A new Silurian trilobite Dalmanites oklahomae

UIV' RSITY OF
ILLIi.^iS LIBRARY

mTtV.1BANA-CHAMPAIGN
NATURAL HIST. SURVEY

FIELDIANA • GEOLOGY

Published by
CHICAGO NATURAL HISTORY MUSEUM

Volume 10 June 6, 1949 No. 7

A NEW SILURIAN TRILOBITE
DALMANITES OKLAHOMAE

Eugene S. Richardson, Jr.

Cukator of Fossil Invertebrates

In the collections of Chicago Natural History Museum there is
a single, fairly well-preserved, partly exfoliated cephalon that differs
from any previously described form. It is here made the type of
a new species of phacopid trilobite.

Dalmanites oklahomae, sp. nov.

(P.)Dalmanites, sp. nov. Reeds, Amer. Jour. Sci., 32, (4), p. 263, 1911 (in faunal
list).

Holotype. — C.N.H.M. P10435, an incomplete cephalon.

Horizon and locality. — Probably Silurian, probably Henryhouse
shale. Collected at Bromide, Coal County, Oklahoma.

Description. — Cephalon about twice as wide as long, nearly flat
in transverse profile across glabella, steeply convex at cheeks, below
eyes. Genal angles or spines unknown. Border very wide in front
of eyes, narrow in front of glabella, projecting anteriorly in process
of unknown length, apparently narrowing behind eyes. Eyes long,
semicircular, extending from first glabellar furrows to nuchal furrow,
high, rising well above glabellar level and above palpebral lobes;
facets about 320 in number, arranged in 39 vertical rows. Distinct
groove beneath eye (see fig. 15). First glabellar furrows pronounced,
rounded, directed anteriorly outward, slightly deepened proximally;
second glabellar furrows directed posteriorly outward, deepened
into pits proximally, almost obsolete distally; third glabellar furrows
parallel to second, proximally deepened, distally shallow. Nuchal
furrow parallel to second and third glabellar furrows, deepened in
line with proximal deepening of those furrows, shallow where it
crosses axis. Frontal lobe large, smooth, and nearly flat on top,
elliptical, about twice as wide as long, gently sloping anteriorly,
strongly arched at sides, bearing fine pustules on front and sides.

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FIELDIANA: GEOLOGY, VOLUME 10

Glabellar lobes smooth on top, flat, bearing fine pustules on ends.
Nuchal ring high, circularly arched, smooth. Dorsal furrow pro-
nounced, with scattered pustules. Cheeks and border minutely
pustulose. Facial suture apparently close below subocular furrow
posteriorly, not seen elsewhere. Doublure as wide as widest part
of border, continuing thus across front of cephalon. The hypostoma
of the unique specimen was evidently lost before fossilization.

Remarks. — The specimen was presented to the Museum in 1908
by Mr. W. W. Newberry, of Chicago, with a notation stating that

Fig. 14. Dalmanites oklahomae, sp. nov. Holotype, an incomplete cephalon.
C.N.H.M. No. P10435. Upper left, dorsal view of test; Xl.5. Lower left, front
view, showing high eyes, flat-topped glabella, and high cheeks; Xl.2. Upper right,
lateral view; Xl.7. Lower right, back view, showing strongly arched nuchal ring
and flat-topped glabella; Xl.2. The line at the top represents one inch on the
scale of magnification of the two lower photographs.

it came from the Silurian Clinton group at Bromide, Oklahoma. A
small amount of matrix adheres to the cephalon, permitting com-
parison with published information on the rocks of the Bromide
region. Considering the apparent stage of evolution of the trilobite,
it seems to be similar to Silurian and Devonian species of the Dal-
manitina socialis lineage, with glabellar lobation and eyes very similar
to those of Dalmanitina arkansana (Van Ingen), of the St. Clair
limestone (middle Niagaran). Lithologically, the specimen might
have been taken from either the Silurian (lower Cayugan) Henry-
house shale or the Devonian (Helderbergian) Haragan shale. The
matrix of the specimen is a light tan, dense, earthy limestone. Ac-
cording to the original description (Reeds, 1911, pp. 261-263), the

RICHARDSON: SILURIAN TRILOBITE 45

Henryhouse consists, in part, of "bluish to yellowish, thin- to
moderately thick-bedded earthy limestone and intercalated shale"
in the lower part of its exposure on Chimneyhill Creek, in the
Arbuckle Mountains of Oklahoma. This formation, however, is
missing by an erosional unconformity below the Haragan shale at
Bromide (Sulphur Springs), the locality from which the fossil is
said to have been taken. Here, the Haragan contains "thin-bedded
earthy limestones which weather to yellowish shales on long ex-
posure." Though the piece of matrix at hand is weathered and of

eye

Fig. 15. Dalmanites oklahomae, sp. nroovp t'V".'-

nov. Sketch of vertical section through ** /^uH>'-: :

eye and cheek. cheeh^0'

border

a somewhat earthy texture, it is not shaly. Such a small sample,
of course, is not necessarily typical of the major features of the
formation from which it was collected, so that whether this trilobite
is Silurian rather than Devonian must remain uncertain.

The lack of a hypostoma prevents an accurate generic assignment,
wherefore the species is here included in Dalmanites, sens, lat., to
which it is, at all events, very closely related. Unpublished work
of Dr. Frank M. Swartz, concerning the subocular grooves of the
dalmanitids, may have an important bearing on its relationships.

In spite of its close resemblance to Dalmanitina arkansana (Van
Ingen), the species is excluded from that genus by the presence of
a frontal border. The width and the broad anterior curvature of
the cephalon, together with the flat-topped glabella and high cheeks,
differentiate this species from other known dalmanitids. Reeds
(1911) listed Dalmanites arkansus (Dalmanitina arkansana?) and
D. werthneri from the lower Silurian (Albion) Chimneyhill formation;
Dalmanites, sp. nov., from the Henryhouse; and Synphoroides
pleuroptyx (as Dalmanites) from the overlying Devonian Haragan.
D. arkansana is much more elongate than D. oklahomae, sp. nov.;
D. werthneri has wider cheeks and more nearly transverse glabellar
furrows; ''Dalmanites, sp. nov." may be the same; and S. pleuroptyx
is more nodose, has wider cheeks, and has somewhat smaller eyes.

REFERENCE

Reeds, C. A.

1911. The Hunton formation of Oklahoma. Amer. Jour. Sci., 32, (4), pp.
256-268.