Harris et al., eds., 2006, The Triassic-Jurassic Terrestrial Transition. New Mexico Museum of Natural History and Science Bulletin 37.
BASAL JURASSIC NONMARINE OSTRACODS FROM THE MOENAVE FORMATION OF ST. GEORGE, UTAH MICHAEL E. SCHUDACK Palaeontology Branch, Institute of Geological Sciences, Freie Universität Berlin, Malteserstrasse 74-100, 12249 Berlin, Germany, E-mail: [email protected]
Abstract—From the basal Jurassic Whitmore Point Member of the Moenave Formation near St. George, Utah, four red mudstone layers have yielded a rich, but low diversity ostracod fauna. The fauna almost exclusively consists of Darwinula, a stratigraphically long-ranging freshwater genus. Some other carapaces (largely indeterminate due to their poor preservation) resemble other freshwater forms. There are no indications at all, as far as the ostracod fauna is concerned, for any marine or lagoonal conditions. A permanent water, limnic (lacustrine) environment is suggested for this site. INTRODUCTION Ostracods are microscopically small crustaceans (arthropods) that protect their soft bodies with a pair of hard, calcareous valves that are arranged on the left and the right sides of the organism and that open ventrally. They are typically 0.5-2.0 mm long in the adult stage, but some interstitial forms are as small as 0.2 mm. Some freshwater species attain almost 1 cm, and pelagic marine forms reach more than 3 cm in length. Figure 1 provides an impression of living ostracods. These are typical freshwater forms with thin, calcified valves that have smooth surfaces (no ornamentation, such as spines or bumps) from the Cypridoidea, the major group of freshwater ostracods. Like all arthropods, ostracods change their skins several times during their lives; after sloughing off the old shells, the primarily soft, chitinous valves rapidly calcify again. Because of this, the valves of individual animals from different larval stages are often very common in sediment and sedimentary rocks, and sometimes even form distinct bodies of rock. The Ostracoda is one of the most diverse groups among living crustaceans. The number of known living and fossil species is estimated to be 33,000 (Horne et al., 2002). A realistic estimation of the true number of living and fossil species (including all those that have yet to be discovered and/or described) is even higher. The majority of the almost exclusively aquatic ostracods (only very few are adapted to semi-terrestrial life) live in the benthos, where the animals crawl over the bottom or along plants with their legs, but many of them can swim as well. Many species are pelagic. The diversity of species and abundance of individuals, together with short stratigraphic ranges, low ecological tolerances, and specialized valve shapes that reflect their ecology, have made fossil ostracods one of the most important microfossil groups for biostratigraphic and paleoecological/paleoenvironmental investigations. GEOLOGIC AND PALEONTOLOGIC CONTEXT The Moenave Formation, which produced the ostracods under discussion here, was deposited at the very beginning of the Jurassic period (see Kirkland and Milner, this volume and Lucas et al., this volume, and references cited therein). These deposits are now well known to vertebrate paleontologists. They have produced a diverse vertebrate fauna and, especially, various tracks. However, the invertebrates and microfossils, tools that are more typically useful for more refined biostratigraphic and paleoecological analyses, have not yet been investigated. Early Jurassic nonmarine ostracods have only rarely been described from the southwestern United States; some were described from Arizona by Kietzke and Lucas (1994). Ostracod faunas similar to those in the Lower Jurassic of Arizona and Utah (present paper) have been
FIGURE 1. Two individuals of Bradleystrandesia fuscata (Jurine, 1820), a living freshwater ostracod species, from a small duck pond located in Vancouver, BC, Canada (belonging to the suborder Cypridocopina). Note the thin calcareous shells with smooth surfaces, the antenna and antennula in the front of the female to the right, and the light eye spots near the anterodorsal margin in all shells. In fossil material (especially from strata older than a few hundred thousand years), only the calcareous shells are available for specific determinations. Courtesy of Ron Neumeyer (Delta, BC, Canada).
presented more often from the Upper Triassic of North America (e.g., Kietzke and Lucas, 1991, and references cited therein). SAMPLES AND MATERIAL The stratigraphic section under investigation has been described elsewhere (Kirkland and Milner, this volume). The ostracods under study come from four samples: Sample 1: Whitmore Point Member, beds 16-17 (courtesy of Jim Kirkland); Sample 2: Whitmore Point Member, beds 18 (courtesy of Jim Kirkland); Sample 3: basal part of the Whitmore Point Member, 12.22 meters above the top of the Dinosaur Canyon Member (courtesy of Andrew Milner); and Sample 4: uppermost part of the Whitmore Point Member, 18.51 meters above the top of the Dinosaur Canyon Member and 1.78 meters below the contact with the Springdale Sandstone Member of the Kayenta Formation (courtesy of Andrew Milner). Thus, all the ostracod samples are from the Whitmore Point Member, which is the uppermost member of the Moenave Formation in this
428 region; this unit consists almost exclusively of mudstones and shales. The samples were processed using standard micropaleontological methods for calcareous microfossils (i.e., treatment with hydrogen peroxide, washing and sieving, and picking and mounting on slides under a binocular microscope). Scanning electron microscope images have been made with a Cambridge S 360 at the Freie Universität Berlin. The four samples yielded abundant ostracods; however, and unfortunately, they are poorly preserved. The ostracod-bearing sediments were mostly red mudstones, a type of sediment in which calcareous microfossils, such as ostracod shells, are typically poorly preserved, if they are preserved at all due to the high amount of diagenetic alteration (oxidation and lime solution). This effect is quite typical for such sediments: macrofossils – calcareous or otherwise – may still be well preserved, but recrystallization of minerals that comprise microfossils frequently destroys their distinctive and very tiny microstructures. These microfossils may even look quite well preserved under low magnification (i.e., the outlines are clear), but upon closer examination (such as with a scanning electron microscope), most of the taxonomically important characters, such as adductor muscle scars (with a few exceptions, see Figs. 2E-F), hinge teeth, and normal and marginal pores, have been obliterated. Unfortunately, this is the case with the Moenave ostracods. Because of these effects, the principally rich (but low diversity) ostracod fauna under discussion here permits only some very tentative determinations, with the single exception of the genus Darwinula (see below). SYSTEMATICS The classification used here refers to the most recent – and, among specialists, widely accepted – subdivision of the class proposed by Horne et al. (2002), not to the traditional, though still widely used, system from the ostracod volume of the Treatise of Invertebrate Paleontology (Benson et al., 1961). CLASS Ostracoda SUBCLASS Podocopa ORDER Podocopida Most members of the Podocopida are marine, but there are three main suborders within this group that include nonmarine species: (i) Cytherocopina, (ii) Darwinulocopina, and (iii) Cypridocopina. By far, the greatest part of the St. George ostracod fauna belongs to the Darwinulocopina, though some others of the poorly preserved specimens are likely members of the Cypridocopina. SUBORDER Darwinulocopina SUPERFAMILY Darwinuloidea Darwinuloids have been one of the major groups of nonmarine ostracods since their origin in the late Paleozoic (about 300 million years ago) and remain so today. They have been asexual since at least the Jurassic, most probably since the Triassic or even earlier (Martens et al., 2003). This means that there are only female individuals. FAMILY Darwinulidae Darwinula Brady and Robertson 1885 The small, elongate, smooth ostracods of the genus Darwinula occur as far back as the Early Carboniferous (Mississippian). The genus is very conservative morphologically and thus of relatively little biostratigraphic utility. It has, however, living representatives and thus is helpful in paleoecological analysis. Species of the genus are only known from nonmarine waters, where they range from freshwater to mesohaline conditions, and are often found in high abundance. Darwinula sp. Figure 2A-G Description: The carapace is very characteristically wedge-shaped
in lateral view, with the anterior margin narrower than the posterior. The left valve is slightly larger than the right one. The typical rosette with the adductor muscle scars is situated slightly in front of mid-length (Fig. 3), but among the Whitmore Point specimens, it is only visible (but still vague) in a few, single valves (arrows on Figs. 2E, F). The larger (taller and broader) posterior part of the carapace holds the brood cavity. More refined details of the adductor muscle scar pattern or other muscle scars, of the hinge, or the normal and marginal pores are not visible. However, the shape of the outline and the muscle scar rosette makes assignment of these specimens to this genus unquestionable. Material: Eight microslides from four samples containing about 500 single valves and complete carapaces. Mostly adult females, but also several younger instars. Dimensions: Adult carapaces (20 representative specimens measured): Length 0.75-0.95 mm, height 0.37-0.54 mm, width 0.30-0.45 mm. Discussion: Due to the combined effects of poor preservation and general problems with the morphological definition of Darwinula species (see below), I prefer not to present a specific determination here. The specimens largely resemble D. sarytirmensis Sharapova, 1947, a species that has also been described by Kietzke and Lucas (1994) from the Lower Jurassic Kayenta Formation of Arizona. But the differences between these forms and other species – for example, D. oblonga and D. leguminella (now Alicenula leguminella) from the Upper Jurassic of Germany (Schudack, 1994), Darwinula sp. from the upper Jurassic Morrison Formation of the USA (Schudack et al., 1998) and many other species from Triassic, Jurassic, Cretaceous, and younger strata worldwide – are so minute (see also Kietzke and Lucas, 1994, p. 27) that it seems almost useless to assign a fossil to a Darwinula species with nothing more than the outlines of the valves and a few badly preserved muscle scars. In this context, it may be interesting to note that Kempf (1980) listed 363 species (including a few subspecies) of Darwinula in his wellaccepted ostracod database; since then, many other “new” species have certainly been added. This dimension demonstrates very well the substantial confusion concerning fossil Darwinula species. SUBORDER Cypridocopina SUPERFAMILY Cypridoidea Most ostracods of this superfamily live in nonmarine environments; specifically, in inland waters with freshwater conditions. However, several representatives can tolerate comparatively high levels of salinity (brackish conditions). Cypridoidea indet. Figure 2H Description: Various carapaces and valves with rounded anterior and posterior margins, more rounded dorsally than ventrally. Different general outlines (Fig. 2H shows just one example of many). No ornamentation or reticulation visible, but obviously originally smooth (though highly recrystallized). No hinges, muscle scars, or pores visible due to high recrystallization. Material: Four microslides from three samples containing about 150 single valves and complete carapaces, obviously from different taxa. Discussion: It makes little sense to attempt generic or specific determinations with this type of material and preservation. The overall impression, however – despite the poor preservation – is one of a typical, nonmarine ostracod association containing different types (genera, species) of freshwater Cypridoidea with their highly rounded outlines and lack of shell ornamentation. Another argument for this interpretation is their close association with abundant Darwinula specimens. BIOSTRATIGRAPHY Unfortunately, with the ostracod fossils thus far recovered from
FIGURE 2. A-G, Darwinula sp. All valves are from female adults. A, Right view of carapace. Length 0.86 mm. Sample 4. B, Right view of carapace. Length 0.86 mm. Sample 4. C, Left view of carapace. Length 0.78 mm. Sample 4. D, Left view of carapace. Length 0.85 mm. Sample 4. E, Interior view of right valve. Length 0.90 mm. Sample 1. The arrow points to the adductor muscle scars. F, Interior view of right valve. Length 0.89 mm. Sample 1. The arrow points to the adductor muscle scars. G, Oblique view of a carapace from the bottom showing the two valves with their front to the left. Length 0.75 mm. Sample 4. H, Cypridoidea indet. Strongly recrystallized valve allowing no refined determination of genus or species within this typically freshwater ostracod group (see text). Length 0.72 mm. Sample 4.
FIGURE 3. General appearance of the genus Darwinula (live animal with appendages) and its characteristic adductor muscle scar pattern; anterior to the left. Not to scale, but the carapace may be around 0.8-0.9 mm in length (modified from Horne et al., 2002).
the Moenave Formation, a refined biostratigraphic resolution of the track site is not yet possible. The 300 million year age range (Horne, 2003) of the only genus positively identified in the Moenave samples, Darwinula, renders moot any biostratigraphic interpretation based on ostracods. Because the genus still exists today (Darwinula stevensoni is one of the most common extant freshwater species), Darwinula is really a “living fossil” in the traditional sense. A more refined age determination would, perhaps, be possible with specific determinations. But because of the poor preservation of the material, these are currently impossible to make. All of the very minute characters that – at least in a modern sense – define a Darwinula (or Alicenula, a closely related, very similar genus) species, such as pore pattern, hinge structure, and muscle scars, are not visible in the Whitmore Point Member specimens because the shells are too strongly recrystallized. Species assignments comprise many more characters than just the form and outline of a specimen. Although other authors have attempted it, I, (in agreement with many others; see also the very high number of fossil species mentioned above) prefer to avoid making a Darwinula species determination on the basis of the outlines alone. At the very least, such determinations would be merely academic and surely could not form a reliable basis for biostratigraphic conclusions. In addition, there are a few other genera of limnic ostracods in the material, but again due to their poor preservation, a specific or even generic determination (which would be necessary for refined dating) is not yet possible. PALEOECOLOGY/PALEOENVIRONMENT The strata in the St. George area have been interpreted as being everything from lagoonal (i.e., marginal marine) to lacustrine (Davis, 1977; J. Kirkland, personal commun.). Ostracods prove to be good tools to resolve some questions about the paleoecology of the site. In general,
the paleoenvironmental information provided by ostracods (recently summarized by Boomer et al., 2003) far outstrips their biostratigraphic utility. In the present instance, the most telling datum is the abundance of the genus Darwinula. The other valves also appear to originate from typical freshwater associations, but due to the lack of reliable genus or species-level determinations, they should not be used for more than just a “general impression.” In the four samples examined for this study, Darwinula occurs in undoubtedly autochthonous associations This attests to the former existence of freshwater lakes in the area during the earliest Jurassic (Hettangian stage). The genus Darwinula is typical for freshwater; it has also been reported from interstitial groundwater. Meisch (2000), in his standard textbook on the European freshwater ostracods, reports that living Darwinula prefer freshwater ponds, lakes, and small streams. In some instances, however, Darwinula has been observed tolerating increased salinities up to a maximum of 15 ppm (mesohaline range). In recent lakes, Darwinula lives at depths of 0 to 12 meters, with a maximum at a depth of 6 meters. There are no living animals below 12 meters (Meisch, 2000). In some lakes, Darwinula seems to be most common, and most abundant, at 1.5 meter depth in the soft mud in front of the macrophyte belt. Fossil Darwinula, described in abundance by many authors from all around the world and from strata ranging from the Early Carboniferous through the Recent, exhibit similar environmental tolerances and preferences as Recent Darwinula. The only exception is that some authors have described the genus from hypersaline environments or, more specifically, salt or gypsum layers deposited by continental lakes or lagoons from which the water completely evaporated, leaving behind only salt and gypsum. However, in the present author’s opinion, this does not necessarily mean that Darwinula really lived (or reproduced) in a hypersaline environment (see also Neale, 1988). More probably, the valves represent the remains of individuals that died prior to significant amounts of evaporation and whose carapaces have simply been left in the salts that formed later by evaporating brine. In summary, in the area around St. George, Utah, freshwater lakes (or possibly even a single, long-lasting lake) must have existed. In these shallow lakes, a rich ostracod fauna (very similar to a typical modern freshwater lake) flourished. It is possible that these lakes may have been subject to periodic evaporation events, as is demonstrated by common trona casts in several strata (Kirkland and Milner, this volume), leaving behind a slightly increased salinity of the remaining water body. But from the composition of the ostracod fauna, this was not necessarily true. There are no indications at all to support an interpretation of the mudstones of the Whitmore Point Member of the Moenave Formation (e.g., Davis, 1977, which actually describes a portion of the Kayenta Formation) as having been deposited in a marginal marine or lagoonal environment. From the ostracod point of view, it was exclusively nonmarine (lacustrine).
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