Description of a Silurian phyllopod mandible with related notes

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GEOLOGICAL SERIES

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FIELD MUSEUM OF NATURAL HISTORY

Volume VI Chicago, May 15, 1935 No. 11

DESCRIPTION OF A SILURIAN PHYLLOPOD
MANDIBLE WITH RELATED NOTES

By Sharat Kumar Roy
Assistant Curator of Gboixwy

Messrs. J. Mann and A. Lees, of Oak Lawn, Illinois, have recently-
donated to the Museum two specimens (part and counterpart) of
a mandible referred here to a new species of Ceratiocaris. The
mandible was found in the Lecthaylus shale. Upper Lockport group
(Niagaran), near Blue Island, Illinois, within one hundred feet of
Ceratiocaris markhami Roy (1935, fig. 29a-e). The specimen is
completely carbonized but much of the carbonized material has
been broken away.

The writer wishes to acknowledge his indebtedness to both
the donors for giving him the opportunity of describing this rarely
found fossil, and takes pleasure in naming it after one of the donors,
Mr. Lees.

The drawings are by the Staff Illustrator of Field Museum,.
Mr. Carl Gronemann.

Ceratiocaris leesi Roy sp. nov. Fig. 33a and b.

Description. — Transversely elongated, widest anteriorly; posterior
end roundly pointed. General outline somewhat resembles that of
a mitre with the "tooth" row forming the base. Upper and lower
margins rounded, giving a rim-like appearance; both broadening
anteriorly; the upper one arching and ending in a narrow pointed
projection which overhangs the "tooth" row, the lower one curving
slightly inward and meeting the anterior margin to form a triangle.
Anterior or oral margin thickest, supporting seven toothed ridges,
the uppermost of which is the longest, then decreasing uniformly in
size downwards. The elongated base, representative of the pro-
topodite, is not well preserved. Its central portion is missing and
its posterior end, the position for the external articular process, may
or may not have been outlined (see fig. 33a and b) correctly.

Dimensions. — Length and width at the center 22.5 mm. and
10 mm. respectively.

No. 343 155

f«r«tia:a] Hiiit«rT Survey

156 Field Museum of Natural History — Geology, Vol. VI

Remarks. — No record of gastric teeth or mandibles of phyllopod
crustaceans in geologic literature is to be found prior to 1843. In
that year J. E. Portlock first figured the gastric teeth, dextral and
sinistral, of Dithyrocaris from Tyrone or Londonderry, Ireland (pi.
12, fig. 6, of Portlock's report and fig. 33c^ of this paper). On page
315 (Portlock, 1843) he states, "Fig. 6 represents bodies which are
frequently found on the specimens of this crustacean, and in this
instance together, as represented in the figured specimen, they each
exhibit a single row of tubercles, and were in all probability con-
nected with the masticatory apparatus, which it is probable, there-
fore, was highly developed in this large species."

Following the report of Portlock, several other teeth of Dithyro-
caris, presumably all gastric, have been found, in comparative
abundance, at Campsie and East Kilbride, Lanarkshire, Scotland,
and at Orchard Quarry, near Thornliebank, Renfrewshire, Scotland.
These teeth have been described and illustrated by Woodward
(1865, pp. 401-404, pi. 11) and more adequately by Jones and
Woodward (1898, pp. 194-198, pi. 26, figs. 1-44).

The "teeth" of Ceratiocaris (from Lesmahagow, Lanarkshire)
were first reported by J. W. Salter in 1860. He observes (1860,
p. 155): "A further exploration in the same rich deposits at Les-
mahagow, where Ceratiocaris occurs . . . has disinterred the rostrum,
hard jaws, and also the antennae or some of the thoracic appendages
of the animal. The last are obscure, but were detected by Professor
Huxley, who also found a clearly articulated hinge uniting the two
flaps of the carapace. This is a very important character, and will
remove Ceratiocaris from any very near alliance with Nebalia, from
which also the solid jaws, like those of Apus . . . tend further to
separate it."

Salter's brief report was supplemented by more adequate explana-
tion and illustrations by Woodward (1865, pp. 401-404). Referring
to plate XI of his paper Woodward states (p. 401) : "Fig. 1 represents
a perfect carapace of Ceratiocaris, at the anterior end of which
(at a) the two opposing mandibles may be seen in their proper
place,^ compressed on the surface of the left valve. A detached

* Copied from a figure which was drawn more accurately from the original
specimen by Jones and Woodward (1898, pi. 26, fig. 44). A facsimile of Portlock's
figure is also given by Woodward (1865, pi. 11, fig. 8).

^ The mandibles were not in their proper place as was explained later by Jones
and Woodward (1888, p. 147). In normal position they would be opposed to each
other horizontally. That they appear as being opposed one to the other vertically
is due to the compression of the carapace, and to their being squeezed against
the inside of the valves.

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\ Description of a Phyllopod Mandible

157

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Fig. 33. a, h, Ceratioearis kegi Roy sp. nov. Mandible (part and counterpart). P.M. No. P23676.
Natural size, c-e, Gastric teeth of Dithyroearis (Jones and Woodward, 1898, pi. 26, fijfs. 23, 43, and 44 ) : c,
Sinistral lying on dextral; d, Sinistral; e, Dextral. f-l, "Teeth" of Ceratioearis? f. Chevron-shaped
regular cusps; g, h. Cusps longer at one end; t, Cusps with more flexuous connecting ridge; i. Cusps
regularly alternate; k, Cusps irregularly alternate; I, Cusps in single row.

158 Field Museum of Natural History — Geology, Vol. VI

mandible (fig. 2) from the same locality as fig. 1 (Upper Ludlow Rock,
Lesmahagow, Lanarkshire) is represented in the plate of the natural
size." On page 402, he further states: "That these detached fossil
remains are the teeth' of crustaceans there cannot now be the least
doubt; for, in addition to the evidence afforded by the remains
of the allied genus Ceratiocaris (pi. XI, fig. 1), and by the fragment
of Dithyrocaris (pi. XI, fig. 6), one needs only to compare the gastric
teeth of the common lobster (Homarus vulgaris), figured in our
plate (fig. 12), with the most perfect teeth of Dithyrocaris (figs. 3,
36), to see the striking similarity between them, both in general
form and in minuter details."

The most important paper on the present subject, "The Gastric
Teeth of Ceratiocaris" was published by Jones and Woodward in
1888. The paper is an excellent survey of all the important specimens
known from Bohemia, Scandinavia and Great Britain and contains
two important conclusions reached by the authors as the result of
their comparative studies. These may be stated as follows:

I. The "teeth" are divisible into two distinct groups:

(a) Gastric teeth. Thick and solid.

(b) Mandible. Trenchant character of cutting edges; broad, com-

pressed, transversely elongated bases of attachment.

II. The "teeth" have at least six different forms.^

(a) Cusps, chevron shaped, regular in size (fig. 33/).'

(6) Cusps longer at one end of the tooth than at the other (fig. 33sf
and h).^

(c) Cusps with a more flexuous connecting ridge (fig. 33i).'

(d) Cusps in two parallel rows, but somewhat alternate (fig. BSj).^

(e) Cusps irregularly alternate in two rows (fig. 33 A:).'
(/) Cusps in a single row (fig. 330-'

As to the first conclusion, my own inference has been the same
but I have hesitated to express it because of lack of actual specimens
to make a comparative study. With regard to the second, the
authors, it appears, have based their groupings on criteria which
have frankly many possibilities of error. It is difficult to believe

1 It is quite apparent that up to this time at least, no special effort was made
to differentiate between mandibles and gastric teeth. Frequently, both organs
were included in the general term "teeth."

* The authors have included mandibles in differentiating these forms. See
II (6).

3 These figures refer to the figures reproduced in the present paper. Fig. 33/
(Jones and Woodward, 1888, pi. VI, fig. 8); g and h (Jones and Woodward, 1888,
text figs. 5, 6); i (Jones and Woodward, 1888, pi. VI, fig. 10c) ;j (Barrande, 1872,
pi. 31, fig. 21); k (Jones and Woodward, 1888, text fig. 4a); I (Jones and Wood-
ward, 1888, pi. VI, 96).

Description of a Phyllopod Mandible 159

that a single genus would possess six widely different forms of teeth.
No such parallel exists among living forms. The gastric teeth or
the mandible of the lobster, Homarus, or of the crayfish, Astacus,
show but little variation, irrespective of the species to which they
belong. The same holds true of fossil forms. The gastric teeth of
Dithyrocaris (Jones and Woodward, 1898, pi. 26, figs. 1-44, and
fig. SSc-e of this paper) found at widely separated localities do not
show any appreciable variations. The evident conclusion, there-
fore, is that some of the teeth that have been identified with Ceratio-
caris because they were found in association with other remains of
Ceratiocaris are not referable to this genus. These teeth must have
come from other crustaceans which have not left any remains other
than teeth. This and other factors which have impeded proper
identification may be summed up as follows:

(1). Commingling of gastric teeth of more than one genus.

(2). Commingling of gastric teeth and mandibles whose bases have not been
preserved or whose cutting edges have been worn or blunted, thus
causing them to simulate gastric teeth.

(3). Relative age of individuals.

(4). Conditions under which fossilization has taken place.

The above factors at once create a situation which does not admit
of identifications that can be relied on. They, further, definitely
imply that unless and until other specimens of the various tjrpes of
teeth hitherto identified with Ceratiocaris are found in such intimate
association with remains of individuals as to leave no doubt that
they are referable to those individuals, there can be no escape from
the confusion now existing.

It has been mentioned in the beginning that the mandible from
Blue Island was found a short distance from C. markhami. I have,
nevertheless, hesitated to identify it with that species for the reason
that the mandible is relatively too large, and hence can hardly be
referred to C. markhami unless it is assumed that the latter has not
reached its adult stage. There is, however, no valid reason for such
assumption. Structurally, C. markhami shows all the features hither-
to regarded as characterizing an adult specimen (S. K. Roy, 1935,
pp. 142-144).

Fossil mandibles of Paleozoic phyllopod crustaceans are not of
common occurrence. The mandible here figured and described is
the only one yet recorded from the Silurian of North America.
Whitfield (1896, pi. 13, fig. 5, and pi. 14, figs. 1-4) has figured
five supposed mandibles from the Lower Helderberg (Devonian)

160 Field Museum of Natural History— Geology, Vol. VI

of Wisconsin, three of which are referred to Ceratiocaris monroei
Whitfield, and two to Entomocaris telleri Whitfield. Whitfield's
specimens have massive cusps and lack entirely the trenchant
character of the sharp cutting edges of mandibles. There is a possi-
bility that they are gastric teeth which have retained their zygo-
cardiac ossicles. The fact that mandible-like organs have been
found attached to the ventral surface of the anterior region of the
carapace does not necessarily imply that they are mandibles. Gastric
teeth, sinistral and dextral, may easily become detached from the
pyloric ossicle and, dropping out of the stomach (during its decom-
position) become lodged inside the valves of the carapace, occupying
the position of mandibles.

The mandible described in this paper, with the exception of its
elongated base, compares very favorably with the two from Les-
mahagow, Lanarkshire (Jones and Woodward, 1888, text figs,
and 6). The toothed ridges of all three specimens are strikingly
similar, and, judging from the similarity of the bases of the Scottish
specimens, it seems highly probable that the base of the present one
might have been somewhat distorted.

Horizon and locality. — Lecthaylus shale. Upper Lockport group
(Niagaran), Blue Island, Illinois.

P23676 Field Museum.

Holotype.

REFERENCES
Barrande, J.

1872. Systeme silurien du centre de la Bohdme. 1, Suppl.
Jones, T. R. and Woodward, H.

1888. The Gastric Teeth of Ceratiocaris. Geol. Mag., New Ser., Dec. 3, 5,

No. 4.

1898. The Gastric Teeth of Dithyrocaris (Mong. Brit. Paleozoic Phyllopoda,
pt. III). Paleont. Soc, London.

PORTLOCK, J. E.

1843. Report on the Geology of the County of Londonderry.
Roy, S. K.

1935. A New Silurian Phyllopodous Crustacean. Field Mus. Nat. Hist.,
Geol. Ser., 6, No. 9.

Salter, J. W.

1860. On New Fossil Crustacea from the Silurian Rocks. Ann. Mag. Nat.
Hist., 3rd Ser., 5, No. 27.

Whitfield, R. P.

1896. Notice and Description of New Species and a New Genus of Phyllo-
caridae. Bull. Amer. Mus. Nat. Hist., 8.

Woodward, H.

1865. On Some Crustacean Teeth from the Carboniferous and Upper Ludlow
Rocks of Scotland. Geol. Mag., 2.