Re: mammalian reptiles

[cnedin@geology.adelaide.edu.au (Chris Nedin)]

M F Hanson wrote:

>What specific details determine that a fossil is from a reptilian mammal?

I think you mean what makes a mammal-like reptile? However here is a sample
of *some* of the characters and *som* of the forms which make up the
therapsis -> mammal transition .

          Reptiles                                Mammals
1  Single occipital condyle          Double occipital condyle
2  Undifferentiated dentition       Differentiated dentition
3  No secondary palate                secondary palate
4  No diaphram                          Diaphram
5  Uncrowned, uncuspate          Crowned, cuspate teeth
      teeth
6  Teeth with single root            Teeth with multiple roots
7  Lower jaw of several bones    lower jaw of dentary bone only
8  Jaw joint quadrate-articular    Jaw joint dentary-squamosal
9  Lumbar region with ribs        Lumbar region free
10 Separate clavical ribs            Fused clavical ribs
11 Flat scapular                         Strong spine on scapular
12 Pelvic elements separate       Pelvic elements fused
13 Limbs out from body           Limbs under body
14 Cold blooded                      Warm blooded
15 Scales                                  Fur/hair
16 Joined external nares           Separate external nares

The reptiles evolved into four major groups; the anapsids, which produced
the turtles, the diapsids which produced the dinosaurs, and an offshoot
group, the eurapsids, which produced the icthyosaurs. 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 group 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

This is by no means exhaustive and there have been some changes since I
drew up this list. However, the basic principle still holds.

Chris

cnedin@geology.adelaide.edu.au