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Makhluk laut raksasa yang mengisyaratkan evolusi arthropoda awal

Baru ditemukan fosil raksasa , makhluk laut punah yang menunjukkan telah dimodifikasi nya kaki , insang di punggungnya , dan sistem filter untuk makan - memberikan bukti penting tentang evolusi awal arthropoda ....read more

Giant sea
creature hints at early arthropod evolution

Date:

March 12, 2015

Source:

Yale University

Summary:

Newly discovered
fossils of a giant, extinct sea creature show it had modified legs, gills on
its back, and a filter system for feeding -- providing key evidence about the
early evolution of arthropods.

.......................

newly discovered fossils
of a giant, extinct sea creature show it had modified legs, gills on its back,
and a filter system for feeding -- providing key evidence about the early
evolution of arthropods.

The new animal, named Aegirocassis benmoulae in honor of its
discoverer, Mohamed Ben Moula, attained a size of at least seven feet, ranking
it among the biggest arthropods that ever lived. It was found in southeastern
Morocco and dates back some 480 million years.

"Aegirocassis is a truly remarkable looking creature,"
said Yale University paleontologist Derek Briggs, co-author of a Nature paper
describing the animal. "We were excited to discover that it shows features
that have not been observed in older Cambrian anomalocaridids -- not one but
two sets of swimming flaps along the trunk, representing a stage in the
evolution of the two-branched limb, characteristic of modern arthropods such as
shrimps."

Briggs is the G. Evelyn Hutchinson Professor of Geology and Geophysics at
Yale and curator of invertebrate paleontology at the Yale Peabody Museum of
Natural History. First author Peter Van Roy, an associate research scientist at
Yale, led the research; Allison Daley of the University of Oxford is co-author.

Since their first appearance in the fossil record 530 million years ago,
arthropods have been the most species-rich and morphologically diverse animal
group on Earth. They include such familiar creatures as horseshoe crabs,
scorpions, spiders, lobsters, butterflies, ants, and beetles. Their success is
due in large part to the way their bodies are constructed: They have a hard
exoskeleton that is molted during growth, and their bodies and legs are made up
of multiple segments. Each segment can be modified separately for different
purposes, allowing arthropods to adapt to every environment and mode of life.

Modern arthropod legs, in their most basic form, have two branches. Each is
highly modified to cater to a specific function on that leg, such as
locomotion, sensing its surroundings, respiration, or copulation; or it has
been lost altogether. Understanding how these double-branched limbs evolved has
been a major question for scientists.

A long-extinct group of arthropods, the anomalocaridids, is considered
central to the answer. The youngest known anomalocaridids are 480 million years
old, and all of them looked quite alien: They had a head with a pair of
grasping appendages and a circular mouth surrounded by toothed plates. Their
elongate, segmented bodies carried lateral flaps that they used for swimming.
Until now, it was believed that anomalocaridids had only one set of flaps per
trunk segment, and that they may have lost their walking legs completely.

But the recent discovery of Aegirocassis benmoulae tells
another story. The new animal shows that anomalocaridids in fact had two
separate sets of flaps per segment. The upper flaps were equivalent to the
upper limb branch of modern arthropods, while the lower flaps represent
modified walking limbs, adapted for swimming. Furthermore, a re-examination of
older anomalocaridids showed that these flaps also were present in other
species, but had been overlooked. These findings show that anomalocaridids
represent a stage before the fusion of the upper and lower branches into the
double-branched limb of modern arthopods.

"It was while cleaning the fossil that I noticed the second, dorsal
set of flaps," said Van Roy, who spent hundreds of hours working on the
specimens. "It's fair to say I was in shock at the discovery, and its
implications. It once and for all resolves the debate on where anomalocaridids
belong in the arthropod tree, and clears up one of the most problematic aspects
of their anatomy."

Aegirocassis benmoulae is also remarkable from an ecological standpoint, note the
researchers. While almost all other anomalocaridids were active predators that
grabbed their prey with their spiny head limbs, the Moroccan fossil has head
appendages that are modified into an intricate filter-feeding apparatus. This
means that the animal could harvest plankton from the oceans.

"Giant filter-feeding sharks and whales arose at the time of a major
plankton radiation, and Aegirocassis represents a much, much
older example of this -- apparently overarching -- trend," Van Roy said.







Story Source:

The above story is based on materials provided by Yale
University. The original article was written by Jim Shelton. Note: Materials
may be edited for content and length.







Journal Reference:

1.    Peter Van Roy, Allison C. Daley, Derek
E. G. Briggs. Anomalocaridid trunk limb homology revealed by a giant
filter-feeder with paired flaps. Nature, 2015; DOI: 10.1038/nature14256

http://www.sciencedaily.com/releases/2015/03/150312083651.htm

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