Cycloidea of the Mississippian Bear Gulch Limestone of central Montana

2006·9.2 MB·English

by  Natalie Thomas

Cycloidea of the Mississippian Bear Gulch
Limestone of Central Montana
Frederick R. Schram,i Arjan C. Boere,^ and
Natalie Thomas^
x^BSTRACT.Anewspeciesofcycloidcrustacean,Halicynemontanaensis,isdescribedfromtheuppermost
Mississippian locality at Bear Gulch, in central Montana. Although the species is ararecomponentofthe
fauna and the specimens at hand are not well preserved, the material does allow comparison to other
speciesinthegenus.TheadditionofH. montanaensistothespecieslistoftheBearGulchfaunastrengthens
similarities to other Carboniferous Konservat-Lagerstatten in Europe and North America and reinforces
ideas about a persistentnearshore marine chronofauna in this time period.
INTRODUCTION 2003). The most recent cladistic analysis places the
The strata oftheBear GulchLimestonedepositsare cycloids near the Copepoda (Schram et ai., 1997).
part ofthe Big Snowy Group dated as Late Missis- There is a moderate degree ofdiversity in shapes
and numbers of taxa within the group. Three dis-
sippian, Chesterian, in age (Cox, 1986; Jenner et tinct morphotypes are now recognized within the
ai, 1998) just below the Mississippian-Pennsylva-
genus Cyclus alone and, in addition, Halicyne
nian boundary. TheBear Gulchfossilsarenotedfor showsyetanotherdivergentmorphology. Flowever,
their generally high quality of preservation (Mel- some ofthese divergences in shape may be a result
ton, 1971) and suggest an anoxic environmentand
oftaphonomy, since the vaulted, convex specimens
rapid burial ofthe organisms, although this would
seem to be contradicted by the presence of a large are ingeneral found inlimestones, whereastheflat-
tenedspecimens aremainlyfoundinshales (Schram
array of burrowing and benthic fishes. The history et al, 1997). Nevertheless, a number of distinct
ofdeposition has been reconstructed in greatdetail
characters unite the cycloids, for example, the
(Williams, 1983; Cox, 1986; Feldmanetal, 1994);
it represents a bay at latitude approximately 10°N ushniierldalmiokuescaanrtaepnancaee,fulsaerdgetosuablclhetlhaotreacmiacxislelgameenatnsd,
in an arid, monsoonal climate comparable to the
maxillipeds, posterior thoracopods as robust uni-
present-day Sahel. Depending on the prevailing
ramous walking limbs, and a short one- or two-
winds and circulation, the bay appears to have de-
segment abdomen.
veloped seasonally an anoxic bottom layer associ-
Cycloids are found in Austria, Belgium, Bosnia,
ated with high rates of cyclic deposition. This
would explain the extraordinary preservation, the France, Germany, Ireland, Italy, Russia, the Neth-
erlands, the United Kingdom, central Asia, and the
presence of those problematic bottom-dw'elling
UnitedStates (Giaessner, 1969;Schrametal, 1997)
fishes and other benthos, andthepeculiar,regularly
and are mainly associated with marine to brackish
alternating size and coloring of the sediments.
environments. Cycloids frequently co-occur with
AgnTohsetufisrstracdymciltosid(Pdheilslcirpisb,ed18w35a)s.uFnrdoemr tthheatntaimmee cplloaindtsmmaitgehritahl.avSechhardamanethearl.bi(v1o9r9o7u)sosrugsgceasvteendgicnyg-
on, the higher taxonomic placementofcycloidshas
shifted from group to group with numerous con- life style, thatis, thattheyperhaps occupiedaniche
equivalent to modern crabs (Brachyura).
flicting suggestions as to their affinity (for an over-
view, see Schram et a!., 1997). It appears that the MATERIALS AND METHODS
cycloids belong within the crustaceans and most
likely within the class Maxillopoda. They m^ere This study is based on only six specimens: five collected
quite diverse in the Paleozoic (see the reviews in in the 1970s near Beckett, Montana, USA (Natural His-
Clark, 1989; Schram et a!., 1997; Brambilla etal., tory Museum of Los Angeles County, Invertebrate Pale-
2002} and are now known to have survived ontology [LACMIPj locality 15424), and one specimen
through the close of the Cretaceous (Fraaije et a!., o(fCNMat4u5r8a1l6)Hifsrtoomryth(eCcMo)l.lecTthieonsspoefctihmeenCsarwneergeieeMxuamsienuemd
with a variety of light microscopic techniques by using
1. Zoological Museum, University of Amsterdam, various lighting strategies in both a dry state and sub-
Maurtiskade 57, 1092 AD Amsterdam, the Netherlands. merged under water. Digitized photographs and camera
2. Department of Geology, University of Leicester, lucida drawings were made under these varying condi-
Leicester LEI 7RH, United Kingdom. tions.
Contributionsin Science, Number 504, pp. 1-8
Natural HistoryMuseum ofLos Angeles County, 2005
2 Contributionsin Science, Number 504 Schram etal.: Cycloidea ofCentral Montana
Figure 1 Halicyne montanaensis n. sp. 1, 2, LACMIP 7310 part and counterpart (holotype), underdirectlight,showing
vaulted trunk, thoracic appendages, segmentation, and rostral plate, X5 (see Figs. 2.1, 2.2). 3, 4, CM 45816 part and
counterpart, underwater, showingcephalic appendages,compoundeyes, and rostralplate, X4. 5,6,LACMIP7311 part
andcounterpart, underdirectlight, showingpartoftheposteriorthoracicsegmentationandcaudalrami, X4.5 (seeFigs.
2.3, 2.4). ABBREVIATIONS: abd = abdomen, an = antenna fragment, ce = compound eye, cr = caudal rami, ms =
marginal shelf, mx 2 = second maxilla, mxpd = maxilliped, on = optic notch, pap = papillae, pn = posteriornotch,r
= rostral plate, s == posteriorthoracic segmentation, 2/6 = thoracic appendages
Contributions in Science, Number 504 Schram etai: Cycloidea ofCentralMontana 3
Figure2 CameralucidadrawingsofspecimensofHaiicynemontanaensisn.sp. 1,2,LACMIP7310partandcounterpart
(holotype),correspondingtothephotographsofFigures 1.1, 1.2.3,4,LACMIP7311 partandcounterpart,corresponding
to photographs ofFigures 1.5, 1.6. ABBREVIATIONS; abd = abdomen, an = antenna fragment, ce = compound eye,
cr = caudal rami, ms = marginal shelf, mx 2 = second maxilla, mxpd = maxilliped, on = optic notch,pap = papillae
on carapace shield, pn = posterior notch, r = rostral plate, s = posterior thoracic segmentation, 2/6 = thoracic ap-
pendages
SYSTEMATICS in size and relatively narrow, penultimate segments
Maxillopoda Dahl, 1956 wide.
HOLOTYPE. LACMIP 7310 (Figs. 1.1, 1.2)
Cycloidea Glaessner, 1928 MATERIAL EXAMINED. LACMIP 7310,
CM
7311, 7312, 7313, 7314; 45816
Cyclidae Packard, 1885
DESCRIPTION. The carapace shield is round to
Haiicyne von Meyer, 1844 oval in outline ranging from 10.8 to approximately
mm
25 inlength,witha distinctthickenedmarginal
Haiicyne montanaensis new species shelf (LACMIP 7310a, Figs. 1.1, 1.2; LACMIP
CM
DIAGNOSIS. Carapace round to oval in outline, 7313, Fig. 3.2; 45816, Figs. 1.3, 1.4) and dis-
slightly vaulted in cross section. Surface papillose, tinct cross-sectional vaulting (LACMIP 7310a,
marginal shelf distinctly thickened with a smooth LACMIP 7313) characteristic of the genus. The
to slightly irregular edge. Carapace excavated pos- edge of the carapace shelf is smooth (CM 45816)
teriorly. Distinct optic notches with large, stalked, to irregular (LACMIP 7310) in outline. Thesurface
compound eyes. Rostral plate broadly fused with of the carapace is distinctly papillated (LACMIP
carapace and anteriorly truncated. Anteriormost 7311a, Figs. 1.5, 1.6; LACMIP 7313, Fig. 3.2), the
part of carapace bearing rounded knobs or bosses. midanterior surface especially so (CM 45816)
Terminal segments ofall geniculateclaws moderate where the patterning consists of almost rhombo-
4 Contributions in Science, Number 504 Schram etal.: Cycloidea ofCentral Montana
Figure 3 Halicyne montanaensis n. sp. 1, LACMIP 7314, photographed with green filter to enhance contrast, showing
thoracicappendages,abdomen, andgutcast, X6.8 (seeFig. 4.1). 2,LACMIP7313,underdirectlight,showingpapillated
carapace surface, fragment of marginal shelf, maxillipeds, and antenna, X9.5 (see Fig. 4.2). 3, 4, LACMIP 7312 part
andcounterpart,revealingpartofposteriorthoracicsegmentation,surroundingplantmaterial,3,underdirecthigh-angle
light (see Fig. 4.3), 4, under direct low-angle light, X5.3. 5, CM 45816, close-up of compound eye with facets, X30.
ABBREVIATIONS: abd = abdomen, an = antenna, g = gut cast, ms = marginal shelf, mxpd = maxilliped, pap =
papillae carapace surface, pi = plant material, r = rostral plate, s = posterior thoracic segmentation, 2/6 = thoracic
appendages
CM
hedral knobs or bosses. The center ofthe carapace 7313, 45816). The carapace directly over the
shield displays two faint ridges running anterior- optical notches is slightly raised; between these ar-
posteriorly and delineating a V-shaped area (LAC- eas is a semioblong elevated area just posterior to
MIP 7310). The carapace shieldcoversvirtuallythe the rostral plate (CM 45816). The anteriormost
entire body (Figs. 1.1-1.4) and includes an anterior rostral aspect ofthe carapace has an irregularmar-
extension or rostral plate that is truncated andcov- gin.
ers the cephalon proper (LACMIP 7310a,LACMIP Optical notches (Figs. 1.3, 1.4) are located just
Contributions in Science, Number 504 Schram etai: Cycloidea ofCentral Montana 5
Figure 4 Camera lucida drawings of specimens of Halicyne montanaensis n. sp. 1, LACMIP 7314, corresponding to
photographinFigure3.1.2,LACMIP7313,correspondingtophotographinFigure3.2.3,LACMIP7312,corresponding
to photograph in Figure 3.3. ABBREVIATIONS: abd = abdomen, an = antenna, g = gut cast, ms = marginal shelf,
mxpd = maxilliped, pap = papillae carapace surface, pi = plant material, pn = posterior notch, r = rostral plate, s =
posteriorthoracic segmentation, 2/6 = thoracic appendages
anterior to either end of the marginal shelf (CM and maxilliped back onto the serrate anterior edge
45816, LACMIP 7310). Within these notches are of the propodus probably facilitated grasping.
stalked compound eyes that are exceptionally well The uniramousthoracic limbs2 through 6 (LAC-
preserved, withupto45 hexagonaltoroundedom- MIP 7310a, LACMIP 7310b, Fig. 1.2; LACMIP
matidia visible (CM 45816, Fig. 3.5). 7314, Fig. 3.1) are robust, short, and terminally
Both pairs ofantennae areverypoorlypreserved. pointed, with the narrow dactyls generally directed
One setmay be partlyvisible onthe anteriorlateral anteriad, especially on thoracopods 2 and 3. The
left side of specimen LACMIP 7313 protruding penultimate segments are relativelythick (LACMIP
from where the edge of the rostrum and carapace 7310). Little is discernable concerning the more
would have been (Figs. 3.2, 4.2). These seem to proximal segments of these limbs. Only on the
consist of the peduncular segments and the proxi- right-hand side ofLACMIP 7310,wherepartofthe
malportionsoftheflagella.Theholotype,LACMIP carapace is missing, can one see parts of the tho-
7310, also shows a slight trace of antennae (Figs. racic tergites and proximal segments of the thora-
1.1, 2.1). copods (Figs. 2.1, 2.2), which appear to be less ro-
Anteriorly, a pair of large uniramous second bust, yet longer than either of the two distal seg-
maxillae (Figs. 1.1-1.4) extendanterolaterallyfrom ments. These thoracopods were probably used for
the frontal portions of the head. Also, at least one walking or pushing along the substrate.
set of rather broad maxillipeds is present. Both The major feature in common among all the
these limbs bearserratearmaturealongtheanterior specimens is the fan-shapedpatternoftheposterior
margins of the dactylus and propodus (LACMIP thoracic segments, clearly evident on LACMIP
7310b, CM 45816). It is not entirely clear if the 7311a (Fig. 1.5), LACMIP 7312a (Figs. 3.3, 3.4),
second set ofthoracopods were developed as max- and LACMIP 7313. LACMIP 7311 shows only a
illipeds as well. The segmentation of the cephalic small part of the posterior end of the animal,
appendages is obscured, but the cuticle of these whereas the anterior half was not well preserved.
limbs seems to be finely papillated (LACMIP 7313, More anteriorly, the trunk shows traces ofsegmen-
Fig. 3.2). The flexion of the dactyl of the maxillae tation thatiseitherchevronshaped,withthelateral
6 Contributionsin Science, Number 504 Schram etai: Cycloidea ofCentralMontana
Figure 5 ReconstructionofHalicyne montanaensis n. sp.
aspect directed anteriorly (LACMIP 7313, Fig. 2.3) rounding rock matrix and the degree of preserva-
or parallel, laterally directed segmentation (LAC- tion of associated plant material (LACMIP 7312,
MIP 7310a, LACxMIP 7312, Fig. 4.3). What one Fig. 3.3).
actually sees in all these instances are the endo- A reconstruction ofHalicyne montanaensis is of-
phragmal partitions between the segments them- fered in Figure 5.
selves.
A large subtriangular area is visible (Fig. 2.2) on DISCUSSION
the central aspect of the trunk (LACMIP 7310a,
CM
45816), which corresponds to the V-shaped Currently, five species are easily recognized in Hal-
area ofthe carapace and probablymarkswherethe icyne, andmostofthecharacteristicfeaturesforthe
carapace was attached to the underlying segments. genus are clearly present in the specimens in this
The gut or digestive tract (Figs. 3.1, 4.1) is pre- study including the round to oval shape, the mar-
served in only one specimen as a dark, irregularly ginal shelf and the papillated surface of the cara-
shaped cast (LACMIP 7314). pace, the distinctoptical notches, andthetruncated
There is a short, apparently two-segment abdo- anterior part of the rostral plate. [The affinities of
men (LACMIP 7311a, Figs. 1.5, 2.3; LACMIP a sixth species, H. plana (Muller, 1955), is uncer-
7314, Figs. 3.1, 4.1), exposed because of the pos- tain; the describedmaterialconsistedonlyofpoorly
terior notch of the carapace. Only one specimen preserved carapace shields.] The large maxillipeds,
(LACMIP 7311b) displays distinct caudal rami which also are found in H. max Schram,Vonk, and
(Figs. 1.6,2.4).Theseramiappeartobeuniramous, Hof, 1997, and H. gondwanae Brambilla et aL,
long (up to 5 mm), thin, bladelike, and attached to 2002, are preserved in other species of the genus
the posteriormost abdominal segment. such as H. ornata Gall and Grauvogel, 1967, H.
REMARKS. Most specimens at hand are of rel- agnota (von Meyer, 1838), or the enigmatic genus
atively poor quality. Only the holotype specimen is Carcinaspides (Schafhautl, 1863).Theelevatedpat-
fairly complete. Specimens LACMIP 7310, 7311, tern ofbosses onthe dorsal surfaceofthecarapace,
CM
7312, and 45816 comprise both parts and including the anteriormost bosses or knobs, is sim-
counterparts. ilar to those in H. ornata and H. agnota from the
One can note differences among the specimens, Triassic Muschelkalk. Although it is not certain
for example, in quality or amount of organic ma- whether the edge ofthe marginal shelfis smoothor
terial preserved, and these are undoubtedly due to irregular, there is no indication of a regularly
taphonomy, which probably also explains the var- sculpted margin as seen in H. max. The postero-
iation visible in the differential coloring ofthe sur- median edge of the carapace, which is rounded in
Contributions in Science, Number 504 Schram etai: Cycioidea ofCentral Montana 7
H. max but pointed in H. ornata and H. agnota, ota ofthe Middle-Pennsylvanian Mazon Creek de-
although obscured in most of the specimens at posits (Illinois, USA). These similarities are also re-
hand, possesses a posterior notch. There is a small flected in other taxa, including polychaete worms
abdomen or telson with two caudal rami, bearing and other invertebrates (Schram, 1979) andsuggest
some resemblance to the posterior partofH. gond- that the chronofauna had existed virtually un-
wanae (Brambilla et al, 2002). changed for millions of years. However, note that
The stout,robustthoracopodsonH. montanaen- chronofaunas (Olson, 1966) did not remain com-
sis are quite different from the limbs of H. max, pletely unchanged, especially at the species level,
which are longer and thinner. The thoracopods of but rather only at higher taxonomic levels. Species
H. montanaensis resemble the robust, supposedly did evolve, became extinct, and were replaced, but
biramous limbs of H. ornata, but there is no indi- the general higher taxonomic compositionand par-
cation of any biramous limbs on the Bear Gulch ticular combination of feeding types remained sta-
species. The presence or absence ofbiramous limbs ble (Schram, 1981; Briggs and Gall, 1990). The
has been both important and controversial in dis- crustaceans of the Bear Gulch Konservat-Lager-
cussions of the affinity of the cycloids (Glaessner, statte have long figured as among the most inter-
1969; Schram et al, 1997), but there is no indica- esting assemblages in the Carboniferous (Schram
tion of exopods in H. max or H. gondwanae, nor and Horner, 1978; Factor and Feldmann, 1985;
in the specimens under study here. In addition, the Schram, 1985; Jenner et al., 1998). We can now
orientation of the limbs with respect to the trunk add the Cycloidea to the faunal list of the crusta-
differs. In H. max they are concentrated in the an- ceans of Bear Gulch, although cycloids at Bear
terior portion of the thorax, whereas in the speci- Gulch remain very rare.
mens here they are positioned more evenly along In conclusion, the specimens at hand provide
the trunk. The strongest resemblance of H, mon- support for two hypotheses about the Cycloidea.
tanaensis seems to be to H. ornata as reconstructed First, the pattern of the segments in the posterior
by Gall and Grauvogel (1967) and Gail (1971) and thorax reinforces analogous similarities between
H. gondwanae. crabs and cycloids proposed by Schram et al.
The radial lamellae or radiating grooves ob- (1997). Second, the presence of a member of the
served in H. max and H. ornata are not preserved genus Halicyne in Bear Gulch supportsthe Late Pa-
on H. montanaensis. Schram et al. (1997) referred leozoic chronofaunal continuum described by
to these structures as a possible diagnostic feature Schram (1981).
to help distinguish between Halicyne and othercy-
ACKNOWLEDGMENTS
cloid genera. Itisconceivable thatthe specifictaph-
onomic conditions at the Bear Gulch locality were WewouldliketothankourcolleaguesattheNaturalHis-
unsuited to preserve these particular soft tissues. tory Museum ofLos Angeles County section for Inverte-
Little is known about segmentation patterns in brate Paleontologyformakingavailablemostofthespec-
cycloids, except some clues fromthepositionofthe imens, and the Carnegie Museum (in particular Albert
proximalmost segments ofthe limbs (Schrametal., Koller) for loan of the other specimen. Special thanks to
1997). The distinctly fan-shaped segmentationseen Jan van Arkel of the University of Amsterdam for help
in these specimens, for example, LACMIP 7311 with photography. Professors Rod Feldmann and Derek
(Fig. 1.5), might be a generalcharacter ofthegenus Briggs offered constructive suggestions for improving an
(or even the family) that was not recognized pre- early version ofthe manuscript.
viously. Reinvestigation ofthe specimens described
LITERATURE CITED
in Schram et al. (1997) showed that this feature is
present in Cyclus americanus, but it was not ex- Brambilla, S., A. Garassino, G. Pasini, and G. Teruzzi.
plicitly recognized or mentioned. This pattern is 2002. Studies on Permo-Trias of Madagascar. 6.
probably convergent to the segmentation seen in First record of Cycloidea from the Lower Triassic
crabs (Brachyura), where the strongly reduced ab- (Olenekian) ofAmbilobe region (NWMadagascar).
domencausesthelaterallyoriented,parallelpattern Atti della Societd Italiana di Scienze Natnrali e del
of the endophragmal structures to be disrupted in Miiseo Civico di Storia Naturale de Milano 143:
105-115.
the posterior part of the thorax. In crabs, the solid Briggs, D.E.G., and J.-C. Gall. 1990. The continuum in
ventral endophragms between the segments like soft-bodiedbiotasfromtransitionalenvironments:A
those seen in LAGMIP 7311 (Fig. 1.5) are used for quantitative comparison of Triassic and Carbonif-
attachment ofthe extrinsic musculature ofthe legs. erousKonservat-Lagerstatten. Paleobiology16:204-
Based on the fauna of malacostracan crusta- 218.
ceans, for example, a characteristic diversity ofho- Clark, N.D.L. 1989. A study ofa Namurian crustacean-
plocaridans and primitive eumalacostracans, the bearing shale from the western Midland Valley of
Bear Gulch biota was recognized by Schram (1985) Scotland. UnpublishedPh.D. dissertation.University
and Briggs and Gall (1990) as part ofa spatiotem- of Glasgow, Glasgow, Scotland, 341 pp.
Cox, R.S. 1986. Preliminary report on the age and paly-
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