An ichthyofauna from the subsurface Devonian of northwestern Iowa and its biostratigraphic and...

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An Ichthyofauna from the Subsurface Devonian of Northwestern Iowa and ItsBiostratigraphic and Paleoecologic SignificanceAuthor(s): Glenn W. StorrsSource: Journal of Paleontology, Vol. 61, No. 2 (Mar., 1987), pp. 363-374Published by: Paleontological SocietyStable URL: http://www.jstor.org/stable/1305328 .

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JOURNAL OF PALEONTOLOGY, V. 61, NO. 2, P. 363-374, 5 FIGS., MARCH 1987

AN ICHTHYOFAUNA FROM THE SUBSURFACE DEVONIAN OF NORTHWESTERN IOWA AND ITS BIOSTRATIGRAPHIC

AND PALEOECOLOGIC SIGNIFICANCE

GLENN W. STORRS Department of Geology and Geophysics and Peabody Museum of Natural History,

Yale University, New Haven, Connecticut 06511

ABSTRACT-A fauna of disarticulated Devonian fossil fish remains includes osteolepid, palaeoniscoid, acanthodian, placoderm, and possible agnathan material. Lithologic and biostratigraphic evidence for a lacustrine or paludal environmental interpretation is examined. Palynomorphs from the deposit suggest a Middle (Givetian) to early Late (Frasnian) Devonian age which is compatible with the faunal evidence.

INTRODUCTION

DURING the course of recent hydrogeologic studies by the Iowa and U.S. Geological Sur- veys, a well-preserved assemblage of disarticulated fish remains was recovered from a well core taken near Hawarden, Sioux Coun- ty, Iowa (Figures 1, 2). This well (#D-7, NE?/4, NE/4, SE?4 sec. 5, T95N, R47W), part of the Dakota Aquifer Project of the I.G.S. and U.S.G.S., encountered at depths of 195.4- 197.0 m a thin, previously unknown deposit of silty shales above a thin sandstone over- lying Precambrian gneissic basement and overlain unconformably by the Nishnabotna Member of the Cretaceous Dakota Forma- tion (Ludvigson and Bunker, 1979). The unit presents a varve-like, laminated appearance and consists predominantly of light-brown to dark-grey, slightly silty, organic-rich shale, with some fine sand, disseminated pyrite, thin siltstone intercalations, and phosphatic lam- inae.

Portions of the 5.1 cm diameter rock core from the shale unit were repeatedly processed in household bleach to oxidize the organic shale binder, the residue screened and dried, and systematically picked. A moderately well- preserved palynomorph assemblage of low species diversity was obtained from the shale by standard processing techniques.

GEOLOGIC SETTING

The spores recovered from the Hawarden core are difficult to examine in detail because of the presence of amorphous kerogen in the samples but a Middle or Late Devonian age is suspected (Ravn, personal commun.). A

similar unit of brown, sandy, organic-rich shale was encountered approximately 20 km to the east, though not cored, in well D-44, Iowa Geological Survey, Boyden, Sioux County, Iowa (NE/4, SE/4, NE/4 sec. 9, T96N, R44W, Sioux Co.) (Figure 2). Here the shale overlies Cambrian dolomite, probably the Upper Cambrian Bonneterre Formation (Witzke, personal commun.). Although no fish debris was observed, spores from well cut- tings confirm both a Devonian age and a sim- iliarity to the Hawarden shale unit for these sediments. The lithologies and the spore assemblages of both shales are so similar that the rocks probably represent the same stratigraphic unit.

Palynomorphs present in well D-44, all tri- lete spores, are relatively free of kerogen and include, most abundantly, Geminospora cf. G. svalbardiae (Vigran, 1964), but also Ver- rucosisporites premnus Richardson, 1965, Dictyotriletes sp., Samarisporites sp., and Convolutispora sp. (Figure 3.1-3.4). Verru- cosisporites premnus was originally reported from the Givetian (Old Red Sandstone) of Britain, while Geminospora svalbardiae is well known from the Givetian and the Frasnian. The entire floral assemblage thus seems to indicate a Middle to early Late Devonian, probably Givetian or Frasnian, age (Ravn, personal commun.). The lack of pollen in the sample supports this determination, as does the absence of the major components of the common Late Devonian (Famennian) Reti- spora lepidophyta-dominated assemblage. Ravn (personal commun.) also suggests that, while for paleoecologic reasons such data are far from conclusive, the present spore assem-

Copyright ? 1987, The Paleontological Society 363 0022-3360/87/0061-0363$03.00



364 GLENN W. STORRS

Devonian

Erosional Limits of Paleozoic (Pre-Devonian) Rocks

-r Strike and Dip of Paleozoic Rocks

7r-- Erosional Limits of Subsurface Devonian Rocks

Present Devonian Outcrop Pattern

Precambrian - Sioux Quartzite Ridge

SStudy Area

FIGURE I--Relationship of study area to Sioux Quartzite Ridge and the structural setting of mid- continental Devonian deposits (modified from Bunker, 198 1; and Ludvigson and Bunker, 1979).

blage differs markedly from those of the Em- sian and Eifelian of the Gulf Coast (see McGregor, 1979, for an overview of Devo- nian spore biostratigraphy).

Both shales apparently represent an outlier approximately 40 km northwest of the main body of Devonian shales and carbonates found throughout most of the subsurface of western Iowa (Figures 1, 2). No Devonian rocks are exposed at the surface in western Iowa. However, the Devonian marine sequence onlaps to the northwest in Iowa and is probably entirely Late Devonian in age near its northwestern margin. Middle Devonian rocks are found in central and southwestern Iowa. Additionally, both new sections are lo- cated near to the southern margin of the Sioux Quartzite Ridge. The quartzite body forming this paleotopographic high is composed of gently folded, highly resistant Proterozoic rocks which locally formed the topographi- cally highest portion of the Transcontinental Arch throughout the Paleozoic and Mesozoic (Bunker, 1981; Johnson, 1970). The presence of quartzite clasts in the basal sands (Devo-

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from Figure 1 showing subcrop patterns of pre- Mississippian age rocks. These rocks are now largely overlain by widespread Cretaceous sediments; only the Sioux Quartzite Ridge is presently exposed. -C, undifferentiated Cambrian rocks (subsurface); 0, undifferentiated Ordovi- cian rocks (subsurface); P-C, undifferentiated Precambrian rocks (subsurface); 1, Iowa Geo- logical Survey well #D-7, Hawarden; 2, Iowa Geological Survey well #D-44, Boyden.

nian) of the Hawarden core suggests the past existence of fluvial networks draining the ridge. Because of its high local relief, the ridge was probably an island during the maximum extent of the Late Devonian marine trans- gressions in the area. Erosional stripping has since removed all Devonian carbonates from around the Sioux Ridge (Witzke, personal commun.).

DESCRIPTION OF FAUNA

The faunal assemblage consists of a wide range of skeletal elements but is particularly rich in scales, teeth, and miscellaneous fragments of bone. All specimens are small (1.0- 4.0 mm) and their delicate nature makes ex- amination difficult. Nevertheless, remains of several fish taxa can be recognized, although some specimens, due to their fragmentary nature, are indeterminate. All specimens are in the paleontology repository of the Depart- ment of Geology, University of Iowa, lot number 51487.

Acanthodii. --Acanthodians are represent-



DE VONIAN ICHTHYOFA UNA FROM IOWA 365

ed in the fauna by two sections of fin spine and dozens of scales (Figure 3.5-3.8). Only slight proportional and size variation is present in the scales, which are the nonimbricat- ing studs peculiar to the Acanthodii. All are less than 1.0 mm in length. The flat, diamond-shaped crown of each scale is separat- ed from the bulbous base by a uniformly wide constriction or neck. The crown is areally larger than, and extends caudad to, the domed, ovate base and thus the latter cannot be viewed in dorsal aspect. The crown is smooth and unsculptured. Concentric growth rings can sometimes be seen under high magnification on the surface of the bony base. The scales are distinguishable from the gross- ly similar scales of thelodonts on the basis of the lack of a pulp cavity in the present specimens. Additionally, they are of the Acan- thodes histologic "type" (Gross, 1947; Orvig, 1951, 1967a, 1967b) in possessing a crown of true dentine and an acellular base, as op- posed to the Nostolepis condition (Gross, 1971; Moy-Thomas and Miles, 1971).

These scales are identical to, and can be assigned with certainty to, Acanthodes? dublinensis Stauffer, 1938. This species was proposed on the basis of scales recovered from the Upper Devonian (Frasnian) Olentangy Formation of central Ohio (Denison, 1979; Stauffer, 1938). It seems unlikely, however, that this species actually belongs to the Car- boniferous genus Acanthodes Agassiz, 1833. According to Denison (1979), the scales of A.? dublinensis resemble those of Acanthodes but are "surely not Acanthodes." Acanthodes scales can perhaps be distinguished by their somewhat concave coronal surface and their smaller posterior overhang of the crown (Wells, 1944). However, Gross (1973), in ex- amining the scale histology ofA.? dublinensis, indicates that Wells (1944) inadvisedly referred the Devonian scales to Acanthoides, a genus improperly erected by Brotzen (1934), and which is now considered a synonym for Gomphonchus Gross, 1971. Wells (1944) proposed two new scale form species ofAcan- thoides, both of which probably represent body region scale variants of Acanthodes? dublinensis.

Acanthodes? dublinensis is also known from Eifelian bone beds in the Boyle Formation of east-central Kentucky, the Jeffersonville For- mation of southeastern Indiana, and the Del-

aware and Columbus formations of central Ohio (Denison, 1979; Wells, 1944). Similar scales have been reported from the Givetian Marcellus Formation of central New York (Smith, 1910). According to Gross (1973), scales of this species are also present in the uppermost Devonian (Famennian) Maple Mill Formation of Iowa. In Canada, this species has been found in the Emsian Eids and Blue Fiord formations of Bathurst and Ellesmere islands, respectively (Vieth, 1980). Valyukyavichyus (1979) reports the presence of Acanthodes? dublinensis from the Eifelian Forkdalen Formation of Spitsbergen and the Lyaday and Kyarnava formations of the So- viet Baltic region.

A large section of an acanthodian fin spine is characterized by a rough, tuberculated surface and a serrated edge (Figure 3.7). As no other acanthodian species are represented in the assemblage by scales, the spine might be assumed to belong to Acanthodes? dublinensis. If so, it represents the first find of a spine of this species. However, a portion of a sec- ond, possibly distinct, spine may also prove to be of acanthodian origin. This is the distal tip of a pronged spine whose external surface is smooth (Figure 3.8). Although only a frac- tion of the entire spine, it most closely re- sembles specimens that have been referred to the acanthodian organ genus Gyracanthus (Eastman, 1907; Wells, 1944). Denison (1979) notes that such spines range from the Lower Devonian through the Carboniferous and that several genera are probably represented by them.

Palaeoniscidae.--A major component of the assemblage is the large number of palaeoniscoid flank scales (Figures 4, 5.1, 5.2). These primarily rhomboidal scales are often fragmentary, probably as a result of the processing method. A size range of 1.0-3.0 mm is estimated. The scales bear the typically variable ribbed external surface and spiked posterior margin of palaeoniscoid actinop- terygians and possess both a thick isopedin base and a transparent ganoine crown, all of which are primitive features.

The ridges on the glossy enamel crown run horizontally from the posterior spines for- ward until they are deflected dorsally along the anterior margin of the scale. This is in contrast with, for example, the diamond- shaped scales of Cheirolepis in which each



366 GLENN W. STORRS

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FIGURE 3-1-4, representative palynomorphs recovered from Iowa Geological Survey well #D-44, Boyden, Sioux Co., Iowa, each magnified to x 750. 1, Verricosisporites premnus; 2, Geminospora cf. G. svalbardiae; 3, Dictyotriletes sp.; 4, ?Convolutispora sp. 5-8, acanthodian remains recovered from Iowa Geological Survey well #D-7, Hawarden, Sioux Co., Iowa. 5, scanning electron micrograph of




ridge intersects the anterior margin (Gross, 1947; Pearson and Westoll, 1979). On the present scales the anterior upturned portion of each ridge is itself ornamented with small, subhorizontal striae (Figures 4, 5.1). Each scale exhibits a dorsal articulatory peg, when not broken, and a low median ridge runs an- terodorsally-posteroventrally across the smooth inner surface. No prominent articulatory facet is found along the anterior border of the scale.

The overall scale morphology of the Iowa material differs dramatically from those of most known Devonian palaeoniscoid genera, whose scales are either not rhomboidal in outline (e.g., Cheirolepis and Orvikuina), are not of similar proportions (e.g., Ligulalepis), or else display striking and unique ornamentation (e.g., Dialipina, Mimia, and Oso- rioichthys (Stereolepis)) (Casier, 1952, 1954; Gardiner, 1984; Giffin, 1980; Pearson and Westoll, 1979; Schultze, 1968, 1977). Con- versely, the Iowa scales are most similar to taxa such as Moythomasia (Aldingeria) or Stegotrachelus, although a wide range of scale ornamentation is known from the many species assigned to the first of these genera. Tegeolepis (Actinophorus) is known from the Famennian Ohio Shale (Gardiner, 1963; Dunkle and Schaeffer, 1973) and has rhomboidal scales, but these are very poorly known and are perhaps too thin to be equated with the Iowa specimens.

Stegotrachelus is known only from a single species, S. finlayi, from the Givetian of Scot- land (Woodward and White, 1926). While Gross (1942) suggested that this fish is possibly congeneric with Moythomasia, Gardi- ner (1963) maintained, on the basis of skull morphology, that the two are quite distinct, although he placed them together in the family Stegotrachelidae, along with the Missis- sippian Kentuckia, and later with Mimia from the Frasnian of Australia (Gardiner, 1984; Gardiner and Bartram, 1977). The rhomboidal flank scales of Stegotrachelus are similar proportionally to those of Moythomasia and to those of the present specimens, but

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are each marked by a single, coarse, diagonal rib. A typical dentine vascular system exists internally (Aldinger, 1937).

Two poorly known species of what appears to be Moythomasia are known from North America from the Frasnian "Portage shales" of western New York (Clarke, 1885; East- man, 1907; Gardiner, 1963; Hussakof and Bryant, 1918; Williams, 1886). However, the scales of neither Mothythomasia devonica (Clarke) nor M. antiqua (Williams) (both originally described as Palaeoniscus, and later as Rhadinichthys) exhibit external ornamentation which is identical to that of the well core specimens. The scales of Moytho- masia devonica are characterized by numerous punctae that are arranged in single rows between the horizontal ribbing. These punctae are an external expression of the prominent vascular canals inhabiting the median dentine layer of the scale (Orvig, 1957). This is a pattern similar to that seen in M. nitida (Gross, 1953; Jessen, 1968) and M. perforata (Gross, 1942, 1953) of the Middle Devonian of central Europe, and in M. durgaringa of the Upper Devonian of Australia (Gardiner,

Acanthodes? dublinensis flank scale, lateral aspect, anterior to left, bar scale equals 100 microns; 6, Acanthodes? dublinensis flank scale, ventral aspect, anterior to northeast, bar scale equals 100 microns; 7, regular light photograph of acanthodian fin spine fragment, approximately 2.5 mm long; 8, fin spine tip, ?Gyracanthus sp., approximately 1 mm long.



368 GLENN W. STORRS

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1984), but no punctae are found in the present specimens. The scales of M. antiqua are apparently dissimilar in that they are essen- tially smooth with little or no ribbing, much as in the German M. laevigata (Gross, 1953), while possessing relatively few punctae. Moy- thomasia antiqua is known only from a few scales and, if distinct, remains a problematic species. Moythomasia striata (Gross, 1933a, 1933b, 1953; Jessen, 1968), like M. laevigata from the lower Upper Devonian of Germany, has no punctae and displays scale ribbing similar to that of the Iowa material, but the horizontal ribs are not upturned at their anterior ends. The scales of M. striata also possess large anterior articulatory facets which are noticeably lacking from the Iowa specimens.

While a large degree of body scale vari- ability is known among palaeoniscoids, it is apparent that the Iowa scales cannot be equated with any known species of Moytho- masia. However, because of the general sim- ilarity of the present specimens to scales of this genus, they can be most prudently clas- sified as ?Moythomasia sp.

Devonian palaeoniscoids, as the earliest representatives of the group, are extremely important from an evolutionary and a paleo- biogeographical point of view. Eastman (1907) indicated that indeterminate Devo- nian palaeoniscoid scales have also been reported from Kentucky, and scales and teeth can be locally common in the acetic acid res- idues of many Devonian rocks (Schultze, personal commun.). However, the Devonian forms are still poorly understood, especially those known only from isolated scales. Fur- ther interpretations must await better material and, with it, the opportunity for a more comprehensive study.

A delicate fragment of tooth-bearing bone (Figure 5.3) is present in the Iowa sample. This is presumably palaeoniscoid in origin, and probably pertains to the same taxon as the scales. Three teeth, probably part of the marginal dentition, are preserved, each with the characteristic acrodin enameloid cap of palaeoniscoids.

Osteolepidae. - Some of the largest and most common elements in the assemblage are the scales of rhipidistian crossopterygians (Figure 5.4-5.6). All such scales are noncy- cloid and belong to the family Osteolepidae. They all bear a full external covering of cosmine as is typical of most members of the family and the minute pore openings are clearly visible (for details of cosmine morphology see Gross, 1956; Meinke, 1984; Or- vig, 1969a; Thomson, 1975). The scales are largely rhomboidal and range from approximately 1.0 to 3.0 mm in length.

The anterior border of each scale usually corresponds in shape to the margin of the finished cosmine surface. An interesting fea- ture of the cosmine margin is the ribbed pattern observed at high magnification (Figure 5.5). The smooth medial surface of the scale displays the typical diagonal ridge of Devo- nian osteolepids.

A single bone fragment from the well core appears to be part of an osteolepid scale in which cosmine resorption has taken place, leaving a single tubercle of cosmine on an otherwise naked bone surface. Other osteolepid scales in the sample possess portions of the lateral line system as a string of large pits recessed into the external surface of the scale (Figure 5.6).

Scales of this general type are common among Devonian osteolepiform genera. Jar- vik (1948) has pointed out that few differ-

FIGURE 5-1-3, palaeoniscoid remains. 1, scanning electron micrograph of representative flank scale minus articulatory peg, possibly ?Moythomasia sp., anterior to left, bar scale equals 100 microns; 2, regular light photograph of similar scale, anterior to left, approximnately 3 mm long; 3, fragment of tooth-bearing bone, approximately 1.5 mm long. 4-8, osteolepid \remains. 4, representative flank scale of osteolepid, genus indeterminate, anterior to right, approximately 3 mm long; 5, scanning electron micrograph of anterior cosmine margin of similar scale, anterior to northwest, bar scale equals 10 microns; 6, scanning electron micrograph of osteolepid scale with portion of lateral line system, anterior to left; bar scale equals 100 microns; 7, indeterminate osteolepid tusk, approximately 1 mm long; 8, indeterminate cosmine-bearing bone, possibly part of an osteolepid lepidotrichium, approximately 2.5 mm long. 9-12, problematic bone fragments. 9-11, possible placoderm fragments, 9 and 10, approximately 1 mm long, 11, approximately 2.5 mm long; 12, possible agnathan fragment, approximately 1 mm long.



370 GLENN W. STORRS

ences exist among the scales of known osteolepid taxa, and that such isolates are almost useless in taxonomy. Glyptopomus from the Famennian of Scotland (Agassiz, 1844) and Panderichthys from the Frasnian of Latvia (Gross, 1941) can be ruled out in this instance because the scales of these fish are characterized by bony ornaments and no cosmine. Megistolepis from the Frasnian of Russia (Obruchev, 1955) is similarly unusual in that it is a very large form having scales averaging 2.0 cm in length.

The known Middle Devonian genera all bore small, nondescript, rhomboidal scales with cosmine. Gyroptychius, Osteolepis, and Thursius of the Eifelian and Givetian had such scales, are part of the circum-North At- lantic fauna discussed by Thomson (1976), and could theoretically have been present in Iowa. All were first reported from Scotland, but Gyroptychius is particularly well known from Greenland (Jarvik, 1950). Our knowl- edge of Devonian osteolepid distributions is limited by the presently small number of useful fossil localities. A similar North Atlantic taxon is the Frasnian Latvius which, in ad- dition to the Baltic region, has also been reported from New Brunswick (Greiner, 1977). Devonian osteolepid occurrences have previously been reported in North America from Pennsylvania (Thomson, 1968, 1972), Ne- vada (Gregory et al., 1977), and Colorado (Denison, 1951). All of these are from the Upper Devonian, few have been generically identifiable, and in none are the scales re- markably different from typical osteolepids. The Iowa scales must also be considered genus indeterminate.

Numerous isolated teeth in the collection (2.0 mm or less in length) can be attributed to crossopterygian fish. Most were probably contributed by osteolepids. Figure 5.7 shows a palatal tusk presumed to have come from an osteolepid. As is typical for Middle De- vonian osteolepids (Schultze, 1970), there is no labyrinthine infolding of the outer enameloid layer. One additional bone or scale in the sample bears an external cosmine layer. This is a relatively long, narrow element which may be part of an osteolepid lepidotrichium (Figure 5.8).

Placodermi. -A small number of problematic bone scraps in the collection seemingly

belong to placoderms, but because of their fragmentary nature these are of uncertain af- finities. One fragment of bony exoskeleton (Figure 5.9) displays several raised cylindri- cal tubercles on its surface. The bases of these tubercles are stellate, or supported by radial struts. This pattern is superficially similar to the peculiar dermal ornamentation of Tra- chosteus clarkii Newberry, 1889, a poorly known placoderm from the Famennian Cleveland Shale of Ohio. Two additional dermal bone fragments (Figure 5.10, 5.11) display rounded tubercles or nodes on their external surfaces which are very like those found in antiarchs such as Pterichthyodes from the Middle Devonian, although certain arthrodires, for example Coccosteus (also Middle Devonian), exhibit similar nodes.

?Agnatha. -What may be a small cephalic shield polygon is the only indication of possible agnathans in the sample (Figure 5.12). This polygonal plate displays six raised tubercles on its outer surface which are smooth and hemispherical. While the plate appears similar to those figured by Stensi5 (1932) for various species of Cephalaspis, its isolated nature prevents identification. There is also some question as to whether the plates of Cephalaspis (well known from the Lower and Middle Devonian of the circum-North At- lantic region) constitute independent tesserae or rather only polygonal areas of the cephalic shield which have become separate through fracture (Wells, 1944). The cephalic shield exoskeleton is known to have been contin- uous in most cephalaspids.

Ohioaspis tumulosus Wells, 1944, of the Middle Devonian of the Midwest and Canada displays similar isolated plates, but these possess quite different stellate tubercles. Ohioas- pis may not be a cephalaspid but is perhaps congeneric with the rhenanid placoderm As- terosteus (Gross, 1973; Orvig, 1969b). Rhen- anids do possess isolated tesserae and it is therefore possible that the Iowa plate is from a placoderm. Histological studies have not been made on placoderm tesserae (Denison, 1978).

DISCUSSION

As the Devonian stratigraphic sequence of northwestern Iowa is virtually unstudied, this assemblage presents potentially important




information for analysis of mid-continental paleoenvironments and paleogeography. Un- fortunately, while there is ongoing progress in the field, Devonian vertebrate biostratigraphy is currently a crucial but poorly refined science. Devonian stage determinations, for example, are always difficult to make, and accurate correlations between marine and nonmarine sections are still needed (Dineley, 1984). Similarly, the biostratigraphic reso- lution provided by the Iowa faunal assemblage is, at present, insufficient to distinguish between a Middle and a Late Devonian age for the Hawarden shale unit. However, the recovered fish material is certainly compatible with the Givetian-Frasnian age suggested by the preserved palynomorphs, and this interpretation should be accepted.

Acanthodes? dublinensis is known com- monly, though not exclusively, from the Mid- dle Devonian, and Moythomasia is found in both Middle and Upper Devonian rocks. Os- teolepiform rhipidistians are especially common in Middle Devonian faunas, although the work of various authors has begun to doc- ument a broad diversity of osteolepiforms in the Late Devonian as well (Denison, 1951; Lebedev, 1983; Obruchev, 1955; Thomson, 1968, 1972; Vorobyeva, 1977). Among osteolepids possessing scales similar to the present specimens, Latvius, for example, is from the Frasnian but Gyroptychius, Osteo- lepis, and Thursius are all Middle Devonian in age. Thomson (1976) has suggested that the traditional paucity of cosmine-bearing osteolepids from the Upper Devonian rocks of Europe and Greenland (where the osteolepids are well represented in the Middle De- vonian) is perhaps related to environmental factors and may have bearing on the func- tional and evolutionary significance of cosmine. The Cephalaspida is one of the few agnathan groups to have survived into the Givetian but it disappears in the Frasnian (Westoll, 1979). As previously stated, the present agnathan occurrence is questionable.

The paleoenvironment represented by the Hawarden shale unit cannot be determined through faunal evidence alone, as the habitat of few Paleozoic fish groups is known un- equivocally. Acanthodians were marine fish during the Silurian, but they had invaded freshwater habitats by the beginning of the

Devonian and became very successful there (Moy-Thomas and Miles, 1971). Denison (1979) noted that the Acanthodidae, to which Acanthodes? dublinensis may perhaps belong, was a predominantly freshwater family. However, A.? dublinensis has been found to date only in presumed nearshore marine deposits. After the Lower Devonian, crossopterygians are believed to have been over- whelmingly freshwater forms, although Thomson (1969) pointed out that they were not exclusively so and has suggested that crossopterygians may actually have been primarily associated with proximal marine environments in the Devonian (Thomson, 1980). The presence of cosmine-bearing osteolepids may, at times, indicate a close association with nearshore marine conditions (Thomson, 1976). Palaeoniscoids also are typically found in both freshwater (e.g., Ste- gotrachelus) and marine (e.g., Tegeolepis and Moythomasia) deposits.

Certain placoderm groups were, as far as is known, restricted to marine environments but others thrived in fresh water, particularly in the Middle Devonian. Many arthrodires lived in fresh water as did most antiarchs (Denison, 1978). While the Agnatha began as marine organisms (Denison, 1956; Rob- ertson, 1957; White, 1958), some became freshwater forms in the Silurian or Devonian, and osteostracans were predominantly so. In either case, freshwater or marine provenance, the preservation and delicacy of many of the Iowa specimens would seem to preclude the possibility of long-distance transport.

Lithologic and stratigraphic evidence is more useful and argues in favor of a restricted lacustrine or paludal facies for the well core shale. The "varved," low energy appearance of the unit, its dissimilarity to the Devonian shale/carbonate marine sequence to the south, and its position as an outlier on basement so near to the Sioux Ridge are relevant. So too is the richness of organic material and the abundance of disseminated pyrite, suggesting not only bottom sediment anoxia, but also a rich supply of terrestrially derived iron. The presence of terrestrial spores and the lack of typical marine invertebrates and conodonts also favor a fresh water interpretation. The possibility that the true environment of de- position was part of a similarly restricted



372 GLENN W. STORRS

nearshore marine or estuarine environment cannot, however, be ruled out at this time.

ACKNOWLEDGMENTS

I want to thank particularly K. S. Thom- son, who suggested the topic for this paper and provided invaluable advice and assis- tance during its preparation. B. J. Witzke kindly brought the existence of this deposit to our attention, provided the samples, and supplied extremely helpful information throughout the course of my research. Thanks are due also to R. L. Ravn, who generously furnished the palynomorph identifications and photographs, and to H.-P. Schultze, L. J. Hickey, B. Schaeffer, P. Forey, and M. Wil- liams, who contributed useful help and/or advice. A. S. Pooley, W. K. Sacco, R. Board- man, and R. Purdy provided technical assis- tance. The manuscript was kindly reviewed by K. S. Thomson, B. J. Witzke, D. L. Dine- ley, and H.-P. Schultze. This study was supported by National Science Foundation grant no. DEB 79-21746 to K. S. Thomson, by a Yale University Louis Pirrson Fellowship, and by the Department of Geology and Geo- physics, Yale University.

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