Discussion of

A Devonian tetrapod-like fish and the evolution of the tetrapod body plan

by

Edward Daeschler – Academy of Natural Sciences, Philadelphia

Neil Shubin – University of Chicago and the Field Museum

Farish Jenkins – Harvard University and Museum of Comparative Zoology

Nature

440:757-763

BIO101, Oct. 26, 2011

Serious questions remain in the evolution of animals.

How did terrestrial animals with four-legs (tetrapods) evolve from marine animals with fins?

Big evolutionary transition!

Major changes in morphology (legs vs. fins) and respirationMany more smaller changes (ears, head shape, etc.)

Lobe-Limbed FishSarcopterygians – Living members include coelacanths, lungfish and tetrapods!

Rod-shaped bones with muscles in the pectoral finsnot simple rays supporting a very thin fin

Big evolutionary transition!

?

tiktaalik.uchicago.edu

Should be able to find transitional species

Transitional species should beapproximately 365-375 MYA

Middle to late Devonian age

Where are mid-to-late Devonian rocks exposed on the surface?

In addition:sparsely populatedfreshwater in Devonian

Three sites in North America,only one was largely unexplored

Welcome to Northern Canada!

Welcome to Northern Canada!

Fossil Site

Part of the Fram Formation

Laid down in late Devonian

Alternating sandstone and siltstone layers. Probably a meandering stream.

Fossil Site

Fossil Site

Fossils

28 partial fossils of this species found.

Each gets a name NUFV 108 through NUFV 135 (Nunavut Fossil Vertebrate Collection)

Three were fairly complete, with skulls and pectoral girdles and articulated

NUFV 108 is the holotype

All specimens are currently in the Canadian Museum of Nature in Ontario

Tiktaalik as a Transitional Species – Ancestral Characteristics

Body was covered in scales

Dorsal view

Tiktaalik as a Transitional Species – Ancestral Characteristics

True fins

Tiktaalik as a Transitional Species – Ancestral Characteristics

Gills

Ceratobranchial Element

Tiktaalik as a Transitional Species – Ancestral Characteristics

Probably had lungs too (like coelacanths and lungfish)

Large Gular Plates

Tiktaalik as a Transitional Species – Derived Characteristics

Flat skullEyes are more on top of the head, not on the sides

Tiktaalik as a Transitional Species – Derived Characteristics

Flattened Ribs

Tiktaalik as a Transitional Species – Derived Characteristics

Functional, flexible neck.

Pectoral girdle (collar and shoulder bones) are tetrapod-like.

Loss of opercular, subopercular and extrascapular bones.

Tiktaalik as a Transitional Species – Derived Characteristics

Wrist bones

Tiktaalik as a Transitional Species – Derived Characteristics

Wrist bones

Tiktaalik as a Transitional Species – Derived Characteristics

Wrist bones

Tiktaalik Ecology

Life in shallow water has it’s advantages

Tiktaalik PhylogenyEvaluated 114 morphological characteristics for Tiktaalik and others

Tiktaalik PhylogenyEvaluated 114 morphological characteristics for Tiktaalik and others