Evolution & Adaptations of PlacodermsJustin 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.

What are placoderms?Had a set of bony platesthat covered the anterior of the body.

One of the first groups to evolve a complex set of jaws.

Why study placoderms?Bony plates make well-preserved 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

ExtinctionExtinction

Dunkleosteus terrelli• Arthrodira• 380-360 million years ago• Large size, "tank-like,"

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• Found worldwide, specifically in shallow marine and freshwater

environments• head and trunk shield (anti-predator)• bottom feeders• spiral digestive tract

Lungs of Bothriolepis• 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 PlacodermsJohanson’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

ExtinctionWent 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• http://www.youtube.com/watch?v=L0PmA5ZjpOA