archived 08-21-00
Archive file# r082100b
donated by L. Savage

The Reptile to Mammal Transition


"Now to evolution. When the first mammal, for example, evolved it probably was alone. Or did a whole lot of mammals just appear? How long did it take before it was able to reproduce? It had to get the gestation period right. Looks like it would have died before it could get sex right, much less gestation."

This highlights a common misconception of evolution which is caused to some degree by the inadequacy of the classification system. The modern classification system was created for and therefore heavily influenced by, extant organisms. These organisms have a distinct series of characters which make them unique. These characters have been honed by millions of years of evolution such that modern organisms tend to clump into groups of similar body plan. Each organism within these groups shares certain basic (or definative) characters with other members of that group, but not with any other group. For the most part the intermediaries or stem stock of each group - or phyla - have long since disappeared from the biological record and what we see today are derived forms which tend to cluster heavily around specific body plans with very few if any intermediate forms. This has given rise to the fact that modern forms in one phyla are very different from forms in another phyla. Whilst this is true, it is very important to note that modern forms in one phyla are not only different, but very distant, in an evolutionary sense (i.e. time to last common ancester), from forms in another phyla. The morphological differences which are the basis for modern classification have been brought about as much by the evolutionary distance as by anything else. Evolutionary distance has resulted in the clustering on organisms around specific bodyplans and has thus made it easy to classify them (Note: I am only talking about the animal kingdom here). Sure there have been examples of convergence, such as the sabre-tooth tiger, Smilodon and it's marsupial equivalent Thylacosmilus, but Thylacosmilus still had marsupial characters (epipubic bones, marsupial dentition etc.). Also if you allow convergence, logically you must allow divergence.

Thus modern organisms cluster into descrete groups or phyla making for easy classification. However, as I have tried to point out, this situation is primarily a factor of evolutionary distance, and evolutionary distance is a factor of time. Therefore, the theory of evolution predicts that if we go back in time, organisms should begin to loose some of their definative characters and that eventually, phyla should merge together. It is at this point that the modern classification scheme becomes a stumbling block (both physically and mentally) since the definative characteristics of a phyla are smeared over several individuals and groups. Because the modern scheme does not allow intermediate groups, it becomes a process of shoe-horning a grey area into a black and white classification. Nowhere is this better illustrated than with the reptile -> mammal transition.

Before documenting this transition, it is probably a good idea to list various characteristics of reptiles and see how these change with the evolution of the mammals.

Reptiles

1 Undifferentiated dentition

2 No secondary palate

3 No diaphram

4 Uncrowned, uncuspate teeth

5 Teeth with single root

6 Lower jaw of several bones

7 Jaw joint quadrate-articular

8 Lumbar region with ribs

9 Separate clavical ribs

10 Flat scapular

11 Pelvic elements separate

12 Limbs out from body

13 Cold blooded

14 Scales

15 Joined external nares

Mammals

    Differentiated dentition

    Secondary palate

    Diaphram

    Crowned, cuspate teeth

    Teeth with multiple roots

    lower jaw of dentary bone only

    Jaw joint dentary-squamosal

    Lumbar region free

    Fused clavical ribs

    Strong spine on scapular

    Pelvic elements fused

    Limbs under body

    Warm blooded

    Fur/hair

    Separate external nares

The reptiles evolved into three major groups; the anapsids, which produced the turtles, the diapsids which produced the dinosaurs, and an offshoot group which includes the ichthyosaurs and the sauropterygians. The final group, the synapsids, took a radically different path than the other groups and produced the therapsids, which concentrated on osteo- and pysiological changes which eventually produced the mammals. The group called the cynodontia (dog tooth) produced a lineage of forms intermediate between reptiles and mammals.

The following is not a direct lineage, but representatives of successful, related groups which exhibit a gradual aquisition of mammalian characters during the Permian-Triassic.

The phylogeny looks like this:

            A      B      C      D      E,F     G

            |      |      |      |      \/      |

Cynodont ----------------------------------------> Mammal

A) Procynosuchus

Latest Permian-Triassic, South Africa.

Has an expanded temporal region; large zygomatic arch; enlarged dentary, but the lower jaw is still made up of several bones (albeit reduced); the begining of a secondary palate; double occipital condyle (first major mammalian character).

B)Thrinaxodon

Early Triassic, South Africa, Antarctica.

Elaborate cheek teeth; large dentary, with coronoid process(for jaw joint), but still lower jaw of more than one bone; reduction to mammalian number of insisors; almost complete secondary palate - before anyone comes in here with the question "How could an almost complete secondary palate work?" - the palate can function quite adequetely by being covered with a fleshy membrane, which it is in reptiles. Thus the underlying bone can form gradually and support the palate more and more, without delateriously affecting the functioning of the palate, until the secondary palate if fully formed, it then becomes important, because it separates the nasal passages from the mouth - this means you can now eat and breath at the same time or more importantly you can breath whilst chomping something that is struggling to get away, or that something else is trying to steal from you); lumbar ribs reduced to small plates - the specialisation of the lumbar area is indicative of the presence of a diaphram, needed for higher O2 intake and homeothermy; the head of the femur is set at a considerable angle to the shaft - this indicates that the limbs were upright and closer to underneath the body that sprawling; adult/baby fossil assemblages have been found - possibly indicating parental care; fossils found curled up - curling usually indicates an attempt to keep body heat, possible homeothermy.

C)Cynognathus

Early Triassic, South Africa.

Enlarged dentary, 90% of lower jaw, teeth differentiating, large canine, molars with cusps; secondary palate well developed; jaw joint quadrate-articular, but bones very small; scapular transverse and turned out - half way to mammal condition; limbs under body; possible evidence for fur in fossil footprints.

D)Diademodon

Early Triassic, South Africa.

Cheek teeth more specialised, with more cusps, occlude together more efficiently; clavical ribs fused.

E)Probelesodon

Mid Triassic, South America.

Saggital crest for greater muscle attachment; nares separated; lumbar free.

F)Probainognathus

Mid Triassic, South America.

Additional cusps on cheek teeth; teeth double rooted; 'double' jaw joint, the quadrate-articular and the dentary-squamosal bones articulate, but the quadrate-articular bones are very much reduced and only loosely constrained in a groove in the dentary bone; cervical ribs very short; lumbar free; phalangeal arrangement mammalian - loss of some bones.

G)Kayentatherium

Early Jurassic, world wide.

Double occipital condyle; secondary palate; separated nares; dentary bone covers almost all lower jaw; differentiated dentition; double rooted teeth; lumbar free; scapulare with spine; pelvic elements fused; fused clavical ribs; but quadrate-articular although very much reduced, still participate in the jaw joint. This feature classifies the organism as a reptile, even though it has far more mammal characters than reptile ones.

Recap:

    A    B    C    D    E    F    G


1   1    1    1    1    1    1    1 


2   *    1    1    1    1    1    1 
   

3   *    *    1    1    1    1    1 
     

4   0    *    1    1    1    1    1 
       

5   0    0    *    1    1    1    1 
     

6   0    0    0    *    1    1    1 
    

7   0    0    0    0    0    *    1 
      

8   0    0    0    0    0    *    * 
       


O = reptilian state

* = intermediate

1 = mammalian state

The decision as to which was the first mammal is somewhat subjective. If, for instance it is decided that a reduced quadrate-articular jaw joint is an essential precurser and should be classed as mammalian, then Kayentatherium would become the first mammal. However, that title will do nothing to affect the viability of that species, since at this stage in the evolution of the mammals very little separates them from what we have decided are reptiles. The differences between reptile and mammal, at this stage is miniscule compared with the modern day differences between the two groups because in the intervening time the two groups have evolved along separate line, becoming morphologically different (evolutionary distance).

So you do not go from the 'true' reptile state to the 'true' mammal state in one generation, that would be impossible. What has happened is that an itermediate group evolved from the 'true' reptiles, which gradually aquired mammalian characters until a point was reached where we have artificially drawn a line between reptiles and mammals. The group has continued to evolve, but we have given them a new name because they are sufficiently different from the 'true' reptiles and sufficiently unique to have their own classification - mammals. Where we draw the line between the two is a function of what charaters we consider unique enough to be meaningfull and has no bearing on the viability of whichever species we decide is the 'first' mammal, since we are placing an inflexable classification system on a gradational series. As things stand at the moment Kayentatherium is far more mammal-like than reptile-like, but since it does not possess all the characters we have decided a mammal should have, it must be a reptile. The theory of evolution is fine, the theory of taxonomy need a bit of revising.

For more information on this and other transitions I suggest you read Kathleen Hunt's excellent "Transition Fossil" FAQ.

This is a University of Ediacara Palaeontological Contribution.

References

Benton, M.J. (1990) Vertebrate Palaeontology: biology and evolution. Unwin Hyman, london. pp 377. ISBN 0045660018

Colbert, E.H. & Harris, E. (1991) Evolution of the vertebrates: a history of the backboned animals. Wiley-Liss, New York. pp 470. ISBN 0471850748

Kemp, T.S. (1982) mammal-like reptiles and the origin of mammals. Academic Press, New York. pp 363. ISBN 0124041205

Kermack, D.M. & Kermack, K.A. (1984) The evolution of mammalian characters. Croom Helm Kapitan Szabo Publishers, London. pp 149. ISBN 079915349

http://members.tripod.com/~Cambrian

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