Evolution & Adaptations of Placoderms

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Evolution &
Adaptations of
Placoderms
Justin Waraniak, Yovaan Totharam, Hanjie
Mo, Kevin Hsiao
What are placoderms?
Main group of vertebrates
in aquatic environments
during the Devonian Period
(~420-360 Mya).
Diverse Group
found everywhere on earth
adapted to many different lifestyles.
included both the largest and
smallest vertebrates of the time.
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What are placoderms?
Had a set of bony plates
that covered the anterior
of the body.
One of the first groups to
evolve a complex set of
jaws.
Why study placoderms?
Bony plates make wellpreserved fossils; make it
possible to see details in
morphology and bodily
structures.
Why study placoderms?
Interesting Morphologies
• Dunkleosteus - jaws
• Bothriolepis - possible lungs
Unknown
Relationship to
Other Gnathostomes
Extinction
Extinction
Dunkleosteus terrelli
• Arthrodira
• 380-360 million years ago
• Large size, "tank-like,"
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posterior unknown
Gluttonous predator
Jaw diversification
Study Methods
• Difficulty with prehistoric ecological issues
• Biomechanical Models, laws of physics
• Cleveland Shale
• Muscle cast measurements
Results
• 6000N, 7000N
• Suction capability, 1/15s
• Four Bar Linkage
mechanism
(Anderson, 2007)
Evolutionary History
• Short Lifespan
• Sharks
• Gouges and Scratches
• Anterior bite margin
• Evolutionary trend
• Derived fish lower jaws
• T-rex and Alligator bite
Bothriolepis
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Found worldwide, specifically in shallow marine and freshwater
environments
head and trunk shield (anti-predator)
bottom feeders
spiral digestive tract
Lungs of Bothriolepis
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Often cited as the earliest vertebrates with both gills and lungs
large pectoral fins could have been used for support
"if sedimentary structures could be interpreted as traces of some digestive
organs, none could be interpreted as putative lungs." (Goujet, 2011)
discovery of soft-tissue preservation shows that if not "lungs" some how
part of the branchial chamber (Janvier, 2007)
Phylogenetic Relationships
Within the Placoderms
Johanson’s statements:
Antiarchs have blood vessels that go through scapulocoracoid, pectoral fin,
and wall of gill chamber
Rostrocaudal canal goes to the back of the gill chamber
Antiarchs more similar to Osteostraci – armored agnathans
Young’s views:
Johanson’s conclusions were wrong
Rostrocaudal canal does not go to the back of the gill chamber.
Monophyly maintained.
(Young, 2008)
Evolutionary Placement of
Placoderms
3 Hypotheses (Gardiner, 1984)
1. Placoderms are the collateral
descendants of all other gnathostomes
2. Placoderms are more closely related
to chondrichthyans than to other
gnathostomes
3. Placoderms are more closely related
to osteichthyans than to chondrichthyans
(Young, 2010)
Structures
Extinction
Went extinct at the end of the Devonian
Mass extinction between the Frasnian and Famennian
periods.
Allowed Chondrichthyans and Actinopterygians to diversify.
(Sallan, 2010)
Future Work/ Conservation
• More fossils being uncovered.
• Resolve the evolutionary tree of
gnathostomes
• Understand why/how such a diverse group
went extinct; understand how mass
extinctions occur.
Overview/ Questions
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http://www.youtube.com/watch?v=L0PmA5ZjpOA
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