Discussion of
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 respiration
Many more smaller changes (ears, head shape, etc.)

Lobe-Limbed Fish
Sarcopterygians – Living members include coelacanths, lungfish and tetrapods!
Rod-shaped bones with muscles in the pectoral fins not simple rays supporting a very thin fin

Big evolutionary transition!
?

Transitional species should be approximately 365-375 MYA
Middle to late Devonian age tiktaalik.uchicago.edu

Where are mid-to-late Devonian rocks exposed on the surface?
In addition: sparsely populated freshwater 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 skull
Eyes 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 tetrapodlike.
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 Phylogeny
Evaluated 114 morphological characteristics for Tiktaalik and others

Tiktaalik Phylogeny
Evaluated 114 morphological characteristics for Tiktaalik and others