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L161 — O-1096

Field Columbian Museum.

Publication no.

Geological Series. Vol. II, No. 7.

THE CARAPACE AND PLASTRON

OF BASILEMYS SINUOSUS,

A NEW FOSSIL TORTOISE FROM THE
LARAMIE BEDS OF MONTANA.

BY

Elmer S. Riggs,

Assistant Curator, Division of Paleontology.

Oliver Cummings Farrington, Ph. D.

Curator of (leology.

CO

Chicago, U. S. A.
F'ebruary 1, 1906.

THE CARAPACE AND PLASTRON OF BASILEMYS SINUOSUS,

A NEW FOSSIL TORTOISE FROM THE LARAMIE

BEDS OF MONTANA.

BY E. S. RIGGS.

An unusually well-preserved shell of a tortoise (P. 12008) col-
lected by the Museum Expedition of 1904 presents new and inter-
esting characters. The specimen is evidently closely related to a form
described* by Cope from the Judith River Beds of Montana as Comp-
semys variolosus. This, with a more complete specimen from the
Belly River series of Alberta, was later referred! by Lambe to the
genus Adocus, and filially by Hay to a proposed new genus, Basilemys%
The size of the specimen under consideration is almost identical
with that described by Lambe and the dermal markings are very
similar, but it differs notably in the form of the anterior end of the
plastron and the arrangement of gular shields. The median sulcus
of the plastron is remarkably sinuous, crossing and recrossing the
median line and often deviating widely from it. On account of this
character the specific name sinuosus is proposed for it. It is provi-
sionally referred to Basilemys pending the definition of that genus.

The specific characters are as follows: Anterior end of plastron
produced into a thick rounded knob; gular shields long and narrow
and meet at the median line; median sulcus unusually sinuous.

The specimen as a whole is in a fine state of preservation. It
was found in a bed of hard but uncemented sand containing bones
of Triceratops, Trachodon, and other characteristic Laramie fossils.
The shell was lying on the plastron in its normal position. It has
been somewhat flattened by compressure in the vertical direction.
The carapace is complete with the exception of a few fragments miss-
ing from the vertebral region and a section from the right side, in-
cluding the posterior half of the second and the anterior half of the
third pleural plates, together with the greater part of the fifth and
the anterior part of the sixth peripheral plates. Much of the con-
tact between the carapace and plastron is also lost. The remainder
of the plastron is preserved entire. However, the inferior surface
is modified by compressure so as to be deeply concave.

•Pro. Acad. Nat. Sci. Phila. 1876, p. 957.
fThe Ottawa Naturalist, Vol. XVI. p. 63.
tNomen Nudem, Bull. U. S. Geol. Surv. Nov. 179, p. 445.

249 •

250 Pibld Columbian Museum — Geology, Vol. III.

The shell is in general structure broad and low, flat and emargi-
nate at the interior end, but steep posteriorly. The carapace and
plastron are firmly united by a wide bridge which extends more than
half the entire length of the shell. The whole of the free surface is
marked by pittings which vary in depth, structure, and arrangement
in different parts of the shell. The horny shields are outlined by
narrow sulci which are plainly marked in the vertebro-costal region,
and in the plastron, though less conspicuous between the marginals.
The sutures joining the bony plates are everywhere closed, so that
in some places they cannot be traced by the aid of a lens. The centra
of the dorsal vertebrae have been detached and lost from the speci-
men entirely. The neural plates bear on their inferior surfaces an
interrupted median ridge indicating the attachment of the neural
spines. The proximal ends of the pleural plates bear stout tuber-
cular processes for vertebral attachment. Plate LXXVII.

The carapace is relatively thin throughout. The anterior end
is unusually flat, a character which has evidently been accentuated
by compressure in fossilization. The nuchal border is rounded and
indented by a wide concavity. Elsewhere the free margin is drawn
to a sharp angle. The posterior end is markedly convex in the pygal
region. The margin is slightly recurved over the femora, but uni-
formly convex and overhanging at the median line. Plate LXXVII.

The neural plates are irregularly hexagonal, but vary in size and
in outline. A certain amount of asymmetry is also noticeable in
them. The first is broadly coffin-shaped, with the large end directed
backward. It articulates laterally with the first pair of pleurals,
postero-laterally with v the second pair, and posteriorly by a concave
line with the second pleural. The second is sub-ovate in outline
and notably smaller than the first. Its pleural articulation is con-
fined to the second pair. The third neural is a trifle smaller than
the first and has an outline similar but reversed in position. Thus
it articulates antero -laterally with the second pair of pleurals and
laterally with the third pair. The entire fourth and part of the fifth
are missing from this specimen. They were evidently narrower than
the third but similar in form. The sixth is notably asymmetrical.
The anterior end and right side have the usual form, but the left
side is elongate so as to articulate with the whole mesial end of pleural
vi and postero-laterally by a short line with pleural vn. The
seventh neural is thus excluded from its normal articulation on the
left with pleural vn, and is correspondingly asymmetrical. It is
also much smaller. The eighth neural is much shortened antero-

Feb., 1906. Carapace and Plastron of Basilemys Sinuosus. 251

posteriorly and lies opposite the suture between pleurals vn and
viii. A ninth irregularly rounded plate intercolated between the
last pair of costals may be regarded as a post-neural.

The pleural plates extend only about two-thirds of the distance
from the neural suture to the margin of the carapace. Their articu-
lation with the neurals has been described in the preceding para-
graph. Distally they articulate with the marginals, breaking joints
in a mosaic pattern. Pleurals 1 to 111 inclusive are directed obliquely
forward. Pleural iv stands at a right angle to the median line.
From this point the remainder of the series becomes more and more
directed obliquely backward until the last pair conforms to the wide
A-shape of the first pygal. Pleural 1 is much the broadest of the
series, articulating anteriorly with the nuchal and peripherals one
and two. Distally it abuts the whole mesial border of peripheral
three and a small anterior portion of four. Pleural 11 is long and
narrow, meeting only two-thirds of peripheral four. Pleural in is
widest at the distal end, articulating antero-laterally with the fourth
and laterally with the fifth peripheral. Pleural iv is uniform in
width, and has a distal articulation similar to the perceding. Pleural
v is slightly expanded toward the extremity and presents an almost
equal surface to marginals six and seven. The suture between Pleu-
rals v and vi is unusually sinuous. The latter element is noticeably
narrower and has its distal articulation similar to the preceding.
Pleural vn is shorter, and much narrower at the mesial end. It
articulates distally with peripheral eight by a short suture, but chiefly
with nine. Pleural vni is shortest and smallest of the series. It
arises from the eighth and ninth neurals, and is more uniform in
width than the preceding. It meets peripheral nine in a short
antero-lateral suture as well as half the mesial surface of peripheral
ten.

The first pygal plate has the form of a wide A. Its lateral wings
are co-extensive with the last pair of pleurals, with which they articu-
late anteriorly. Laterally they abut a third of the mesial surface
of peripheral ten. The posterior surface is concave to receive the
rounded surface of the succeeding element. The second pygal is a
wide irregular figure, convex on the anterior surface and bounded
by three concave facets posteriorly. These facets articulate with
the eleventh pair of peripherals and the third pygal. Laterally this
bone also presents a small pair of surfaces to the postero -lateral faces
of the tenth pair of peripherals. The third pygal is not more
than half as large as the second and is sub-hexagonal in outline.

252 Field Columbian Museum — Geology, Vol. III.

The peripherals are eleven in number exclusive of the nuchals
and last pygal. The series as a whole is unusually deep, especially
in the lateral region where the plates above the angle of the cara-
pace .are more than half the length of the adjacent pleurals. Only
the last pair is covered entirely by the marginal shields. The nuchal
differs from the common hexagonal type in that the anterior border
forms a reentrant angle at the median line. The peripheral bones
gradually increase in depth from first to fourth. The fourth, fifth,
and sixth are similar in size and are largest of the series. With the
seventh there is a decided reduction in antero-posterior diameter.
The ninth and tenth diminish rapidly in length, the eleventh is much
the smallest of the series. The tenth presents an equal surface to
articulate with the eighth pleural and the first two pygals. Peri-
pherals three to seven inclusive form the bridge.

The vertebral shields are notably variable in outline. The first
has the form of an irregular truncated pyramid with its base at the
anterior marginal suture. In this specimen it is quite asymmetrical.
The second, third and fourth are much longer than wide. The sec-
ond and third are sub -rectangular in outline and nearly equal in size.
The fourth is somewhat smaller, narrower at the posterior end and
more irregular in outline. The fifth is the largest of the series. It
has the form of a wide triangle whose base is at the posterior mar-
ginal suture.

The costal shields are four in number. The first has the form
of a quadrant of a circle whose center is at the mid-lateral border
of vertebral II. It overlies the greater part of the first and half of
the second pleural, as well as parts of the first four peripheral bones.
The second and third costals are rectangular in outline. The second
is somewhat broadest in its antero-posterior diameter and extends
over the posterior half of the second, all of the third and the anterior
half of the fourth pleural plates, as well as parts of the fourth, fifth
and sixth peripherals. The third costal shield bears a similar rela-
tion to the fourth, fifth and sixth pleurals, and the sixth, seventh and
eighth peripherals. The fourth costal shield is an irregular quadri-
lateral and is the smallest of the series. Its relation to the neural and
peripheral plates is similar to that of the second and third. The
intercostal sulci have the usual position opposite the middle of the
vertebral shields.

The marginal shields are twenty-five in all. The sulcus separa-
ting them from the vertebrals and costals falls far without the pleuro-
peripheral suture and is little more plainly marked than those in the

Feb., 1906. Carapace and Plastron of Basilemys Sinuosus. 253

vertebral region. The nuchal shield is very small. It is narrow
in its superior aspect but rapidly widens as it rounds the anterior
margin. The first pair of marginals are long and narrow; the sec-
ond and third increase regularly in breadth. The fourth to seventh,
inclusive, overlie the bridge. Their distal articulation will be
described with the plastron. The eighth, ninth and tenth diminish
slightly in depth in the order named. The eleventh increases rapidly
in depth toward the mesial border. The pygal is deepest of the series
and is paired.

The plastron is suggestive of great rigidity and strength as in-
dicated by the wide bridges with thickened buttresses and firmly
interlocking sutures, the massive anterior process formed by the
epiplastra, and the thickened and rugose borders. The pittings are
more deeply marked than on the carapace, and the arrangement of
shields is unusually complicated. In its entire length the median
sulcus crosses the median suture fourteen times. At no point does
it coincide with the median suture. Compared with the plastron
of B. variolosus as figured by Lambe* there is a great similarity.
However, the epiplastrals in this form are more produced, the ento-
plastron noticeably larger, the gular shields meet at the median line,
and the median sulcus is much more sinuous. Plate LXXVIII.

The epiplastra unite at the median suture to form a thick and
rounded knob, which may be termed the gular eminence. The dis-
tal ends appear on the inferior surface as a pair of wing-like processes
which are directed postero-laterally. The entoplastron is sub-
hexagonal in outline and one-third broader than long. Its anterior
margin joins the epiplastra in two approximately straight lines which
meet in an angle of sixty degrees. The lateral borders are short,
the posterior angle very obtuse. The hypoplastra are somewhat
larger than the hyoplastra. In this specimen there is a break in their
connecting suture at the median line. The former articulate laterally
with peripherals three, four and five, and postero-laterally they pre-
sent a short articulating surface to peripherals six. The hyoplastra
articulate laterally with peripherals six and seven. The xiphiplastra
extend forward almost to the femoral notches. On the superior
surface the gular eminence terminates in a single short median ridge,
and is flanked by a pair of marginal ridges which extend half way
to the humeral notches. The entoplastron bears on a pair of low
converging ridges the facets for articulation with the precoracoids.
The xiphiplastra bear a similar but larger pair of facets for ligamentary

•Geol. Surv., Canada. Vol. III. part II, page 39.

254 • Field Columbian Museum — Geology, Vol. III.

union with the pubic process. From the femoral notch to the median
line the margin is elevated to form a sharp ridge, deeply pitted on
the free surface.

The dermal shields of the plastron are very complex in arrange-
ment. The intergulars are similar on the inferior surface to those
of B. variolosus , but superiorly they are extended to cover the pro-
truding gular process. Between them the median sulcus twice crosses
the median suture. The gulars are long and narrow, and meet at the
median line. The humerals are relatively smaller than in B. vario-
losus. They present for union at the median line a surface barely
a half -inch in length. The pectoral shields as a pair are similar in
outline to those of the species cited, though they extend somewhat
farther backward mesially. Between them the median sulcus de-
parts from the median line in a series of meanderings in which it
crosses the median suture five times. In the abdominals it follows
almost parallel with the suture making a single digression near the
posterior border. The abdominals send backward a pair of peculiar
narrow processes between the inguinals and femorals to the border
of the femoral notch. The femorals are sub-quadrangular in outline,
extending laterally to the border of the femoral notch. In the anals
the median sulcus presents the most fantastic tracery, crossing the
median suture in a series of wide loops, departing from it, in one in-
stance, nearly two inches and finally returning to it at the posterior
margin. On account of portions missing at the lateral angle of
the carapace it cannot positively be determined whether the
marginals are divided into an inframarginal series. From the fact
that no evidence of a dividing sulcus can be made out on either side,
as well as the opposite position of the vertical sulci on each side of
the gap, it is assumed that the series is not divided into marginals and
inframarginals, respectively.

From the narrow lateral extremities of the pectorals radiate
three wedge-shaped shields. The anterior and smallest is the axillary
shield which overlies the axial buttress and extends upward along
the free margin of the carapace to join the second nuchal. The
other shields radiating from this point are the fourth and fifth
marginals. The sixth marginal is very broad at its ventral
border, joining nearly the whole lateral surface of the abdominal
shield. Two pairs of shields converge at the postero-lateral
angle of the abdominals. The most anterior is the seventh mar-
ginal, which narrows mesially almost to a point. Back of this lies

♦Proc. Acad. Nat. Sci. Phila. 1876, p. 257.

Feb,, 1906. Carapace and Plastron of Basilemys Sinuosus. 255

the inguinal shield which overlies the posterior or inguinal
buttress.

The sculpturing of the shell in "members of this genus has been
variously described. In his original description* of the type species
B. variolosus Cope described the dermal markings as follows: "The
sculpture consists of round fossae, which are deeply impressed, and
are arranged quincuncially, so that their borders never form straight
lines. The latter are also more or less angulate on the edges, so that
the surface has a more than usually rugose character." Of the species
B. (Compsemys) imbricarius he writes as follows:* "The character
of this sculpture distinguishes the species and, in the present instance,
in a special manner. It consists in the C. imbricarius of excavations
bounded on the sides by a short ridge each, which alternate with each
other. Thus each bounding ridge terminates abruptly at the fundus
of one of the fossae, while the other end of the fossa rises and con-
tracts to another ridge." In the more complete specimen described
by Lambe as belonging to Adocus variolosus the markings are de-
scribed as follows :f "The sculpture consists, when most rugose,
of well-excavated pits of rounded hexagonal outline arranged quin-
cuncially; the dividing ridges are angular and narrower than the pits
are wide, their angularity and height being more pronounced at the
junction of every three pits with each other."

It will be noted that the quotations from Cope's description of
B. imbricarius and Lambe's characterization of the sculpture in B.
variolosus describe very similar markings. Moreover, the various
types of markings indicated by these three descriptions are all to be
found in various parts of the specimen under consideration. In
fact, the sculpture in various regions of carapace and plastron varies
so widely that no area could be taken as typical with the assurance
that fragments from other parts of the shell could be recognized from it.

The pittings are most conspicuous and most regular on the plas-
tron and about the margin of the carapace. In the vertebral region
they are least conspicuous and without any regular arrangement.
On the superior surface of the gular process and the anterior half of
the femoral shields there is to be found the type of sculpture described
in B. imbricarius in which a sharp ridge terminates abruptly at the
fundus of the succeeding pit. It is noticeable, however, that in those
portions of the femoral shields which overlie the xiphiplastra the
arrangement suddenly changes to a series of irregular diagonal rows.

•Ibid.

tGeol. Surv. Canada. Vol. Ill, part p. 40.

256 Field Columbian Museum — Geology, Vol. III.

Similarly on the lateral surface of the gular process the markings
fall into straight rows in which the pits of adjacent rows alternate
so as to form diagonal lines. Thus the pits are in regular rosettes —
patterns of six around one. In other parts of the plastron may be
occasionally recognized groups of fives, of fours and of threes, but
for the most part no regular arrangement is to be recognized.

It is, therefore, probable that if the whole carapace and plas-
tron of B. variolosus and B. imbricatus were known the same regional
variation would be found to obtain. At all events, little reliance
can be placed upon the depth or arrangement of pittings as bases
for specific determination in this genus.

MEASUREMENTS.

M

Length of carapace at median line 705

Breadth of carapace 565

Length of plastron 650

Breadth of hypoplastra 480

Length of bridge 360

Greatest breadth of nuchal plate 145

Least breadth of nuchal shield 007

Greatest breadth of first pleural plate 112

Greatest length of third pleural plate 205

Greatest breadth of anterior pygal .1 70

Breadth of entoplastron 148

Thickness of epiplastra at gular eminence 060

•■'USEUM.

:v. i;

Pen. I

PI. I

COi. I.

m

BASILEMYS SINUOSUS, , :

Nu nuchal plate. Py. L to 3, pygal plates.

N. 1 to 8, neural plates. V. I to V, vertebral shields.

P. N., post-neural plate.
Per. 1 to 11, peripheral plates

Cos. I to IV, costal shields.
M. I to XII, marginal shields.

,ii\- teueouMie 8YM3jieA8

.?btairia Ifiicfemv ,V oJ I .V

.ablaiifa IsJaoD ,VI oJ I .acG

.abhiiia Isahnsai ,IIZ a) 1 .M

.alfilq Isrfoun ,.uH

.aaifila Isiuaa ,8 oJ I M

.9tslq kiuen-Jaoq ,.VI .4

.asluiq Imarfqhaq ,11 oJ 1 .134

FIELD COLUMBIAN MUSEUM.

GEOLOGY, VOL. II. PLATE LXXVI.

Carapace of Basilemys sinuosus. dorsal view, X %i.

ILUMBIAK M.

BASILEMY8 8INU08U8, X %,.

Ep., e pi plastron.
Ent., entoplastron.
Hyo., Hyoplastron.
Hyp., Hypoplastron.
Xiph., xiphoplastron.
I. G., intergular shield.
G., gular shield.

Ing., inguinal shield.

H., humeral shield.

Pec, pectoral shield.

Ab., abdominal shield.

Fem., femoral shield.

An., anal shield.

Ax., axial shield.

M. IV to VII, marginal thield.

.it* x ,eU80UKllti 8YM3Jl8AtJ

.bbids lsismod ,.H

.btaidg IfiioJDaq ,.094

.bbids Isnimobcls ,.dA

.bbids isioorel ..aisT

.bbide Isns t.nA

.bbirfa Isixs ,.xA

.bbidt bfli§nsm ,11 V ol VI M

.bbide lsaiujjfli ,.§ni

.iKnJ8fiiqiq3 ,.q3

.notJesIqolria ,.Jn3

.noiJaslqoyH ..o^H

.noiJasIqoqYH ,.q^H

.noilaslqoiiqix ,.dqiX

.bbida iBlujmlni ,.0 .1

.bbide islus ..O

FIELD COLUMBIAN MUSEUM.

GEOLOGY, VOL. II. PLATE LXXVII.

Shell of Basilemys sinuosus, ventral view, x -ii.

FIELD COLUMBIAN MUSEUM.

GEOLOGY, VOL. II. PLATE LXXVIII.

Plastron of Basilemys sinuosus. visceral surface, X -\^.

a