Estimates for vertebrate divergences

scarr at kean.ucs.mun.ca scarr at kean.ucs.mun.ca
Wed Feb 10 18:02:44 EST 1993


In article <1993Feb6.042713.15036 at massey.ac.nz>, GEustace at massey.ac.nz (Glen Eustace) writes:
> 
> Hello
> 
> many authors state dates of divergence between taxa but may not cite
> primary (or any) references in support. I am after estimates of
> divergence times between the following vertebrate groups and would like
> references for them:
> 
> ray-finned fish & amphibians
> ray-finned fish and reptiles (esp. lizards)
> ray-finned fish and birds
> ray-finned fish and mammals
> amphibians & reptiles
> amphibians & mammals
> reptiles & birds
> reptiles & mammals
> birds & mammals.
> 
> 80 million years is a frequently cited estimate for mammalian radiation.
> From what reference(s) did this date come.
> 
> with many thanks
> 
> Robert Hickson.

Deart Robert

Fortunately, I've just this morning finished the lecture on ray-finned fish
origins for my vertebrate biology class, and I updated my notes at the
beginning of the semester, so all of the following numbers are at hand. 
Divergence times for any vertebrate groups can be obtained from: 

R. L. Carroll (1988). Vertebrate Paleontology and Evolution. W. H. Freeman.
Pough et al (1989). Vertebrate Life, 3rd ed. Macmillan.

If you think about the problem phylogenetically, you will realize that you 
are really asking only three distinct questions. To know the divergence time, 
you simply have to identify when the last common ancestor lived. This is not
necessarily the same as when the class came into existence, since in
conventional taxonomy one class can change into another. For example, 
mammals are derived from the lineage of synapsid 'reptiles', which last had a 
common ancestor with the other reptiles in the early Pennsylvanian, ca. 300 MYBP, 
but the Class Mammalia is usually dated only as far back as the Triassic, ca. 
220 MYBP. See Chap. 20 of Carroll for the mammalian radiation.

Sarcoptergyii, the evolutionary lineage that includes fleshy-finned fish,
amphibians, reptiles, birds, and mammals, diverged from that leading to the
ray-finned fishes (Actinopterygii) in the late Silurian / early Devonian, 
call it 400 MYBP. That is, the last common ancestor of a ray-finned fish and any
tetrapod lived 400 million years ago. Thus, the answers to your first four
questions are exactly the same. Begging your pardon, the question suggests 
the line of thought that the divergence time between fish and (say) reptiles
should be "shorter" than that between fish and (say) mammals because 
reptiles are "more primitive" / "older" than the "more advanced" / "newer" mammals. 
Not so: look for the common ancestor, which lived at a particular moment in 
time. 

The Amniota (including scaly reptiles, feathered reptiles (AKA birds), and
mammals) diverged from the lineage leading to Lissamphibia (modern 
amphibian orders) in the early Carboniferous, ca. 350 MYBP, the answer to the
 next two questions.

The answer to the last question is more complicated, as it depends on what 
you mean by 'reptile'. Birds are Reptiles. Refer to Figures 10-12 (p. 201), 11-
1 (p. 218), 13-1 (pp. 262-263), 14-1 (p. 287), & 16-23 (p. 347) in Carroll. 
The first undoubted bird fossil, Archaeopteryx, dates back only to the upper
Jurassic, 145 MYBP. However, the divergence of Aves from Saurischian 
dinosaurs may be as far back as the early Jurassic, ca. 210 MYBP. The divergence of
Dinosauria from other archosaurs (including crocodiles) is late Permian, ca.
250 MYBP. The divergence of archosaurs from other diapsid reptiles
(Lepidosauriomorpha, including snake and lizards) is early Permian, ca. 280
MYBP. [i.e., a Tyrannosaurus rex is considerably more closely related to 
your canary than it is to a lizard]. [This date may be one you're most 
interested in]. The relative times of divergence of Diapsid, Anapsid (including 
turtles) and Synapsid (including mammals) lineages is uncertain, opinion favoring a
slightly earlier divergence of Synapsids followed by Anapsids, but both ca.
300 MYBP.

Finally, in response to postings by several other netters: molecular
biologists don't need a data base to tell them this stuff, we just need to
take advantage of basic reference material and/or courses in vertebrate
paleontology and/or evolution. Learning systematic arrangements isn't any 
more tedious than memorizing the Krebs Cycle, possibly less so since the 
component entities have fur, feathers, and scales and are thus cuter. 

Does anyone out there have a copy of the description of a kangaroo for
molecular biologists? It begins, "The kangaroo has a molecular weight of
60,000,000,..." 

Steve Carr
Dept. of Biology
Memorial University of Newfoundland
St. John's NF A1B 3X9
Canada



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