Fish diversity of the Bear Gulch Limestone, Namurian, Lower Carboniferous of Montana, USA

FISH DIVERSITY OF THE BEAR GULCH LIMESTONE, NAMURIAN, LOWER CARBONIFEROUS OF MONTANA, USA RICHARD L U N D & CI~CILE P O P L I N LUND R. & POPLIN C. 1999. Fish diversity of the Bear Gulch Limestone, Namurian, Lower Carboniferous of Montana, USA. [La diversit~ des poissons de Bear Gulch, Namurien, Carbonifere inf~rieur du Montana, USA]. GEOBIOS, 32, 2: 285-295. Villeurbanne, le 30.04.1999. Manuscrit d~pos~ le 25.08.1998; accept~ d~finitivement le 19.10.1998. ABSTRACT - Quarrying operations for fossil fish from the Bear Gulch Limestone (Heath Formation, Namurian, Montana, USA) have occurred from 1968 to 1997 and continue. The Bear Gulch Limestone lens (14 x 9 km) thins to a shore line on all edges except the buried eastern boundary, and contains an entirely marine flora and fauna. Eighty outcrops have been sampled, and 4 547 identifiable fish of 113 taxa have been recorded. This study details aspects of the diversity and distribution of the fishes of the Bear Gulch Bay analyzed on the basis of geographic areas, following facies and sedimentary criteria. The total diversity of the fish fauna is 21.08 (Simpson index) and 12.07 (Margalef index). Diversity decreases from east to west and towards the southern margin. These trends mask dramatic differences in species occurrences and dominant species in each area. The high diversity of the "Depocenter" area can be accounted for by its proximity to the mouth of the bay and access to the epicontinental seaway to the east. The southeast region is comprised of the Blacktail, Allen, and Buchek areas. Diversity declines greatly from the algae - and sponge - rich Blacktail area southwestward to the marginal Buchek area. This region contains several dominant species that are adapted to lurk in or maneuver among plants, feed from the bottom, or burrow. High numbers of larval paleoniscoids and the coelacanth Caridosuctor lower the diversity of the near-shore North Shore area. The western end of the bay is dominated by Acanthodes, scales of Listracanthus and Strepsodus, and large cladodont sharks. Fish distributions, preliminary assessments of morphological adaptations, and facies all support the concept of a heterogeneous set of habitats within this fossil bay. KEYWORDS: BIODIVERSITY, FISH, MARINE, CARBONIFEROUS, PALEOENVIRONMENT, NORTH AMERICA. Rt~SUMI~ - Des poissons fossiles ont ~t~ r~colt~s de 1968 h 1997 (et continuent de l'~tre) dans le Calcaire de Bear Gulch (Formation de Heath, Namurien, Montana, USA). La lentille de calcaire (14 x 9 km) s'amincit jusqu'h une ligne de rivage sur tout son pourtour, sauf le long de sa limite est qui est enfouie. Elle contient une flore et une faune enti~rement marines. Quatre - vingt affleurements ont ~t~ ~chantillonn~s et 4 547 poissons identifiables appartenant 113 taxons ont ~t~ enregistr~s. Les donn~es sur la r~partition des poissons ont ~t~ analys~es par aires g~ographiques d~finies en fonction de crit~res s~dimentaires et de facies. La pr~sente ~tude d~taille les aspects de la diversit6 des poissons de la Baie de Bear Gulch. La diversit~ totale de la faune ichthyologique est 21,08 (indice de Simpson) et 12,07 (indice de Margalefl. La diversit~ d~cro~t d'Est en Ouest et vers la marge sud. Ces gradients masquent des diff,rences importantes dans la r~partition des esp~ces, en particulier des esp~ces dominantes, dans chaque aire g~ographique. La forte diversit~ du "Depocenter" peut 6tre expliqu~e par sa proximit~ du d~bouch~ de la baie sur le chenal ~picontinental vers l'Est. La r~gion du Sud-Est comprend les aires de Blacktail, Allen et Buchek: depuis Blacktail, riche en algues et ~ponges, la diversit~ d~croit fortement en direction du Sud-Est jusqu'h l'aire marginale de Buchek. Cette r~gion comporte plusieurs esp~ces dominantes qui sont adapt~es pour s'embusquer ou manoeuvrer parmi les plantes, ou encore fouir le fond pour chercher leur nourriture ou s'enfouir. Dans l'aire de North Shore, pros du rivage, le grand nombre de larves de pal~onisco~des et du coelacanthe Caridosuctor font baisser la diversitY. L'extr~mit~ ouest de la baie est domin~e par Acanthodes, les ~cailles de Listracanthus et de Strepsodus et de grands requins cladodontes. Les r~partitions des poissons, les estimations pr~liminaires sur les adaptations morphologiques, et enfin les facies ~tayent tousle concept d'une s~rie h~t~rog~ne d'habitats dans cette baie fossile. MOTS-CLI~S:BIODIVERSITI~, POISSONS, MARIN, CARBONIFt~RE,PALI~OENVIRONNEMENT,AMI~RIQUE DU NORD.

INTRODUCTION The B e a r Gulch L i m e s t o n e is a K o n s e r v a t - L a g e r s t~tte t h a t contains an excellently p r e s e r v e d record of occurrences of Carboniferous fishes of a tropical m a r i n e bay, deposited t h r o u g h a v e r y brief interval of geological time ( e s t i m a t e d at 1 000 - 2 000 years,

Williams 1980) at the end of the U p p e r Chesterian. This s t u d y is based u p o n q u a r r y i n g operations for fossils t h a t were conducted from 1968 to 1997, and, d u r i n g these years, 80 outcrops were s a m p l e d w i t h i n the B e a r Gulch L i m e s t o n e a n d 4 547 identifiable fish of 113 t a x a h a v e been recorded from these sites.

286 FIGURE 1 - Namurian paleogeographic setting of the Bear Gulch Limestone, Heath Formation, Big Snowy Group, Fergus County Montana. BGL, location of Bear Gulch Limestone outcrop area; CMT, Central Montana Trough; EQ, 10 °, equator and 10 ° north latitude; M, miogeosyndine; N, paleomagnetic North; WB, Williston Basin. Shaded areas, emergent land; solid lines, current Canadian and US political boundaries; dashed line, approximate limit of continental plate. (After Witzke 1990). Situation paldoggographique au Namurien du Calcaire de Bear Gulch, Heath Formation, Big Snowy Group, Fergus County, Montana. BGL, localisation de l'affleurement du Calcaire de Bear Gulch; CMT, Bassin Central du Montana; IEQ, 10, dquateur et 10 ° de latitude nord; M, miogdosynclinal; N, Nord paldomagndtique; WB, bassin de Williston. Grisd: terres dmergges; lignes pleines: fronti~res politiques actuelles du Canada et des Etats-Unis; ligne pointillge: limite approximative du plateau continental (d'apr~s Witzke 1990).

The Bear Gulch Limestone Konservat-Lagerst~itte is unique among Carboniferous fish faunas in the high number of species, the preservation of the fish and the varied bay habitats, and in the complete absence of freshwater macrofaunal elements. By comparison, Schultze and Maples (1992), in discussing Upper Carboniferous fish faunas, list 13 genera for Essex, 12 for Braidwood, 10 for Kinney, 8 for Hamilton, Robinson and Linton (where there are 11-12 species), 5 for Montceau-les-Mines, and 4 for N~r~ny, each of which have a high percentage of freshwater faunal elements. After Moy-Thomas & Bradley Dyne (1938) 32 species of fish are known from the marine Visean of Glencartholm. The Bear Gulch Limestone fish fauna contrasts strongly with modern fish faunas in its species composition and taxonomic distribution, being composed of about 60% chondrichthyans, about 34% actinopterygians, and 5% coelacanths. On the contrary, in terms of number of individuals, the most abundant Bear Gulch species is a coelacanth, followed by a few actinopterygians and a chondrichthyan. By comparison, about 96% of modern fish are actinopterygians, 3% are chondrichthyans (Elasmobranchii), and there are trivial percentages of agnathans, holocephalans, and a coelacanth; in modern faunas only actinopterygians can be considered to be abundant. This study details aspects of the biodiversity of the fishes of the Bear Gulch Limestone. The Bear Gulch Limestone lens of the Bear Gulch member of the Heath Formation (Upper Chesterian, Namurian E2b, Montana, USA) is 14 km x 9 km in extent, and thins to a shore line on all edges except the eastern boundary, which is buried (Horner 1985; Williams 1983; Feldman et a]. 1994).

O

All indicators suggest that the Bear Gulch Limestone was deposited in a bay whose narrow mouth opened to the epicontinental seaway along the northeastern edge of the deposit. Paleomagnetic evidence indicates that the Bear Gulch Limestone was located at about 12 ° N latitude in the Upper Namurian, that the North American continent was rotated about 45 ° clockwise relative to today's north, while paleogeographical and sedimentary evidence indicates a tropical, semi-arid climate in the region (Williams 1983; Horner 1985; Witzke 1990) (Fig. 1). Climatological evidence indicates t h a t seasonal monsoonal conditions would occur at these latitudes, with prevailing wind conditions normally blowing almost directly across the bay: in the winter from the east and northeast, and in the summer from the west and southwest (Grogan & Lund, ms.). The Bear Gulch limestone lens was very rapidly deposited in the narrow and tectonically active Central Montana Trough. It shallowed upward into terrestrial beds and an unconformity that marks the local Mississippian-Pennsylvanian boundary. Direct marine access to the Pacific Ocean through the Central Montana Trough was blocked immediately west of the Bear Gulch Limestone during its time of deposition, but access to the Pacific ocean was possible through the miogeosyncline south of the Williston basin, and access was also available to the eastern North American epicontinental seaway (Fig. 1). Several facies have been described within the Bear Gulch Limestone on the basis of sedimentological and paleontological data: a basinal facies indicating a clear bottom of fine mud with little or no sessile benthos; an algal facies characterized by large

287 teeth, Strepsodus sp., Psephodus sp., large cladodont (stethacanthid) sharks, and Listracanthus sp., by only admitting one specimen at any outcrop in any one quarrying day into the analysis. Concerning the fragmentary remains, while there is no uncertainty about the identity of any chondrichthyan species because they are based on gross morphological characteristics such as tooth and spine morphology, there are several difficulties inherent in the identification of the actinopterygian taxa, few of which present readily identifiable isolated bones. Many larval actinopterygians may not be identifiable to genus or to species at present, but comprise a significant part of the fish fauna and are treated as distinct ecomorphological units for the determination of species diversity. These larval actinopterygians can only be divided into two groups at present, those that may be attributable to the taxon "Yogo" and those that do not pertain to this taxon. The taxon '~ogo" itself contains two sister species, but few of the specimens can be identified to species at this time. Additionally, about 15 actinopterygian specimens cannot currently be firmly assigned to known species: these are entered into the data base as three tentative unique species. The number of actinopterygian taxa used for the analyses is therefore considered to be conservative.

concentrations of fine filamentous floating algae with small spiny productid brachiopods suspended in the algae; an Arborispongia facies characterized by the dominance of up to l-meter- diameter arborescent sponges along with many associated epizootic and epiphytic forms and many forms of algae; and a marginal facies of dark, irregularly bedded, high-organic-content shales and limestones, showing wave action and poor preservation of most vertebrates (Williams 1983; Horner & Lund 1985; Lurid et al. 1993; Feldman et al. 1994) (Fig. 2). The sessile benthic invertebrate fauna of the Bear Gulch Limestone lens is limited in variety and highly restricted in area of occurrence. Cephalopods, crustaceans, and worms dominate the mobile invertebrate fauna. There are no significant freshwater or terrestrial macrofossils in the Bear Gulch Limestone. MATERIALS AND METHODS

All identifiable specimens from the Bear Gulch lens are included in the data base for the initial analysis; this information is identified as "all Bear Gulch" in the text and tables. Unpublished species are referred to by "code names"; their taxonomic assignments are listed in Table 1. Data from outcrops have been grouped into several areas on the basis of geographic and facies criteria, and the data from all outcrops within each area are merged for this analysis. One area, in the extreme southeast, has been excluded from further analyses because it lacks sufficient numbers of specimens, and specimens not attributable to a specific outcrop or area have also been excluded from f u r t h e r consideration.

The species occurrence data in each area have been analyzed in two manners, first using the absolute numbers, and second as standardized to percent of 100% occurrence at any area to correct for unequal sample sizes. However, unequal sample sizes from area to area, as well as preservational differences, reflect substantive aspects of bay ecology and deposition as well as retrieval rates per person per day of quarrying effort.

Data are corrected against sampling bias for the taxa that are only represented by isolated scales or Class

Chondrichthyes

Acanthodii

Osteichthyes

Acanthodes hmdi

"Subclass - - E1as-mo-brancl~ii-[- -E-uc1~on~tr oc e p-h-a~-"

Ac~nopterygii

Crossopterygii

"Yogo" Wendykhthys dicksoni "Fcop" "Prol" 'Tube snout" "Long pelvic" "Ae" "Yogo"larvae "Garden Eel" "Disc" actinopterygian larvae

Strepsodus sp.

Paraselachii Heteropetalus deganhdus

Inf-raclass

Holocephali

.................................

TABLE :[ - S y s t e m a t i c d i s t r i b u t i o n of t h e t a x a r e f e r r e d to in t h i s paper. Place systdmatique des taxons citds clans le texte.

Order

Psephodus sp. Iarpagofi~tutor volsellorhinu:

Cladodontida

Iniopterygii

large stethacanthid Falcatusfalcatus Incertae sedis Listracanthus sp.

"Iniop spl" "Iniop sp3"

Coelacanthiformes Caridosuctor populosmn Hadrcalector donbairdi Allenypterus montanus

288 Diversity and dominance indices are calculated using the Species Diversity and Richness software program; equations for the indices and references are given in the accompanying manual (Henderson & Seaby 1997). While there are many different diversity indices comparing the numbers of individuals with the numbers of taxa, the MargalefD and Simpson indices have been used here because they offer different but comparable views of the data. The Berger-Parker dominance index compares the abundance of the most dominant taxa to those of the remaining taxa (Henderson & Seaby 1997). In the figures and tables, however, the most dominant taxa are arbitrarily designated as those containing 3.5% or greater of the individuals at any one area of outcrops. The map is drawn with a CADD program using a base of U.S. Geological Survey 7 1/2' topographic maps, employing both aerial photographs and ground observations as checks upon the maps. Graphical representation of the standardized occurrences at each outcrop are drawn with the Corel Draw vector graphics program, and arranged along Rose Canyon

Forest Grove

three transects. These transects cover all areas, and facies, for which adequate occurrence data exists. All data are archived at the Section of Vertebrate Paleontology, Carnegie Museum of Natural History, Pittsburgh, Pa., USA. RESULTS

FAUNAL COMPOSITION Data from the entire Bear Gulch Limestone fish fauna show that Acanthodii occur at 1.6% of the total individuals sampled, Actinopterygii comprise 62% (50% adults and 12% larvae), Coelacanthiformes occur as 14%, and Chondrichthyes as 19% (8% elasmobranchs and 11% Euchondrocephali) (Table 2). Considerable variability in the percent composition of the major taxonomic groups is evident between the regions of the basin and along transects within the regions (Fig. 2). Acanthodii increase to abundance only toward the western margin of the bay. The greatest percent of adult actinopterygians occurs along the northeast

North Shore Depocenter Big Spring

Buchek

Allen

Blacktail 100

Acanthodii 75

Adult Actinopterygii Coelacanthiformes

50

Elasmobranchii

25

Pa rase lachii Holocephali

Strepsodus

l't4

N

CIES (HABITATS) 446'4/` 0

5

10

4"~80EI~4,£I/vM4RGfN4. O/VO/4 L

KM FIGURE 2 - Transects of percent occurrences of major taxonomic groups at each outcrop, across outcrop areas of the Bear Gulch Limestone. N, present day North; NN, North during the Namurian (after Witzke 1990). Profils des pourcentages des groupes taxo-

nomiques majeurs ~t chaque affleurement, le long des aires d'affleurements du Calcaire de Bear Gulch. N, Nord actuel; NN, Nord au N a m u r i e n (d'apr~s Witzke 1990).

289 to southwest transect in the southeastern region (comprised of the Blacktail, Allen-Surprize, and Buchek areas) where larval actinopterygians are scarce; actinopterygians are scarce toward the western margin of the bay. Coelacanths are at their highest percentages in the central eastern region of the bay (comprised of the Big Spring, Depocenter, and North Shore areas) along the South to North transect. Both elasmobranchian and euchondrocephalan (= Iniopterygii + Paraselachii + Holocephali, Lund & Grogan 1997) Chondrichthyes are at their most abundant toward the southern and western margins of the basin. The crossopterygian Strepsodus sp., represented only by scales and teeth, is most common at the southern and western margins of the basin. Generalizing by major taxonomic subdivision, the acanthodian Acanthodes lundi dominates toward the western extent of the Bear Gulch Limestone, along with Elasmobranchii, Euchondrocephali and Strepsodus sp., while the percentages of adult actinopterygians, larval actinopterygians and coelacanths diminish westward. Coelacanths as well as adult and larval actinopterygians dominate in the North Shore area. Data from the entire Bear Gulch Limestone fish fauna show also that, of the 113 taxonomic units, only seven are present at greater than 3.5% of the total: the coelacanth Caridosuctor populosum, larval actinopterygians, the actinopterygians Wendyichthys dicksoni and "Fcop", the stethacanthid elasmobranch Falcatus falcatus, and the actinopterygians "Yogo" and "Prol" (Table 3).

In the northeastern region (Fig. 3) of the Bear Gulch Limestone, the Big Spring area differs primarily in adding the actinopterygian Long Pelvic to the dominant fish. The North Shore area differs in the high percentage of larval actinopterygians (29%) collected, followed by C. populosum, the abundance of the actinopterygian "Garden Eel" and the lower percentage of"Fcop". In the southeastern region (Fig. 3) of the Bear Gulch Limestone, larval actinopterygians are not among the dominant elements of the fauna. C. populosum only occurs among the dominant elements of the fauna at Allen-Surprise, F. falcatus only in the Blacktail area, while at the Buchek area the only dominant elements of the fauna that are shared in common with the fauna as a whole are W. dicksoni and "Fcop". The actinopterygians "Tube snout" and "Long pelvic" appear among the dominants in both Blacktail and Allen-Surprise areas. The "Garden Eel" occurs among the dominants at Blacktail area, along with "Ae". In the western region (Fig. 3) of the Bear Gulch Limestone, the percentages of C. populosum and the numbers of actinopterygian taxa decline, while the numbers of chondrichthyan taxa and the percent ofAcanthodes lundi increase to the westward. "Yogo" continues to be a dominant element of the fauna in this region. The Rose Canyon area is similar in composition to the northeastern region but includes "Yogo" larvae and the holocephalan Harpagofututor volseUorhinus among the dominants. S o u t h to N o r t h

N E to S W

TABLE 2 - Percent composition of Bear Gulch Limestone fish fauna by major taxonomic subdivision for the entire deposit (ALLBG) and by outcrop areas along transects. Composition en pourcentages de la faune ichthyologique de Bear Gulch pour les groupes taxonomiques majeurs, pour la totalitd du dgpSt (ALLBG) et par aires d'affleurement selon les axes d'orientation.

Agnatha

0.07

Acanthodii

1.61

0.76

0.69

Elasmobranchii

8.01

9.09

Euchondrocephali

11.59

Chondrichthyes

East to W e s t

0.20

0.07

0.00

1.78

0.87

1.14

9.80

5.52

6.90

5.92

8.99

5.85

1 7 . 6 5 19.48

15,91

8.28

2 4 . 1 4 10.26 11.59

9.27

1 7 . 6 5 20.78

19.60

25.00

13.79

31.03

16.17

20.58

15.12

35.29

40.26

Actinopterygian Adult

50.19

5 9 . 8 5 6 9 . 6 6 43.38 46,94

53.20

3 5 . 6 1 43.14

20.78

Acfinopterygian Larvae

12.38

3,79

29.59

3.92

0.00

Actinopterygii

62.57

6 3 . 6 4 73.79 46.83 60.75 6 3 . 1 3 65.20

47.06

20.78

Coelacanthiformes

14.34

7.58

11,72 10.34 19.13 13.80 16.91

5.88

2.60

Strepsodus

0.o4

3.03

1.38

1.96

7.79

4.14

3,45

6.90

13.81 9.93

1.97

1.56

1.63

28.57

290 Forest Grove

Rose Canyon

North Shore Depocenter Big Spring

Allen

Blacktail

N

Acanthodes

Prol W. dicksoni Yogo C. populosum Ctadodont E falcatus Iniop 1 INop 3

Buchek

iiii!iiiiii!iiiiiii!

N

:':'::+:':::"

50

l......

hi. volsellorhinus Strepsodua sp.

N

. •

'

Larval Actinop

-

!i!!!:!i!i!i!i!i!i~::x ~\',~ ......... ~,z~i! -:':+:+1";5':" +:':+.

~

"~';

H. elegantulus 0

I

FIGURE 3 - Transects of dominant species (>3.5%) across outcrop areas of the Bear Gulch Limestone. N, present day North; NN, North during the Namurian (after Witzke 1990). See Table 1 for systematic assignments of species. Published species in italics, unpublished species designated by identification codes. Profils des nombres d'esp~ces dominantes (>3,5%) le long des aires d'affleurement du Calcaire de Bear Gulch. N, Nord actuel; NN, Nord au NaTnurien (d'apr~s Witzke 1990). Voir le Tableau 1 pour la place systdmatique des esp~ces. Esp~ces publiges en italiques, esp~ces non publides ddsignges par des codes.

DIVERSITY The total diversity of all Bear Gulch Limestone fish is calculated as 21.08 (Simpson index) and 12.07 (Margalef index) (Table 4). Diversity indices of each area, plotted on a map of the bay, show trends of decreasing diversity along transects westward away from the eastern margin. Diversity of the fish fauna is the smallest in the two areas located in the marginal facies, Buchek and Forest Grove (Fig. 4). The southeast region is fringed along its eastern border by Arborispongia (Rigby 1985) facies, particularly rich in filamentous algae centrally, and extends into the marginal facies in the Buchek area. There is a distinct decrease in diversity in the southeastern region along the transect from Blacktail through Allen to Buchek, corresponding to this facies transition. Blacktail has the highest Simpson number in the Bear Gulch, 23.2, but it is ranked significantly lower by the Margalef index (4.45). This region contains a high number of dominant species that are adapted to lurk in or maneuvet among plants, feed from the bottom, or burrow. Thus the elongate, diminutive paraselachian

Heteropetalus elegantulus and the eel-like holocephalan Harpagofututor volsellorhinus accompany actinopterygians with elongate, eel-like, deep, or flattened-venter body morphologies. Unusually, the coelacanth Caridosuctor populosum is very rare in this area, as are larval actinopterygians. This region was clearly sheltered from a high-energy flow regime but the individual areas within it are quite heterogeneous in habitat. The Allen - Surprize area (Simpson - 16.6, Margalef - 3.8) is noteworthy principally for its low Margalef diversity index, but is unremarkable in terms of the relative abundance of its fish. The Buchek area has a sufficiently low sample number that any statistics calculated here are very uncertain. Notwithstanding, three rare to uncommon species are among the principal species found, the eel-like holocephalan chondrichthyan Harpagofututor volsellorhinus, the discoidal actinopterygian "Disc" and the shortbodied coelacanth Hadronector donbairdi. The three areas of the northeast region, Big Spring, Depocenter, and North Shore, lay along a south-tonorth transect starting in a thin area of the basin facies, passing through the thickest part of the

291 TABLE 3 P e r c e n t occurrences of d o m i n a n t species (>3.5 %), n u m b e r of taxa, a n d n u m b e r s of specimens, for all fish a n d for outcrop a r e a s of t h e B e a r Gulch Limestone. Publis h e d species in italics, u n p u b l i s h e d species d e s i g n a t e d by identification codes. Composition en pourcen-

tages des esp~ces dominantes (>3,5°/o), et pour les aires d'affleurements du calcaire de Bear Gulch. Esp~ces publides en italiques, esp~ces non publides ddsigndes par des codes.

South to North

NE to SW "~

East to West r,

Species =

-~

~

.~

ca

~

-=

Acanthodes hmdi

9.80 28.57

Cladodont sp

12.99

Falcatusfalcatus

5.32

6.12

Listracanthus histrix

11.76 5.19

6.90

Heteropetalus elegantulus

8.33

Harpagofututor volsellorhinus

10.34

5.88

Iniop spl

5.19

Iniop sp2

5.19

Yogo gulo

5.08

Wendyichthys dicksoni

6.91

11.36 8.28 10.34 5.92 7.44

F cop

6.71

15.17 6.90 9 . 0 7 7.37

F prolatus

4.31

5.52

5.30 5.52

Long pelvic

4.55 8.28

Ae erin

4.55

Garden eel

4.55

4.93

4.07

Disc pec

6.90

Larval actinop

9.39

4.14

Strepsodus sp.

!13.81 9.93 29.59 6.90

Caridosuctor populosum

11.50

8.28

Hadronector donbairdi

N:

7.84

4.53 5.53 5.88

Tube snout

NO taxa:

5.40 4.55 9.80 7.79

7.79 16.37 11.38 14.47

6.90 113

38

33

20

54

4547

132

145

29

507

58

26

17

2891 615

84

51

77

Bear Gulch Limestone lens, and ending in Arborispongia facies extremely close to the northern limit of the Bear Gulch Limestone lens. They present a faunal continuum in terms of their species content as well as in terms of their relative diversity profiles. The Big Spring area (Simpson - 15.4, M a r g a l e f - 6.84) is characterized by its high percentages of Caridosuctor populosum, larval actinopterygians, and the unusual actinopterygian "Long Pelvic". The Depocenter area (Simpson - 22.3, Margalef - 10.5) has the highest number of species in all groups. There are fewer, and lower-ranking, dominant species here t han elsewhere in the bay; two of these are the coelacanth C. populosum and large full - bodied larval actinopterygians.

The shallow western end of the bay, Forest Grove (Simpson - 8.7, M a r g a l e f - 2.2) and Rose Canyon (Simpson - 16.8, Margalef - 2.7) areas, has far fewer species and far fewer specimens t han other areas, as well as poorer preservation. Acanthodes lundi, rare elsewhere in the fauna and usually represented by poorly preserved or fragmentary individuals, is the dominant member of the fish fauna at Forest Grove. Scales of Listracanthus and Strepsodes sp., large cladodont shark carcasses and teeth, and the remains of two species ofiniopterygians, "Iniop spl" and "Iniop sp3", are also common at the western margin of the bay.

Extremely high numbers of small slim larval actinopterygians, and the abundant coelacanth C. populosum lower the overall diversity of the reef- like North Shore area (Simpson - 8.36, Margalef- 7.3).

The diversity indices conceal dramatic differences in percentages of species occurrences, and dominant species, in each habitat (Tables 3,4). Two areas in particular, North Shore and Forest Grove, are

DOMINANCE

292

H

SHANNON-WEINER

Variance H

A

S

3.58

3.552

A

S

3.392 3.42 -. 0,0003 0,0002 0,0059 0,0001

A 3.165

S

A

S

A

S

A

S

3.165

2.901

2.901

3.307

3.323

3.584

3.568

0,0065 0,0001 0,0181

0

A 2.982

S

A

S

2.982

2.994

3.032

0,0031 0.0602 0,0005 0.0001 0,0041 0.0002 0,0139

0

A

S

2.628 2.628 i. . . . . . . . . . . i0,0141 0.0001 i

S I g N

21.726 21.079 27.041 23.187 18.643 16.652 36.909 16.52

MARGALEFD

13.298 12.073 8.218

4.451

7.033

3.8

5.643

2.063

9.8

6.84

EQUITABILITY

0.757

0,723

0,591

0,67

0,542

0.614

0.618

0.703

DOMINANCE-BERGER-PARKER

0.115

0.753

0.117

0,634

0.115

0.114

0.152

0.152

0.103 0.103

15.717 15.419 22.608 22.344

0.164

0.164

8.457

8.366 23.558 16.807

13.05 10.535 10.434 7.274 AREA826

AREA836

6.133

2.714

AREA834

9.657 : 8.689 4.604

2.171

AREA827

0,67

0.755

0,557

0.631

0,359

0.641

0,491

0.556

0.114

0.114

0.293

0.293

0.12

0.118

0.286

0.286

TABLE4 - Diversity, equitability and Berger-Parker dominance indices for all Bear Gulch fishes and for outcrop areas of the Bear Gulch Limestone. Data are calculated for absolute numbers of individuals at outcrop areas (A = Absolute) and for numbers of individuals standardized (S = Standardized) to percent of the total individuals at each outcrop area. Indices de diversitY, "~quitabilitd" et de dominance Berger-Parker pour t o u s l e s poissons de Bear Gulch et pour les aires d'affleurement du calcaire de Bear Gulch. Calculs faits pour le nombre absolu d'individus dans les aires (A = Absolute) et pour les nombres d'individus ramen~s au pourcentage du nombre total d'individus dans chaque aire (S = Standardized).

FOREST GROVE 4 0 - -

ROSE CANYON

NORTH SHORE

DEPOCENTER

BIG SPRING

BUCHEK

ALLEN

BLACKTAIL 4O

3O

30

20

2O

10

10 0

N

N~llk

OUTCROP *

N/'

.1" FACIES (HABITATS)

o

~ s,MPso. ~ MARGALEF

~

UAtG'/4 FIGURE4 - Transects of Simpson and Margalef D diversity indices across outcrop areas of the Bear Gulch Limestone, using percent occurrences of individuals in species at each outcrop. N, present day North; NN, North during the Namurian (after Witzke 1990). Profils des indices de diversitg de Simpson et de Margalef D le long des aires d'affleurement du Calcaire de Bear Gulch, calculgs &partir des pourcentages d'individus des esp~ces & chaque affleurement. N, Nord actuel; NN, Nord au Namurien (d'apr~s Witzke 1990). e x t r a o r d i n a r i l y d o m i n a t e d by o n e t a x o n o m i c u n i t , and h a v e B e r g e r - P a r k e r d o m i n a n c e index scores i n t h e .28 to .29 r a n g e , w h i l e B e r g e r - P a r k e r s c o r e s for m o s t a r e a s of t h e B e a r G u l c h L i m e s t o n e is i n

the range these two are larval cimens at

o f .1O to .16. D i f f e r e n t f i s h d o m i n a t e a t a r e a s : 29.5% o f t h e f i s h a t N o r t h S h o r e a c t i n o p t e r y g i a n s , w h i l e 28.5% of t h e speForest Grove are Acanthodes lundi. A.

293 timing of the seasonally fluctuating wind-driven hydrodynamic and nutrient flow conditions near the mouth of the bay. This shallow, high productivity zone provided nutrients as well as shelter for large numbers of larval fish, and proximity to the mouth of the bay and the epicontinental seaway would have provided a source of refreshment of the bay waters and a route of immigration for planktonic, opportunistic, and migratory fish species. These conditions would support the levels of diversity found in the eastern reaches of the Bear Gulch bay.

lundi is a member of the dominant fish group at only one other area, Rose Canyon, where it occurs at 9.8%, and is present at other areas only as a trivial component of the fauna. The Buchek area has a low dominance index of .10, reflecting the collection of 29 specimens from 20 different taxa and again, but numbers of specimens found for each day of quarrying effort were also extraordinarily low in and near this area. DISCUSSION

The southeastern area of the bay is almost as high in diversity by the Simpson index as is its northern counterpart, although it has far fewer species and individuals. It is most noteworthy, however, that a large proportion of the species and the individual fish in this area have body forms that deviate from those of normal swimming, maneuvering fish towards either the eellike or the deepened body form and that neither larval actinopterygians nor C. populosum are dominant parts of the local community.

The fishes of the Bear Gulch Limestone present a faunal continuum throughout their bay environmerit. There are, however, clear gradients of decreasing diversity among the Bear Gulch fishes, with the highest diversity occurring closest to the eastern margin and the entrance from the epicontinental seaway, and the lowest diversity occurring in the near-shore marginal facies to the south and west. A south-to-north transect close to and parallel with the eastern margin of the deposit traverses the deepest, thickest, most speciose, and most diverse area of the bay to terminate close to the northern margin of the Bear Gulch lens in an area dominated by sponges, algae, brachiopods, bivalves, larval actinopterygians, and the coelacanth C. populosum. High concentrations of sponges, brachiopods, and other associated invertebrates at the North Shore and along the eastern edge of the Bear Gulch Limestone indicate a relative]y high energy environment and strong nutrient supply (Grogan & Lund, ms), in association with the direction and Forest Grove

North Shore

Larval Actinops

niop

sp

3

Yogo

~

~

1

Depocenter

C. populosum

Buchek

W. dicksoni

Blacktail

W. dicksonJ

W. dicksoni

H. elegantulus

Yogo ~Larval

FProl

F. falcatus

The east-west diversity gradient culminates to the westward in a marginal facies area dominated by A. lundi, with lesser components of several chondrichthyans and scales and teeth of Strepsodus sp. A. lundi is a toothless filter-feeder that is known to have bred in fresh-brackish water lagoons to the immediate west of the Bear Gulch lens (Zidek 1980; Lund et al. 1993). The increase in abundance of A. lundi to the west can be interpreted to mean a higher level of the fine prey particles necessary for

~

Actinops

C. populosum

Disco

~ ~

~

/~)i~

F

~ Ae

cop Long Pelvic

H. volsellorhinus

Gardeneel

F. cop

H. volsellorhinus

Gardeneel

FIGURE 5 - Body forms of d o m i n a n t species at selected areas (see Table 1 for s y s t e m a t i c a s s i g n m e n t s of species). Acanthodes drawn from A. bridgei after Zidek 1980. Silhouettes des esp~ces dominantes dans quelques aires gdographiques (voir le Tableau I pour la place systdmatique des esp~ces). Acanthodes dessinds selon Acanthodes bridgei d'apr~s Zidek 1980.

294 their maintenance as well as increased seasonal fresh water input from the west and southwest, possibly signifying true estuarine conditions. Strepsodus sp., however, a large - toothed predator, is most common in shallow water marginal deposits regardless of presumed salinity or the availability of fine particulate prey. The trend in diversity indices from the eastern margin, the most diverse area, to the least diverse at the southern and western margins of the Bear Gulch Limestone are accompanied by distinctive differences in faunal composition. These distinctions among the dominant members of each area agree with the habitat patchiness indicated by previously assigned geofacies and biofacies, supporting the concept that different patterns of resource utilization were responsible for the existence of distinctive subcommunities within the bay. It is possible to extract some preliminary ecomorphological information from the fossil fishes of the Bear Gulch Limestone with which to examine some aspects of resource utilization in this bay, albeit in a tentative and preliminary manner. There is, for instance, ample evidence from the contrast between the abundance of larval actinopterygians and the record of life stage histories among the chondrichthyans to indicate that the actinopterygians were R-selected, and that the coelacanths as well as many of the chondrichthyans, as at present, were clearly K-selected and utilized strong ]ife-stage segregation (Pianka 1994; Lurid 1990; Lund & Lund 1984). The distribution of the fish also demonstrates distinctive differences in body form among the areas. Body form relates directly to swimruing function and thus may most reasonably be equated to habitat preference as a measure of niche utilization (Webb 1984; Pianka 1994). In the broadest sense of body and fin form and function, larval actinopterygians and all but one of the coelacanths (Allenypterus montanus LUND & LUND, 1984), with their rounded, low aspect ratio tails, share a common suite of maneuvering adaptations (Webb 1984), with the coelacanths specialized to the highest degree for fine positional adjustments (Lund et al. 1985). The actinopterygians W. dicksoni and '~Yogo", by contrast, typify more open-water swimming adaptations, while "Disc" and "Ae" exemplify shelter - dwelling maneuvering forms. Among the chondrichthyans, the elasmobranchs, that also demonstrate strong size-based resource partitioning (Schoener 1974), typify the body forms of active predatory swimmers, while most of the euchondrocephalans and particularly the holocephalans possess body forms with maneuvering, top-water, benthic, or near-benthic adaptations (Fig. 5). Feeding morphology is intimately related to prey preferences and thus to prey availability. The five

species of coelacanths also epitomize fine niche partitioning on the basis of gape size (prey size) as well as body form (Lund et al. 1985). Acanthodes lundi was an obligate filter-feeder. There is a great morphological differentiation of dentitions, suspensoria, dental histologies, and therefore feeding adaptations among the subclasses of chondrichthyans (Lund 1990; Lund & Grogan 1997). While many of the actinopterygians remain to be described in detail, the most prevalent actinopterygian across all but the most marginal areas of the Bear Gulch Limestone is W. dicksoni, with a unique combination of feeding-mechanism characters of a keyholeshaped notch in the snout, an extralateral bone in the gular series, and fine, forwardly-pointed teeth (Lund & Poplin 1997). CONCLUSION

Over 113 species of fish have been identified from the Namurian Bear Gulch Limestone of Montana, rendering it the largest Paleozoic fish fauna, and one of the largest fossil fish faunas known. This study of the occurrence and the diversity of the fishes of this Konservat-Lagerst~itte has revealed intrabasinal gradients between areas with distinct differences in species composition within the faunal continuum of the bay. These differences correspond to geofacies and biofacies differences in the bay, and are related to seasonal wind-driven hydrodynamics and the consequent patterns of nutrient flow, and can be further correlated to both morphological and life-history adaptations of the fish found in the bay. A preliminary assessment of the morphological adaptations of some of the dominant fish within each area presents ample evidence to support the hypothesis that these adaptations in swimming mechanisms, feeding mechanisms, and life history strategies were critical determinants of resource utilization during the Namurian. Further study of these factors, and finer-grained study of the distribution of the fish within areas, may further clarify the basis for the diversity and distribution of the fishes of the Bear Gulch Limestone. Anyhow, even if only a preliminary study, the present work shows how, in its main lines, the ecology of a marine tropical bay could be organized during Namurian times. Acknowledgments - We are grateful to Henri Lavina for editing the tables, to Eileen Grogan for advice and help, and also to all those who have helped in the excavations t h r o u g h the years, and p a r t i c u l a r l y Ron Bogosh, Carol DeBell, Bill Hopkins, B r i a n Kleinhaus, Myra Moore and B r y a n Snyder.

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Congr~s International de Stratigraphie et de Ggologie du Carbonif~re, Urbana (Illinois), 5: 437-442. LUND R. 1990 - Chondrichthyan life history styles as revealed by the 320 million years old Mississippian of Montana. Environmental Biology of Fishes, 27, 19 p. LUND R., FELDMAN H., LUND W. & MAPLES C. 1993 - The depositiohal environment of the Bear Gulch Limestone. In Montana

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RIGBY J.K. 1985 - The sponge fauna of the Mississippian Heath Formation of Central Montana. Comptes Rendus Neuvi~me

Congr~s International de Stratigraphie et de Gdologie du Carbonil~re, Urbana (Illinois), 5: 443-456. SCHOENER. T.W. 1974 - Resource partitioning in ecological communities. Science, 185: 27-39. SCHULTZE. H - P. & C.G. MAPLES 1992 - Comparison of the Late Pennsylvanian faunal assemblage of Kinney Brick Company quarry, New Mexico, with other Late Pennsylvanian lagerst~tten. New Mexico Bureau of Mines and Mineral Resources Bulletin, 138: 231-235. WEBB P.W. 1984 - Body form, locomotion and foraging in aquatic vertebrates. American Zoologist, 24: 107-120. WILLIAMS L.A. 1981 - The sedimentational history of the Bear Gulch Limestone (Middle Carboniferous, Central Montana). Ph.D. dissertation, Princeton University, 242 p. (unpublished). WILLL~MSL.A. 1983 - Deposition of the Bear Gulch Limestone: a Carboniferous Plattenkalk from Central Montana. Sedimentology, 30: 843-860. WITZK~ B.J. 1990 - Paleoclimatic constraints for Paleozoic Paleolatitudes of Laurentia and Euramerica. In Mc KERROW W.S. & SCOTESEC.R. (eds), Paleozoic Paleogeography and biogeography. Geological Society Memoirs, 12: 57-73. ZIDEK J. 1980 - Acanthodes lundi, new species (Acanthodii) and associated coprolites from uppermost Mississippian Heath Formation of Central Montana. Annals of the Carnegie Museum, 49: 49-78. R. L U N D Biology Department, Adelphi University Garden City New York 11530, U.S.A. E.mail: [email protected] C. P O P L I N Laboratoire de Pal4ontologie, UMR 8569 CNRS 8 rue Buffon F - 75005 Paris