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GEOLOGY

UNIVERSITY OF

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IHE LIBRARY OF THE

SEP 20 1944

UNIVERSITY OF ILLINOIS
GEOLOGICAL SERIES

OF
FIELD MUSEUM OF NATURAL HISTORY

Volume 8 CuHIcAGo, AuGuST 14, 1944 No. 13

THE AELURODON SAEVUS GROUP
By Pau. O. McGREw

ASSISTANT CURATOR, PALEONTOLOGY

The holotypes of the species of Aelwrodon consist almost exclu-
sively of jaws, maxillaries, or fragments thereof. Some (A. feroz,
A. meandrinus, A. brachygnathus, A. wheelertanus) are based upon
inadequate specimens and their specific status cannot be determined.
The characters that have been considered as diagnostic for the vari-
ous species are those of general size, relative size and massiveness of
teeth, and general proportion. While differences in these characters
do exist between the specimens described as representative of dis-
tinct species the assumption that the differences are of specific
significance has been based upon purely subjective evidence.

The known species of Aelurodon fall clearly into two quite dis-
tinct groups. One of these is represented by the types of saevus,
haydent, inflatus, and mortifer and the other by the type of taxoides
and a number of specimens referred to wheelerianus. The taxoides
group is readily distinguished from the saevus group by the following
characters: (1) Horizontal ramus not tapering anteriorly, symphysis
deep, (2) premolars little reduced, (8) heel of M; short, (4) entoconid
of Mz much reduced, (5) paraconid, metaconid and entoconid of Mz
reduced. Actually the differences here are so great that there seems
to be sufficient justification for the separation of the taxoides group
as a distinct genus.

_In the collections of Field Museum are several jaws that are
referable to the saevws group of Aelurodon. These specimens are
from two fossil quarries, one at Big Spring Canyon, South Dakota,
and the other near Ainsworth, Nebraska. As nearly as can be deter-
mined from associated mammals the two deposits are of equivalent
age. The morphologic range among the specimens seems superficially

No. 563 79

vA Ty,
HIST.

_ 80 FIELD MUSEUM OF NATURAL HISTORY—GEOLOGY, VOL. 8

to be extensive. In size and general proportions they range from that
of the holotype of A. saevus, one of the smallest and earliest of species,
to that of the type of A. mortifer, one of the larger species. Two
specimens from Big Spring Canyon—specimens that in all proba-
bility represent the same local population—differ considerably more
in some respects than do types of supposedly distinct species. These
facts naturally create some doubt Onecare the validity of several
of the described species.

In dealing with an extinct phylum such as Aelurodon, in which
few specimens are known, and those from scattered localities, there
is no direct way of determining the range of morphologic variation
within a species. If, however, the range of variability is known in
a related group of the same size, the variations that might be
expected can be determined. At least this kind of comparison offers
objective evidence that is considerably more than guess work. Thus,
the use of the wolf,! Canis lupus, as a standard in assessing the taxo-
nomic significance of characters in species of Aelurodon is justified
because the wolf is closer in genetic relationships and size than any
other species whose morphologic range can be reasonably well
estimated.

Fortunately nearly every character that has been used to dis-
tinguish the species of Aelurodon is measurable, and quantitative
comparison is therefore possible. Measurements of a series of
characters of various species of the saevus group whose types are
lower jaws are here compared with homologous measurements of
specimens of the single species Canis lupus. The variants compared
include nearly all of those that have been used for diagnosis of the
various species.

The specimens used are as follows:

Type A. saevus; Niobrara River

Type A. mortifer; Snake Creek, Nebraska

Type A. inflatus; Big Spring Canyon, South Dakota
F.M. No. 26807; Ainsworth, Nebraska

F.M. No, 26806; Ainsworth, Nebraska

F.M. No. 15800; Big Spring Canyon, South Dakota
F.M. No. 15801; Big Spring Canyon, South Dakota

If it is assumed that all of these fossil specimens represent one
species, values for the coefficient of variation (V) and standard

1 Although the wolves have at times been placed in several species, the most
authoritative recent work (Pocock, 1985; Goldman, 1937) indicates that, with the
exception of Canis rufus of the lower Mississippi basin and the probable exception
of Canis lycaon of eastern Canada, all belong to a single species, C. lupus.

THE AELURODON SAEVUS GROUP 81

deviation (S.D.) should be close to those of the living form. If the
values are significantly greater and no reason therefor may be found
it may be assumed that more than one species is represented in the
sample. The statistics for the specimens studied are as follows:

Aelurodon.

Variant Observed range $s. D. v*
Peng Mee icc Sis 27.9-83.3=5.4 1.794.438 6.0+1.6
Breadth M; eK er ee. Ae 10.9-12.6=1.7 0.68+.17 5.384+1.4
ROU Fie hose cc ge econ Wh 15.6-19.1=3.5 1.50+ .48 8.842.3
Breadth Po) 9.3-10.8=1.5 0.564.16 5.641.5
Length Pz A AES ia Fp 11.1-12.9=1.8 0.60+.17 5.0+1.3
Dent OF jOW: on fh... 26.9-36.38=9.4 3.3384 .96 10.6+42.8

Canis lupus

Variant Observed range Ss. D. vt
MEN Bs 25S oie ae ok 25.2-30.4=5.2 1.57+.30 5.65+41.0
Bresata My... acca. 10.0-12.5=2.5 0.74+4.14 6.6 +1.2
SM Eick sss puck 14.5-17.0=2.5 0.694.138 4.4 +0.8
MON eo) 4 cs 6.8— 8.7=1.9 0.49+ .09 6.2 +1.1
Ne EERE ae ok 11.7-15.1=3.4 0.87+.16 6.2 +1.2
Depth or jaw... ek. 22 .8-31.2=8.4 2.274.448 8.7 41.6

*V (coefficient of variation) is calculated from the standard deviation.

The agreement here is surprisingly close in all variants except
the length of Pz. The differences in the coefficient of variation and
the standard deviation for the depth of the jaw are not great, consid-
ering the variable nature of the character, which is strongly affected
by age, sex, and physical development of the individuals.

_ Ordinarily the differences in the values of the coefficient of vari-
ation and the standard deviation for the length of Pz would indi-
cate that the unknown sample includes more than one taxonomic
unit of the same magnitude as that of the sample with which com-
parison is made. In this particular case, however, there is good
reason to believe that the normal range in this character in a single
species of Aelurodon was much greater than in Canis lupus, and
that more than one species is not necessarily indicated. The three
specimens from Big Spring Canyon in all probability represent
a single local population. The observed range in these specimens is
2.9 mm. (15.8-18.7). This is within 0.6 mm. of the observed range
for the entire sample and is 0.4 mm. greater than the observed
range for the fourteen specimens of Canis lupus. Such high variability
in this character should be expected in Aelurodon, because, being
closely related, or ancestral, to such genera as Osteoborus and Boro-
phagus, with highly modified Pz’s, this character was in a state of

U. OF ILL. LIB.

\ 92 FreLp MusEUM oF NATURAL History—GEoLocy, VOL. 8

rapid change. Contrary is the condition in Canis, which is a long-
established, conservative form with unmodified Pz.

This analysis suggests the possibility that all species of Aelurodon
(except A. haydenz) whose types are lower jaws, should be referred to
A. saevus. It does not prove that all belong with one species but
does prove that on the characters so far used more than one species
cannot be demonstrated.

The specimens here included in Aelurodon saevus range in age
from Barstovian to late Clarendonian. The earlier specimens
appear on the whole to be smaller than the later. It will probably
be shown that there was a progressive increase in size. A popula-
tion sample from the Barstovian will probably prove significantly
different from a similar sample from the Clarendonian, but the gap
theoretically will be filled by intergrading series from the inter-
mediate horizons. The entire morphologic range is of about the
same magnitude as is found in a comparable living species. Thus,
if the extinct group is to be broken into smaller taxonomic units,
the smaller ones will logically be regarded as subspecies. The entire
series, then, may represent a chronocline,! as defined by Simpson
(1948).

Aelurodon haydeni, judging from the original description and the
preservation of the type, was found in the same deposits as A. saevus.
Aelurodon hayden is surely a valid, distinct species. Comparing the
type of the larger form with the group of specimens here regarded as
A. saevus we find that the breadth of Mz and length of Pz are sig-
nificantly greater, with P values? of less than 0.01; the length of Mr
and the breadth of Pz are probably significantly greater, with
P values of 0.02 or less; the length of Pz and the depth of the jaw
are not necessarily significantly greater.

The principal differences between A. haydent and A. saevus are
the large size and slightly different proportions of the former. These
differences correspond rather closely to those between the coyotes
and wolves of today.

An interesting possibility is suggested by the fact that Aelurodon
haydeni, as a contemporary of the smallest member of the saevus

1 Prior to Simpson’s first published use of the term “‘chronocline’’ (1943, p. 174)
Alfred E. Emerson used the term “‘chronological cline’ (Amer. Nat., 77, p. 102,
1943) with the same meaning. Because of advantages in using the shorter,

perfectly clear single word, and with Dr. Emerson’s approval, I prefer to use and
help establish the paleontologically significant ‘‘chronocline.”

2P represents the probability that “the single deviation could have been
equaled or exceeded by one drawn at random from the population represented by
the sample.’”’ (Simpson and Roe, 1939, p. 189.)

Canis lupus

Length Mtr

Breadth Mr

Length Pr

Breadth Pz

Length Pz

Depth of jaw under Mr

Aelurodon saevus (incl)

Length Mr

Breadth Mr

Length Pr

Breadth Pr

Length Pz

Depth of jaw under Mr

09-04 ° O02 ‘0 C2 DAs

Fic. 26. Logarithmic plot showing relative range of morphologic variation
in Canis lupus and Aelurodon saevus (incl.). For Aelurodon those specimens
listed on page 80 were used, and for Canis lupus fourteen specimens in the col-
lections of Field Museum. Horizontal lines show the range of variation and
vertical lines indicate the mean of each character.

83

84 FIELD MUSEUM OF NATURAL HISTORY—GEOLOGY, VOL. 8

group, is distinguished primarily by its larger size. If A. haydeni
had been found in deposits of later age it might be construed as a
terminal member of the A. saevus chronocline. If a contemporary
morphologic intergrading series were found connecting A. saevus
and A. haydent we would have a chorocline more or less duplicating
a chronocline. Such conditions might well be found to exist in some
groups when sufficient data are available.

I am indebted to Dr. Anne Roe Simpson for checking on my
statistical procedure. Any errors that may exist, however, are my
own.

REFERENCES

GOLDMAN, E. A.
1937. The Wolves of North America. Jour. Mamm., 18, pp. 37-45.

Pocock, R. I.

St bias Races of Canis lupus. Proc. Zool. Soc. Lond., 1935, pp. 647-686,
pis. I,

SIMPSON, G. G.
1943. Criteria for Genera, Species and Subspecies in Zoology and Paleozoology.
Ann. N. Y. Acad. Sci., 44, pp. 145-178.

—— and RoE, ANNE

1939. Quantitative Zoology. xviii+ 414 pp., 52 figs. McGraw-Hill, New York
and London.

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