SPINDEL DIAGRAM ON BRACHIOPOD FOSSIL RECORDS
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