Fins to Limbs to Fins: Is Evolution Confused

1
Fins to Limbs to FinsIs Evolution Confused?

• Conner Means
• Biology Department
• Bellarmine University

2
Introduction

• There is a lot of research saying that marine
  animals evolved into terrestrial (or land going)
  animals.
• The evolution of fins into limbs has been an
  integral part of that theory.
• However, one family of reptiles that lived during
  the reign of the dinosaurs has apparently
  reversed evolution, turning limbs back into fins.

3

• The basic structure of the vertebrate limb is
  remarkably conserved among tetrapods.
• It consists of 3 parts
• A proximal part, consisting of 1 skeletal element
  (your upper arm or upper leg).
• A medial part, consisting of 2 skeletal elements
  (your lower arm or lower leg).
• A distal part, consisting of carpus (wrist) or
  tarsus (ankle) and radiating digits (fingers or
  toes).

4

• Animal appendages are outgrowths of the body wall
  that are adapted for specialized functions, such
  as feeding and locomotion.
• The tetrapod limb is just one type of appendage.
• It is a particular example of body wall outgrowth
  that played a key role in the appearance of
  terrestrial vertebrates in the late Devonian
  period.

5

• The key event in the evolutionary transition from
  fins to limbs was the formation of digits, which
  occurred around 360 m.y.a.
• The prevailing theory stated that digits were a
  morphological novelty that appeared as an
  adaptation to the need of supporting the weight
  of the animal during terrestrial locomotion.

6

• However, recent analysis of fossils of one of the
  most primitive known tetrapods have suggested
  that digits evolved in water as an event
  unrelated to the need of terrestrial locomotion.
• Newly discovered fossils have shown that the
  earliest tetrapods had more than five fingers and
  toes.

7

• One intriguing aspect of tetrapod limb evolution
  has received relatively little attention
• The secondary radiation of tetrapods back into
  aquatic environments.
• Called the fin-to-limb-to-fin transition.
• This involves a major morphogenetic
  reorganization of the limb to a paddle-like, or
  fin-like, structure.

8
The gradual evolution of limbs to fins
9
Examples of Fin-Limb-Fin Species

• Among living groups of tetrapods, this process of
  secondary radiation and morphogenetic evolution
  has produced the specialized limbs of
• Cetaceans
• Seals
• Sea lions
• Manatees
• Walruses
• Sea turtles

10

• This aquatic adaptation has also been seen in the
  fossil record in a number of extinct lineages of
  diapsid reptiles
• Mosasaurs
• Ichthyosaurs
• Plesiosaurs
• Pliosaurs
• Extinct
• crocodilians

11

• Our understanding of the fin-to-limb-to-fin
  transition is surprisingly good.
• This is due to the excellent fossil preservation
  provided by aquatic depositional environments.
• There are superb fossils documenting temporal
  sequences of morphological changes to the limb
  skeleton as animals continued to adapt to their
  environments.

12
Diapsid Evolution

• The earliest diapsid reptiles are from
  approximately 310 m.y.a.
• At 260 m.y.a., there is evidence of aquatic
  adaptations in diapsids.
• Over the next 200 m.y., numerous groups of
  diapsids returned to aquatic habitats.
• They subsequently altered their skeletal systems
  for aquatic locomotion, principally the limb
  skeleton.

13

• Sixty-five m.y.a., the last of the giant marine
  diapsids became extinct.
• In less than 10 m.y., whales filled the
  ecological and morphological gap.

14
Method of Limb-to-Fin Evolution

• The general process behind limb evolution to fins
  is the process of perichondral bone loss.
• The first area to lose bone was the ulna.
• Once the limb bones had shortened significantly,
  the first and fifth digits began to get smaller,
  until they completely disappeared in some species.

15

• Subsequent evolution led to the loss of margins
  of the remaining upper limb bones, with the
  exception of the humerus, and the margins of the
  remaining digits 4 through 2.
• In this manner, the bones of the limbs were
  shortened, with some (mainly certain digits and
  wrist bones) being completely lost.
• However, some species also reformed lost digits
  and wrist bones, altering the number and shape of
  each bone as it adapted to its environment.

16

• Research shows that the different species of
  diapsids evolved differently.
• Some evolved with radical changes, like
  eliminating most digits, then reforming more than
  it originally had.
• Others evolved through more minimal changes.

17
Conclusion

• Current research in limb developmental genetics
  is demonstrating the importance of homeobox gene
  complexes in limb pattern formation, which may
  have future bearing on this research.
• One thing that is fascinating is that, although
  each species made the change from limbs to fins
  in slightly different ways, the pattern of change
  and the results of that change were almost
  identical in all species.

18

• What is amazing is that mosasaurs, crocodiles,
  ichthyosaurs, and plesiosaurians were terrestrial
  tetrapods that became secondarily aquatic and
  evolved aquatically adapted limbs.

19
References

• Caldwell, Michael W. From Fins to Limbs to Fins
  Limb Evolution in Fossil Marine Reptiles.
  American Journal of Medical Genetics, (2002) Vol.
  112, pp 236-249.
• Capdevila, Javier. Perspectives on the
  Evolutionary Origin of Tetrapod Limbs. Journal of
  Experimental Zoology, (2000) Vol. 288, pp
  287-303.
• Motani, Ryosuki. Rulers of the Jurassic Seas.
  Scientific American, Mar. (2004), Vol. 14, pp
  4-13.