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Re: Hominid fossils FAQ file
Kathleen Hunt (jespah@u.washington.edu)
1 Jan 1995 23:24:55 GMT
>In article <3e1nv9$ouc@newsbf02.news.aol.com>,
>Steve ThM <stevethm@aol.com> wrote:
>>
>>Granted, there are transitional forms within certain species. But can you
>>list one clear-cut, completely authenticated transitional form between one
>>species and another species? I do not believe you can. If so, please
>>file the name of it to me for study.
I posted several such examples just LAST WEEK. Perhaps you didn't see
them. So here they are again.
I particularly recommend you find Gingerich's 1980 paper and look
carefully at the figure that shows *hundreds* of specimens from entire
*populations* of fossils, slowly diverging in average morphology until
there were two distinct populations, which slowly became two distinct
species, which slowly became two distinct genera, etc. Rose & Bown's
paper also has a very good figure showing gradual transformation of the
teeth. These species-to-species transitions consist of literally hundreds
of individual fossils spanning the transition from one species to another.
Frequently the fossils right in the middle are so exactly intermediate
that it's almost impossible to decide what name to give them. Krishtalka
& Stucky had to invent a new way of naming fossils, with hyphenated names,
because of this problem, because they had so many intermediate specimens
in such a smoothly changing, complete lineage!
Also look up K & S's hyena monograph and carefully read the descriptions of
"Artiodactyl A" and "Artiodactyl B", which were transitional between a
species of _Diacodexis_ and two new *families*. Also carefully read the
description of the population averages for that _Diacodexis_ species.
Notice how the "Artiodactyl A" features were present in occasional
individuals of _Diacodexis_? The new features occurred in a higher &
higher proportion of the _Diacodexis_ population, until the population in
one particular location had those new features in *almost all* of the
individuals. At that point it was not really like _Diacodexis_ anymore and,
in fact, it was a new species. The descendents of that species became
more and more different, and, in fact, became a new family.
The take-home message here is that species-to-species transitions *ARE*
known. Period. If you haven't heard of them it's because:
a) the research is slow and tedious and doesn't consist of a single
dramatic discovery.
b) paleontologists are really unsurprised by the discoveries of these
transitions. It's exactly what everybody expected. So they haven't
bothered to popularize it.
c) most of this research was started in the 1960's, with the papers being
published in the late 70's and throughout the 1980's (it takes about ten
years to collect and analyze that many fossils), and so, much of it is too
new to have gotten into popular books.
Species-to-species transitions are most commonly known for lineages that
have well-preserved and numerous fossils. That means fairly recent large
animals, such as mammals since the Miocene. There is also a pocket of
much earlier Eocene strata in the Rockies that is very complete, and has
also yielded many species-to-species transitions. BTW you need about one
fossil every 25,000 years to document species-to-species transitions
(Gingerich's estimate), which for older (pre-Cenozoic) strata is very
rarely found.
I'll append the information about the papers below. If you can't find
these papers in your library, I would be happy to xerox them (and others)
and send you copies. Just give me your snail-mail address.
Now here is the info, AGAIN, for some of the known species-to-species
transitions. I picked four well-known case studies here (more are known).
I have copied out all the species names involved in every transition. Let
me reiterate that in every case listed below, there are numerous known
fossils that fall *between* each pair of named species and are
intermediate in both time and morphology.
1. _Diacodexis_ species (early artiodactyls)
Krishtalka & Stucky (1985) documented smooth transitions in the common
early Eocene artiodactyl genus _Diacodexis_. The fossil record for
these animals is very good (literally hundreds of new specimens have
been found in Colorado and Wyoming since the 1970's). Analysis of
these specimens found gradual species-species transitions for *every
step* of the following lineage, including the origination of *three*
different familes. The lineage is as follows: _Diacodexis secans-
primus_ is the first artiodactyl species known. Immediately a new
group of animals split off that gave rise to the _Wasatchia_ and
_Bunophorus_ genera (not further discussed by the paper). Meanwhile,
the main lineage of _D. s-primus_ continued, and gave rise to _D. s-
metsiacus. Two species split off from _D. s-metsiacus_: one was _D.
gracilis_, the other was an as-yet-unnamed new species "Artiodactyla
A", which gave rise to "Artiodactyla B"; these two were the first
members of the new families Homacodontidae and Antiacodontidae.
Meanwhile, _D. s-metsiacus_ continued changing and became _D. s-
kelleyi_. Another species forked off, _D. minutus_. Slightly later
another species forked off, _D. woltonensis, which apparently was the
first member of the new family Leptochoeridae. Meanwhile, _D. s-
kelleyi continued changing and became _D. s-secans_.
The authors said "...it appears that different taxa of artiodactyls
-- in hindsight, the most primitive members of originating suborders,
families, and subfamilies -- arose at different times from different
lineage segments of the single species _Diacodexis secans_."
Reference:
Krishtalka, L., and Stucky, R.K. 1985. Revision of the Wind River
Faunas. Early Eocene of Central Wyoming. Part 7. Revision of
_Diacodexis_ (Mammalia, Artiodactyla). Am. Carnegie Mus. 54:413-486.
2. The hyena family.
Though there are only four species now, hyaenids were once *very* common
and have an abundant fossil record. A recent monograph (Werdelin &
Solounias, 1991) discussed over one hundred (!) named species, with
extensive discussion of the eighteen best-known species, and cladistic
analysis of *hundreds* of specimens from the *SIXTY-ONE* "reasonably
well known" hyaenid fossil species. Their core tree shows a main stem
of generally small to medium-sized doglike forms, showing a general
trend toward an increase in size:
_Herpestes antiquus_, a viverrid (civet-like animal) thought to be the
ancestor of the hyenid family.
_Protictitherium crassum_ (& 5 closely related species). Transitional
between the early civet-like viverrids and all the hyenids. Split
into three lines, one of which led to the aardwolf. Another line
eventually led to modern hyenas:
_Plioviverrops orbignyi_ (& 3 closely related species)
_Tungurictis spocki_
_Ictitherium viverrinum_ (& 6 closely related species)
_Thalassictis robusta_ (& 5 other spp.)
_Hyaenotherium wongii_
_Miohyaenotherium bessarabicum_
_Hyaenictitherium hyaenoides_ (& 3 other spp.)
_Palinhyaena reperta_
_Ikelohyaena abronia_
_Belbus beaumonti_
_Leecyaena lycyaenoides_ (& 1 other)
_Parahyaena brunnea_
_Hyaena hyaena_
_Pliocrocuta perrieri_
_Pachycrocuta brevirostris_ (& 1 other)
_Adcrocuta eximia_, which split into:
_Crocuta crocuta_ (the modern spotted hyena), _C. sivalensis_, and _C.
dietrichi_.
The authors said "We view the evolution of hyaenids as overwhelmingly
gradual. The species, when studied with regard to their total
variability, often grade insensibly into each other, as do the
genera. Large specimens of _Hyaenotherium wongii_ are, for example,
difficult to distinguish from small specimens of _Hyaenictitherium
hyaenoides_, a distinct genus. Viewed over the entire family, the
evolution of hyaenids from small, fox-like forms to large,
scavenging, "typical" hyenas can be followed step by step, and the
assembly of features defining the most derived forms has taken place
piecemeal since the Miocene. Nowhere is there any indication of
major breaks identifying macroevolutionary steps."
Reference:
Werdelin, L, and N Solounias. 1991. The Hyaenidae: taxonomy,
systematics, and evolution. Fossils and Strata 30.
Universitetsforlaget, Oslo.
3. The elephant family
The Pleistocene record for elephants is very good, and almost half of
the Pleistocene elephant speciation events have been preserved in
fossils. In general, after the earliest forms of the three modern
genera appeared, they show very smooth, continuous evolution from one
species to another.
For example (quotes are from Maglio, 1973):
_Elephas ekorensis_ An early Asian elephant with rather primitive
molars. "At present, a direct phyletic relationship between E.
ekorensis and E. recki seems certain."
_Elephas recki_ "Can be traced through a progressive series of
stages...These stages pass almost imperceptibly into each other."
_Elephas iolensis_ "In the late Pleistocene a more progressive elephant
appears which I retain as a distinct species, _Elephas iolensis_,
only as a matter of convenience. Although as a group, material
referred to _E. iolensis_ is distinct from that of _E. recki_, some
intermediate specimens are known, and _E. iolensis_ seems to
represent a very progressive, terminal stage in the _E. recki_
specific lineage. "
Maglio also documented very smooth transitions between three Eurasian
mammoth species: _Mammuthus meridionalis_ --> _M. armeniacus_ (now
called _M. trogontherii_) --> _M. primigenius_.
Overall, Maglio showed that at least 7 of the 17 Quaternary elephant species
arose through smooth anagenesis transitions from their ancestors.
Lister (1993) reanalyzed mammoth teeth and confirmed Maglio's scheme of
gradual evolution in European mammoths, and added another branch to
the tree: _M. meridionalis_ also crossed to North America and
evolved into _M. imperator, which then evolved into _M. columbi_.
Carroll (1988) says "Within the genus _Elephas_, species demonstrate
continuous change over a period of 4.5 million years. ...the
elephants provide excellent evidence of significant morphological
change within species, through species within genera, and through
genera within a family...."
References:
Carroll, Robert. 1988. _Vertebrate Paleontology and Evolution_. W.H.
Freeman & Co., New York.
Lister, A.M. 1993. Evolution of mammoths and moose: the Holarctic
perspective. Chapter 9, pp. 178-204, in: Morphological change in
Quaternary mammals of North America, eds. R.A. Martin and A.D.
Barnosky. Cambridge University Press, New York.
Maglio, V.J. 1973. Origin and evolution of the Elephantidae. Trans.
Am. Phil. Soc., New Ser. 63:1-149.
4. Early primates
Gingerich (1977) studied the early lemur-like primate _Pelycodus_. He
traced two distinct species, _P. frugivorus_ and _P. jarrovii_, back
in time, and found that they converged on the earlier _Pelycodus
abditus_ "in size, mesostyle development, and every other character
available for study, and there can be little doubt that each was
derived from that species." Further work (Gingerich, 1980) in the
same rich Wyoming fossil sites found species-to-species transitions
for *every step* in the following lineage: _Pelycodus ralstoni_ (54
Ma) to _P. mckennai_ to _P. trigonodus_ to _P. abditus_. _P.
abditus_ then forked into three branches. One became a new genus,
_Copelemur feretutus_, and evolved into _C. consortutus_. The
second branch evolved into _P. frugivorus_. The third led to _P.
jarrovi_, which evolved further to another new genus, _Notharctus
robinsoni_, which itself split into at least two branches, _N.
tenebrosus_, and _N. pugnax_ (which then changed to _N. robustior_),
and possibly a third, _Smilodectes mcgrewi_ (which then changed to
_S. gracilis_). Note that this sequence covers at least three and
possibly four genera.
Notice here that these species had previously been recognized on the
basis of a few, clearly distinct specimens from widely separated
strata. Not until Gingerich studied the hundreds of specimens from
intervening levels did the transitions become clear. The species
actually form several continuous morphological series that can be
divided into separate species only arbitrarily. Also note that the
intermediate specimens had been there all along; Gingerich was simply
the first researcher to put in the quite tedious work (over ten
years' research!) of collecting and analyzing them. Overall, in this
1980 paper Gingerich described 24 species that have arisen by gradual
phyletic evolution, and 14 cases in which species appear suddenly in
the fossil record (he attributes these to immigration). The species
he studied did not all show progressive change; some showed stasis
for considerable lengths of time.
Another example: Rose & Bown (1984) analyzed over 600 specimens of
primates collected from a 700-meter-thick sequence representing
approximately 4 million years of the Eocene. They found smooth
transitions between _Teilhardina americana_ and _Tetonoides tenuiculus_,
and also beween _Tetonius homunculus_ and _Pseudotetonius ambiguus. Note
that both transitions cross genus lines. "In both lines transitions
occurred not only continuously (rather than by abrupt appearance of new
morphologies followed by stasis), but also in mosaic fashion, with greater
variation in certain characters preceding a shift to another character
state." The T. homunculus - P. ambiguus_ transition shows a dramatic
change in dentition (loss of P2, dramatic shrinkage of P3 with loss of
roots, shrinkage of C and I2, much enlarged I1). These changes are very
"dramatic" when you compare the first fossils to the last fossils. But
when you look at all the fossils in between, you see that the changes
actually occured *gradually* and smoothly during the 4 million years. The
authors conclude "...our data suggest that phyletic gradualism is not only
more common than some would admit but also capable of producing
significant adaptive modifications."
References:
Gingerich, P.D. 1977. Patterns of evolution in the mammalian fossil
record. In: Patterns Of Evolution As Illustrated By The Fossil
Record (ed. A. Hallam), chapter 15, pp. 469-500. Elsevier Scientific
Pub. Co.
Gingerich, P.D. 1980. Evolutionary patterns in early Cenozoic mammals.
Ann. Rev. Earth Planet. Sci. 8:407-424.
Rose, K.D., and Bown, T.M. 1984. Gradual phyletic evolution at the
generic level in early Eocene omomyid primates. Nature 309:250-252.
Kathleen Hunt
--
If we increase the size of the penguin until it is the same height as
the man and then compare the relative brain size, we now find that the
penguin's brain is still smaller. But, and this is the point, it is
larger than it *was*. (Monty Python)
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