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What is a reptile? (A?)

Doug.Eernisse at UM.CC.UMICH.EDU Doug.Eernisse at UM.CC.UMICH.EDU
Fri Sep 25 00:02:46 EST 1992

Malcolm SHUTE ( v_ at _ ) asks:
>My question, though, is what distinguishes the dinosaurs from other reptiles?
>In what ways are crocodiles, alligators *not* surviving dinosaurs (as usually
>seems to be claimed or implied)?
This question is not a simple or noncontroversial one to answer. There are
presently both morphologists and molecular evolutionists who have advocated
the "Haemothermia" (proposed by the staunch anti-Darwinist Sir Richard Owen
in 1866) grouping of birds + mammals. I am not one of these advocates,
although I should preface all the following with the disclaimer that
I work mainly on molluscs, but maintain a broader interest in metazoan
relationships. The following is mostly a superficial summary of the
conclusions of J. Gauthier (Saurischian monophyly and the origin of birds.
Pp. 1-55 In: K. Padian (ed.), The Origin of Birds and the Evolution of
Flight. Mem. Calif. Acad. Sci. 8 (1986)) and Gauthier, Kluge, and Rowe
(Amniote phylogeny and the importance of fossils. Cladistics (1988) 4:
I prefer the conventional hypothesis of birds + crocodiles as members of
Archosauria, which also includes the extinct dinosaurs. Even with this
view, there has been some controversy concerning the competing hypotheses,
either birds + crocodiles as sister taxa within archosaurs, or birds
as dinosaurs. Gauthier (1986) favors the latter based on a detailed
phylogenetic analysis. Consider the following bird-dinosaur similarities
(from Gauthier, 1986:5):
1)  The skeleton is hollow and lightly constructed.
2)  The cervical vertebrae are elongate.
3)  There are more than two sacral vertebrae.
4)  The scapula is elongate and narrow.
5)  The coracoid is short and rounded.
6)  The ilium is prolonged anteroposteriorly.
7)  The acetabulum is roofed above by a supracetabular buttress of the
8)  The bony contribution of the ilium to the acetabulum is more or 
    less replaced by membrane.
9)  The ischium and pubis are much elongated.
10) The femur has a strong anterior trochanter.
11) The femur has a crest on the ventral face of the outer condyle that
    passes between the tibia and the fibula.
12) The proximal end of the tibia is produced anteriorly into a strong
    crest, which is bent outwardly, or towards the fibular side.
13) There is a crest on the lateral side of the proximal end of the 
    tibia for the attachment of the fibula.
14) The tibia has a fossa distally for the reception of the ascending
    process of the astragalus.
15) The fibula is gracile compared to the tibia, and its distal end
    is gracile compared to the tibia, and its distal end is much 
    smaller than the proximal.
16) The astragalus is compressed, its articulation with the tibia is
    concave proximally and it has a convex, pulleylike distal surface,
    and the disparity in size between the tibia and fibula is also
    reflected in the astragalus being much larger than the calcaneum.
17) An "ascending process" (the intermedium) more or less tightly
    connects the astragalus and tibia.
Believe it or not, Gauthier's diagnoses are much more detailed and
technical than this list. These 17 attributes were merely those proposed
by T.H. Huxley (1868-70) that have survived the 12+ decades (18 other
attributes he proposed have not survived). I found it interesting that
Huxley's views were widely favored by subsequent authorities before 1900
(e.g., Cope, Schmidt, Marsh, Gegenbaur, Williston, W.K. Parker, T.J.
Parker, Baur, and Darwin) although other views and the claim that the
dinosaurian affinities" of birds were due to convergence have been
advocated by a minority in the present century. While all of these 17
attributes favor the grouping of birds + other dinosaurs exclusive of
crocodiles (i.e., Crocodylia, incl. alligators), some are diagnostic at a
deeper level within Archosauria than Dinosauria (incl. birds), for
example, diagnosing Ornithosuchia, whose proposed sister taxon is
Pseudosuchia (incl. crocodiles).

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