triassic metoposaurid amphibians

57 Pages·2004·18.82 MB·Italian

TRIASSIC METOPOSAURID AMPHIBIANS
TRIASSIC METOPOSAURID AMPHIBIANS
EDWIN HARRIS COLBERT
Curakar ofFossil Reptiles and Amphibians
JOHN IMBRIE
Research Associate in Fossil Invertebrates
BULLETIN
OF THE
AMERICAN MUSEUM OF NATURAL HISTORY
VOLUME 110 : ARTICLE 6 NEW YORK : 1956
BULLETIN OF THE AMERICAN MUSEUM OF NATURAL HISTORY
Volume 110, article 6, pages 399-452, text figures 1-20, plates 25-28,
tables 1-9
Issued July 9, 1956
Price: $1.00 a copy
CONTENTS
INTRODUCTION 405
.
Acknowledgments. 406
LoCALITIESANDHORIZONSFROM WHICHTRIASSIC METOPOSAURSHAVE BEENCOLLECTED. 408
A TAXONOMIC REVIEW OF THE METOPOSAURIDAE . . . . . . . . . . . . . . 410
. .
The Described Genera and Species of Metoposaurs . 410
. . . . . . . . . . . . .
.
SOME CONSIDERATIONS OF METOPOSAUR OSTEOLOGY . .. . .. . . .. .. . . . 424
Skull and Jaws . 424
. . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Postcranial Skeleton . 426
. . . . . . . . . . . . . . . . . . . . . . . .
. .
BIOMETRIC ANALYSIS OF AMERICAN METOPOSAURS . . . . . . . . . . . . . . . 429
.
Bivariate Characterization . . . . . . . . . . . . . . . . . . . . . . . . . 430
Bivariate Discrimination Test . . . . . . . . . . . . . . . . . . . . . . . . 433
Estimation of Bivariate Overlap . . . . . . . . . . . . . . . . . . . . . . . 434
Multivariate Characterization . . . . . . . . . . . . . . . . . . . . . . . 437
.
Summary of Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . 438
THE GENERA AND SPECIES OF METOPOSAURS . . . . . . . . . . . . . . . . . 443
.
Discussion ..... .443
Conclusions 9
...........
.
BIBLIOGRAPHY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 451
403

INTRODUCTION
DURING LATE TRIASSIC TIMES the labyrin- were descended from the Permian labyrin-
thodont amphibians entered the final stages thodonts of the Order Rhachitomi. But
of their evolutionary history. These were the whereas the Permian forms were strong and
terminal membersofalongphylogenetic line well adapted to life either in the water or on
that extended back to the close of the De- the land, the Triassic types were characteris-
vonian period, and as such they represented tically adapted to an almost complete aqua-
a group of vertebrates that had been persis- tic existence. Some of the Triassic labyrin-
tently successful over a time lapse of more thodontswereeven largerthan their Permian
than 100 million years. Even though the forbears (in fact certain genera among the
labyrinthodonts were destined to become stereospondyls were the all-time amphibian
extinct at the close of Triassic times, these giants, with skulls that were more than a
amphibians were numerous and successful meter in length), but they had weak verte-
throughout the Triassic period. Theirdemise bral columnsand comparatively small, feeble
was certainly not a slowand gradual process; limbs, which would seem to indicate that
rather the evolutionary development of they may never have leftthewater. Yet they
these interesting amphibians continued with wereobviouslyverysuccessfulintheenviron-
seeming vigor up to the end of the Triassic ment to which they were adapted, for their
period, at which time they disappeared sud- remains in some regions are among the most
denly and completely fromallthecontinents. common of Triassic fossil vertebrates.
The first land-living vertebrates were One group of Triassic stereospondyl laby-
labyrinthodonts, which appeared as direct rinthodonts is that of the family Metopo-
descendants from certain crossopterygian sauridae, so named from the European genus
fishes duringthe transition from Devonian to Metoposaurus. These amphibians were all of
Carboniferous times. For a period of geo- late Triassic age. The skeleton in the meto-
logically brief duration the ancient labyrin- posaurs is commonly 2 meters or more in
thodonts had the land to themselves, but length and is characterized by the relatively
very soon the first reptiles arose from enormous, flatskull. Asistypicalof thelaby-
amphibian ancestors to share the land. rinthodonts, the skull roof is complete, being
Even with these active reptilian competitors pierced only by the openings for the orbits
around them, the labyrinthodonts continued and the nostrils, and by the single pineal
as very successful tetrapods; indeed, they opening, placed along the median line near
reached what was perhapsthe culmination of the back of the skull roof. The jaws arelong
their evolutionary history during the Per- and bear numerous labyrinthodont teeth, as
mian period, when there were many large, do the premaxillary and maxillary bones in
aggressive reptiles in the world. Some of the the skull. There are also teeth upon the pal-
rhachitomous labyrinthodonts of Permian ate. The palate is pierced by very large
times were themselves rather aggressive ani- palatal vacuities, separated by a long para-
mals, and it is likelythat they could compete sphenoid bone which is suturally and firmly
actively and directly with many of their rep- connected at the back with the pterygoid
tilian contemporaries. These amphibians boneson either side.
were large and strong, well able to live either The bones of the skull roof are extraordi-
in streams and ponds, or out on the land. In narilythickand heavyanddeeplysculptured
some parts of theworld the Permian labyrin- on theirdorsal surfaces. Certainly one reason
thodonts were among the most numerous of for the abundance of metoposaur remains in
theanimalsthatconstituted thefaunaswhere upper Triassic continental sediments is to be
they lived. found in the thick,comparatively indestruct-
The success of the late Paleozoic labyrin- ible nature of the skull bones. The same is
thodonit amphibians was carried over into true of the ventral bones in the shoulder
Triassictimes,butonabasisdifferent from its girdle, namely, the interclavicle and the
Permian expression. The Triassic labyrintho- clavicles. These are thick bones, heavily
donts belonging to the Order Stereospondyli sculptured.
405
406 BULLETIN AMERICAN MUSEUM OF NATURAL HISTORY VOL. 110
In contrast to the thick bones of the skull ACKNOwLEDGMENTS
and of parts of the shoulder girdle, the other We wish to express our deep appreciation
parts of the skeleton in the metoposaurs are to Prof. Alfred S. Romer, Director of the
remarkably weak. The centra and the spines Museumof Comparative Zo'ologyatHarvard
of the vertebra are solid enough, but other- College, for facilities and assistance that
wise there is evidence of a strong, secondary made possible the study and measurements
trend towards cartilage in the axial skeleton.
of the metoposaurs found near Lamy, New
For instance, although the limb bones and Mexico, in that institution. Likewise, similar
bones of the fingers and toes arewell formed, thanks and appreciation are due to Drs. C.
they are comparatively small, while there is Lewis Gazin and David Dunkle of the
little evidence as to the structure of the wrist United States National Museum for making
and ankle, theseportions of the skeletonhav-
possible the study and measurements of the
ing been evidently mainly cartilaginous.
metoposaurs, alsofrom theLamylocation, in
Considerable numbers of metoposaur Washington. Again, ourthanksand apprecia-
skulls, interclavicles, and clavicles (and to a tion go to Mr. Boone McClure and Mr. J. T.
lesserextentother partsof the skeleton) have Hughes, Director and Curator, respectively,
been found in the upper Triassic sediments ofthe Panhandle-Plains Museumat Canyon,
of Europe and North America. These fossils
Texas, for the opportunity to study and
give us an opportunity to learn something measure their unusually large collection of
about the metoposaurs and about the world metoposaurs from Potter County, Texas.
they lived in. For example, enough metopo- To Prof. Charles L. Camp andto Dr. S.P.
remains have been found in close
saur as- Welles of the University of California we
sociation at certain localities to give fair are indebted for permission to make meas-
samples of populations.
urements of some metoposaurs in the Mu-
No fewer than 10 genera and 19 species of seum of Paleontology at Berkeley, which
metoposaurs from upper Triassic sediments were found near St. Johns, Arizona. We are
have been described, of which eight
genera also indebted to Dr. E. C. Olson of the Uni-
and 14 species are North American. Manyof versity of Chicago and to Dr. Rainer Zangerl
the descriptions were based on single speci- of the Chicago Natural History Museum for
mens, as was frequentlynecessary, and many permission to study and measure the meto-
of them were made without adequate com- posaurs from Wyoming that are now in the
parisonstogeneraand species already known. collections of the Chicago Museum. Data on
This multiplication of names gives a variety specimens in the universities of Michigan,
to the listsof upper Triassic faunas of North Missouri, and Texas, and in Princeton Uni-
America and creates the appearance of dif- versity, were taken from publications, as
ferences between the several faunas that
were those for specimens in European mu-
may be more imaginary than real. Conse- seums.
quently the differences between the faunas
Abbreviations for the above-mentioned in-
have received undue stress, while the simi-
stitutions, and for the American Museum of
larities have sometimes been overlooked.
Natural History, as used in this paper, are as
The purpose of the present paper is to follows:
survey the upper Triassic metoposaurs of
North America, in an attempt to place their A.M.N.H., the American Museum of Natural
relationships on as objective a basis as pos- History
sgiebnlee.raFiarsntd tshpeerceieiss oafsmuertvoepyoosfautrhse okfnothwins CM..CN..ZH..,M.M,usCheiucmagoofNCaotmupraarlaHtiisvteorZyo5Mlougsy,euHmar-
vard College
continent. Then there is a study of certain M.P.U.M., Museum of Paleontology, University
populations, with an attempt to evaluate of Michigan
their relationships tooneanother. Finally, on P.U., Princeton University
the basis of these population studies, an at- U.C., University of California, Berkeley
tempt is made to determine the validity of U.M., University of Missouri
the various genera and species and to indi- U.S.N.M., United States National Museum
cate their interrelationships. U.T., University ofTexas
1956 COLBERT AND IMBRIE: TRIASSIC AMPHIBIANS 407
The drawings that illustrate this paper Texas; cast in Texas Technological College
made by Mr. Michael Insinna, and Museum.
were
of the graphs prepared by Mr. SAMPLE 5: The type of Buetineria perfecta,
some were
William G. Heaslip. M.P.U.M. No. 7475.
In tables through 9 the sample numbers SAMPLE 6: The type of Buettneria bakeri,
1
M.P.U.M. No. 13055.
itinsdtiiccaaltepgorpuoluaptsioonfs.spTechiemeenxspltarneaattieodn aosf stthae- SAMPLE: 7: Fourspecimens from the Popo Agie
formation, between Lander and Bull Lake Creek,
sample numbers is follows:
as Wyoming, at the Chicago Natural History Muse-
SAMPLE 1: Specimens from a quarry in the um and at the University of Missouri.
Chinle formation, near Lamy, New Mexico, in SAMPLE 8: Four specimens from the Chinle
the Musuem of Comparative Zoology and the formation, between St. Johns and Cameron,
United States National Museum. Arizona, at the Museum of Paleontology, Uni-
SAMPLE 2: Specimens from a quarry in the versity of California.
Dockum formation, Potter County, Texas; Pan-
handle-Plains Museum, Canyon, Texas. In tables 1 through 4, groups of measure-
SAMPLE 3: Specimens from a quarry in the ments of skulls, clavicles, and interclavicles
tDhoecUknuimverfsoirtmyatoifonT,exaHso.ward County, Texas, at are made on individual specimens, and there
SAMPLE 4: A single small skull from Howard is no assurance of association among skulls
County, Texas: original at the University of and postcranial elements.
LOCALITIES AND HORIZONS FROM WHICH TRIASSIC METO-
POSAURS HAVE BEEN COLLECTED
THE FIRST METOPOSAUR to be made known metoposaurs were made in North America
to the scientific world was Metopias diag- until after the turn of the century. Then in
nosticus, described in 1842byvon Meyer, the 1904 Lucas described the interclavicle of a
type specimen of which was found in the metoposaur from the Chinle formation of
upper Triassic or Keuper beds of Germany. northern Arizona, the specimen having been
In later years additional fossils have been found by Barnum Brown. At about the same
discovered in southern Germany and de- time E. B. Branson described some skulls of
scribed, but up to the present time not very metoposaurs from the Popo Agie formation,
many specimens of these amphibians have west of Lander, Wyoming; on these skulls he
been found in the European region. Such based his new genus Anaschisma.
fossils as have been found seem to be isolated Sinclair described a fragmentof a jaw from
specimens, most of them fragmentary. In the Newark group in 1917, and in 1920 Case
spite of the intensive scouting of European described an interclavicle with associated
Triassic exposures no associated populations clavicles from the Dockum formation of
have come to light (at least so far as can be western Texas. Shortly thereafter, in 1922,
determined from the literature); consequent- Case described very fine materials from the
ly it is necessary to turn to the North Amer- Dockum formation, and on them he founded
ican discoveries in order to obtain an ade- a new genus Buettneria. This was the first
quate knowledge of probable metoposaur re- adequate description of metoposaurs from
lationships. The name Metopias, not being the southwestern Triassic.
valid, was replaced by Metoposaurus by Since that time numerous discoveries of
Lydekkerin 1890. metoposaurs have been made in the Dockum
Metoposaurs are known in the upper Tri- and ChinleformationsofTexas, New Mexico,
assic Maleri beds of central India. These fos- and Arizona, some of which have been de-
sils, reviewed by von Huene in 1940, are very scribed. In addition, Branson and Mehl de-
fragmentary and consist of vertebral inter- scribed in 1929 some supplementary skulls
centra, portions of some occipital condyles of and other skeletal materials from the Popo
the skull, some pieces of the skull roof, and Agie formation of Wyoming.
some sections of interclavicles and clavicles, At this place it might be well to mention
one interclavicle being fairly completely three of the more important discoveries from
represented. Von Huene did not attempt to the Triassic of the southwest, which receive
give these fossils a formal name, a sound de- special attention in this paper. These dis-
cision inview of the incomplete nature ofthe coveries were made during the 1930's, two of
fossils. them in the Dockum formation in western
Thefirst metoposaurs from North America Texas, one in the Chinle formation in north-
were discovered and described about a hun- ern New Mexico. In all three instances the
dred years ago by Leidy and by Cope. Leidy discoveries led to the opening of very rich
described Dictyocephalus, which is a very quarries, from which abundant fossils, de-
small and fragmentary specimen that may posited in the greatest profusion, were col-
be of metoposaurid relationships, from the lected.
sediments of the Newark group in North The New Mexico locality is about 16miles
Carolina. Cope described Eupelor, an un- south of Lamy, at a place where outcrops of
doubted metoposaur, from the Newark beds the Chinle formation are not far from High-
of eastern Pennsylvania. Since then very way 285. This locality was first excavated by
little additional material has been found in Robert V. Witter and T. E. WVhite in 1938
the Newark sediments. for the Museum of Comparative Zodlogy at
After Leidy's description of Dictyocephalus Harvard. A large and impressive block con-
in 1856 and Cope's descriptions of Eupelor taining many skulls and other bones (illus-
in 1866 and 1868, no further discoveries of trated in pI. 28, fig. 1, of the present paper)
408
1956 COLBERT AND IMBRIE: TRIASSIC AMPHIBIANS AII
was taken from the quarry. Subsequently museum in Canyon. Data for the analysis of
David Dunkle of the United States National this population were gathered by the senior
Museum obtained a second block (p1. 28, fig. authorat Canyon in thespring of 1954.
2) from this locality. The specimens in these In addition tothese three large undoubted
two blocks represent one of the populations population samples, two suites of fossilscited
analyzed on subsequent pages of this paper, in the present paper are here considered as
on the basis of measurements and studies of representingpopulationsamples. Oneofthese
the original material made by the senior au- consists of several skulls, four of which were
thor. described and figured by Branson and by
During the years 1939-1942 some excava- Mehlfrom the Popo Agie formation of Wyo-
tions were made in Howard County, Texas, ming. These specimens, assigned to three
by government project workers, under the genera and four species, were collected
direction of paleontologists at the Univer- along the strike of the Popo Agie beds, as
sity of Texas. This work was done about 3 exposed in the area between Lander and
miles north of Otis Chalk, which is southeast Bull Lake. This exposure stretches over a
of Big Spring. As a result quite a few meto- linear distance of about 35 miles, which we
posaurskulls andotherboneswererecovered, consideraseasilywithinthelimitsthatmight
and this material was described bySawin in have been occupied by a single population.
1945. The analysis of the population repre- Theothersuiteoffossilsconsistsofanumber
sented by these fossils is based on the data of undescribed metoposaur skulls from Ari-
presented in Sawin's publication. zona, found betweenSt. Johnsand Cameron.
At about the same time an extraordinarily Here the distance is about 150 miles, but
large series of skulls, jaws, and postcranial again we do not consider such a distance as
skeletal parts was excavated, also by govern- excessive for therangeofa single population.
ment project workers, under the direction of These two suites are treated statistically in
C. Stuart Johnston of the Panhandle-Plains the same manner as are the large population
Museum at Canyon, Texas, from the Her- samples from Potter County and Howard
ring Ranch, Potter County, west ofAmarillo, County, Texas, and from near Lamy, New
Texas. These fossils are now housed at the Mexico.