Middle Permian (Guadalupian) Fusulinids from Subsurface Midland Basin, West Texas, USA, Including Paleoenvironmental Microspheric Parafusulina

Well-preserved fusulinids are not common in Middle Permian (Guadalupian Series) subsurface carbonate strata of the Permian Basin because of pervasive dolomitization of the strata through most of the region. Most Guadalupian fusulinid data from the Permian Basin region are derived from outcrop studies of limestone strata in the Guadalupe, Delaware, Apache, and Glass Mountain ranges of far West Texas (Fig. 1A). It is important, therefore, to document preserved fusulinid assemblages from subsurface cores in Guadalupian carbonate strata of the region to further build the biostratigraphic database, improve stratigraphic correlations, and enhance the ability to unravel the stratigraphic architecture and history of the Guadalupian Series of the Permian Basin stratotype region.

The present study documents moderately well-preserved specimens of the fusulinid genus Parafusulina from a Middle Permian (Guadalupian) progradational carbonate unit deposited within the central part of the Midland Basin during a sea level lowstand (Fig. 1). The basinal lowstand carbonate unit correlates to a marked unconformity capping the strata of the upper part of the San Andres Formation (commonly called the Upper San Andres, e.g., Ramondetta, 1982) on the adjacent Central Basin Platform and Northwest Shelf paleotopographic highs (Figs. 1, 2).

Subsurface conventional cores from three wells through the lowstand carbonate unit were described and interpreted by the senior author, and facies ranging from peritidal to slope/basin were recognized (Fig. 3). Nearly all of the peritidal to outer ramp facies are thoroughly dolomitized, and fusulinids occurring in foreshoal middle to outer ramp facies are mostly skelmoldic. The preserved fusulinids of the genus Parafusulina occur in the lower 20 ft (6.1 m) of the outermost ramp packstone facies that is only partially dolomitized. The outermost ramp interval containing preserved fusulinids is overlain by mid-to-outer ramp fusulinid-skeletal dolopackstone facies with skelmoldic fusulinids and is underlain by upper slope facies composed of dark-gray quartz siltstone (Figs. 3, 4, and 5). Significantly, the lower 1 ft (0.3 m) of the outermost ramp packstone facies interval contains microspheric morphotypes of Parafusulina.

The senior author described subsurface cores from three wells through a Middle Permian (Guadalupian) lowstand carbonate reservoir unit in the central part of the Midland Basin (Fig. 1). The lowstand carbonate unit was previously discussed by Todd (1976). On the request of the well operator, the well names are not used herein, and the three cored wells are referred to as Well No. 1, Well No. 2, and Well No. 3 (Fig. 1B). The purpose of the core descriptions, and the facies and sequence stratigraphic interpretations, was to aid in characterizing the regional reservoir quality of the lowstand carbonate unit.

All three cores described are composed of mostly pervasively dolomitized carbonate strata, but the core from the most downdip Well No. 1 contained an outermost ramp facies interval that is only partly dolomitized and contains moderately well-preserved fusulinids (Figs. 1, 3, 4, 5). The outermost ramp dolomitic limestone shallows-upward into completely dolomitized middle to outer ramp facies with common skelmoldic fusulinids (Figs. 4, 5). Subsequent to the carbonate reservoir study, additional core samples were taken from the outer ramp fusulinid-bearing dolomitic limestone in the Well No. 1 core for this fusulinid biostratigraphic study. The fusulinids in that interval are preserved well enough for confident species identifications and age dating. In addition, the samples from the lower 1 ft (0.3 m) of the outermost ramp facies in the core from Well No. 1 contained microspheric morphotypes of Parafusulina. The preserved fusulinids from the outermost ramp facies in the Well No. 1 core are illustrated in Figures 6–15.

Fusulinid assemblages discussed herein are from Well No. 1 of the three Middle Permian (Guadalupian) subsurface cores taken in Midland County, West Texas. Paleogeographically, the three cored wells are located in the central part of the Late Paleozoic Midland Basin of the Permian Basin region (Fig. 1).

The lowstand carbonate stratigraphic unit discussed herein is underlain and overlain by basinal siliciclastic facies. The carbonate unit was originally assigned to the Upper San Andres, but Todd (1976) recognized that the unit was deposited within the structurally low Midland Basin during a post-Upper San Andres sea level lowstand, and it actually correlates with the marked unconformity on the top of the Upper San Andres on the adjacent Central Basin Platform paleotopographic high. That well-known unconformity capping the Upper San Andres on the adjacent platform and shelves is characterized by leaching, erosion, karst, and associated hydrocarbon reservoir development (Kerans & Ruppel, 2020). Correlating the lowstand carbonate unit on the basin floor with the post-Upper San Andres unconformity suggests that the unit might be better assigned to the Grayburg Formation, which overlies the Upper San Andres on the shelves (Fig. 2). Wilson et al. (2019) recently discussed this lowstand carbonate reservoir unit and dealt with the lithostratigraphic nomenclatural problem by referring to the lowstand carbonate package as the Grayburg-San Andres (GYBG-SNDR) reservoir unit.

As shown by the upper Leonardian and Guadalupian lithostratigraphic chart of Kerans & Ruppel (2020, fig. 3), the Lower San Andres Formation (lower Guadalupian) on the Northwest Shelf and Central Basin Platform is capped by a major unconformity, which is represented in the adjacent Delaware and Midland basins by the slope-to-basin siliciclastic facies of the Brushy Canyon Formation and equivalent units (Fig. 2). Overlying that major unconformity on the shelves are the carbonate strata of the Upper San Andres Formation, which are capped by another, more minor unconformity that has features indicating subaerial exposure, leaching, and karst development (Fig. 2). During that hiatus, the lowstand carbonate unit discussed in this study was deposited on the westward sloping floor of the Midland Basin. Overlying the Upper San Andres capping unconformity on the adjacent Central Basin Platform, the lower part of the overlying Grayburg Formation is composed of mixed sandstone and dolomite facies (Kerans & Ruppel, 2020). Barnaby & Ward (2007) documented a post-Upper San Andres sea level fall of more than 30 m in the Guadalupe-Brokeoff Mountains outcrop area of far West Texas and described the mixed siliciclastic-carbonate facies and sequences of the Grayburg Formation in the Permian Basin region. Kerans et al. (2013) described the Grayburg-Queen composite sequence CS11, which is composed of high frequency sequences HFS G10-G14 in the Guadalupe Mountains, and they designated the basal Grayburg Formation HFS G10 as a lowstand (LST) forced regression wedge (Fig. 2).

Based on recent seismic stratigraphic analyses in the Midland Basin, and on comparisons with Middle Permian carbonate successions in other parts of the Permian Basin, Kerans & Ruppel (2020, figs. 3, 30) and Nance (2020, fig. 1) have suggested that the Midland Basin lowstand carbonate unit represents the Guadalupian G10 sequence and should be assigned to the Grayburg Formation (Fig. 2). That stratigraphic assignment is supported by the stratigraphic and fusulinid biostratigraphic interpretations herein.

The Midland Basin Grayburg G10 lowstand carbonate unit discussed herein consists of the same shallow-water carbonate ramp facies assemblage as the well-known San Andres and Grayburg carbonate successions deposited on the adjacent Central Basin Platform and Northwest Shelf paleotopographic highs. As described by Todd (1976), the lowstand carbonate strata are composed of prograding oolite shoals and associated inner through outer ramp facies. The study area of Todd (1976) was located in Upton and Reagan counties, Texas, just south of the current study area in Midland County, Texas (Fig. 1).

The three cored wells studied herein confirm the interpretation by Todd (1976) that the Midland Basin lowstand carbonate unit is composed of prograding shallow-water carbonate ramp facies, including oolitic dolograinstone shoal facies (Fig. 3). The most downdip cored section in Well No. 1, which is the subject of the present study, consists of cycles composed of prograding oolite shoals overlying more open marine foreshoal mid-to-outer ramp facies and deeper-water slope-to-basin facies (Figs. 4–6). The outermost ramp facies in the Well No. 1 core contains the preserved fusulinids discussed in this study. The most updip cored section in Well No. 2 consists of mostly inner ramp, restricted marine, oolitic shoal, lagoon, and peritidal facies (Fig. 3). The cored section in Well No. 3, which was located paleogeographically between the other two wells, contains inner ramp lagoon, oolite shoal, and middle ramp dolomitic facies (Fig. 3).

Fusulinids are common throughout the more normal marine foreshoal mid-to-outer ramp carbonate facies in the cores studied, but most of the carbonate strata are dolomitized, and the fusulinids are skelmoldic or recrystallized. Moderately well-preserved fusulinids occur only in the lower 20 ft (6.1 m; 4769–4789 ft = 1453.1–1459.7 m) of an outermost ramp packstone facies interval in the Well No. 1 core (Figs. 3–5). The outermost ramp facies with preserved fusulinids underlies a completely dolomitized skelmoldic fusulinid dolopackstone facies, and it immediately overlies dark-gray quartz siltstone upper slope facies (Figs. 4, 5) that contain sparse, downward decreasing, crinoid ossicles and other small bioclasts (Figs. 3–5). The lowermost 1 ft (0.3 m) of the outermost ramp fusulinid-bearing packstone facies contains microspheric specimens of Parafusulina, which are discussed in a separate section below.

The Permian Basin of West Texas-New Mexico is the stratotype region for the global Middle Permian Guadalupian Series and its component Roadian, Wordian, and Capitanian Stages (Fig. 2), which have been generally considered to be equivalent to lower, middle, and upper Guadalupian (Glenister et al., 1992, 1999; Wardlaw et al., 2000). The history of the chronostratigraphic classification of the three Guadalupian stages are discussed by Wardlaw et al. (2004), Henderson et al. (2012), and Henderson & Shen (2020).

Most of the published taxonomic and biostratigraphic data regarding Guadalupian fusulinids in the Permian Basin region come from limestone outcrop studies in the Guadalupe, Delaware, Apache, and Glass Mountain ranges of far West Texas. The Guadalupian lithostratigraphy and sequence stratigraphy of the Permian Basin region have been recently summarized by Kerans & Ruppel (2020) and Nance (2020).

Before further discussions, changes in the nomenclature of the classic Middle Permian lithostratigraphy of the Glass Mountains should be noted. King (1931) originally described the Permian Word Formation in the Glass Mountains as including four limestone units referred to as the First, Second, Third, and Fourth Word limestones. Cooper & Grant (1964, 1966) later gave those units formal lithostratigraphic names, the First Word Limestone being named the Road Canyon Formation, and the Second, Third, and Fourth Word Limestones being named the China Tank Member, Willis Ranch Member, and the Apple Ranch Member of the Word Formation, respectively (Fig. 2).

Dunbar & Skinner (1931, 1937) described several Guadalupian fusulinid species from the Permian Basin outcrop areas mentioned above and outlined the first Guadalupian fusulinid zonation of the region. Ross (1963, 1964) later described additional Guadalupian fusulinid species from the Word Formation in the Glass Mountains. Ross (1963, fig. 4) recognized Parafusulina deliciasensis Dunbar & Skinner as the only species of Parafusulina to range throughout the Word Formation. He cited P. sullivanensis Ross, P. bosei Dunbar & Skinner, P. lineata Dunbar & Skinner, P. ironensis Ross, and P. wildei Ross as typifying the lower part of the Word from the First Word Limestone (= Road Canyon Formation) up to the base of the Third Word Limestone (= Willis Ranch Member of Word Formation; Fig. 2). He reported P. sellardsi Dunbar & Skinner from the Second Word Limestone (= China Tank Member of Word Formation) through the Third Word Limestone, where it reached its peak abundance. And the uppermost Fourth Word Limestone (= Apple Ranch Member of Word Formation) was characterized by the large elongate P. antimonioensis Dunbar.

There are also other applicable studies on Leonardian-Guadalupian fusulinid faunas from Mexico, Central America, and the northern part of South America in which new species were described that were later found to also occur in the Permian Basin region (e.g., Dunbar, 1939, 1944; Thompson & Miller, 1944, 1949; Kling, 1960; Wardlaw et al., 1979; Vachard, 1993; Vachard & Fourcade, 1997; Vachard et al., 1997, 2004). Wilde (1955, 1975, 1977, 1986a, 1986b, 1988, 2000), Wilde & Todd (1968), and Wilde & Rudine (2000) discussed various aspects of the Guadalupian fusulinid biostratigraphy of the Permian Basin. Wilde (1990) published an important summary paper that proposed a fusulinid zonation for the Pennsylvanian and Permian strata of the Permian Basin region. In the Guadalupian Series, he proposed six fusulinid zones, which were, in ascending order, the Zone of Parafusulina boesei–Skinnerina (Zone PG-1, lower Roadian), the Zone of Parafusulina rothi–P. maleyi (Zone PG-2, “Brushyan”, middle Roadian), the Zone of Parafusulina lineata–P. deliciasensis (Zone PG-3, lower “Cherryan”, upper Roadian), the Zone of the latest Parafusulina (Zone PG-4, lower Wordian), the Zone of Polydiexodina (Zone PG-5, upper Wordian), and the Zone of Paraboultonia splendens (Zone PG-6, Capitanian; Fig. 2). Wilde (2000) later discussed the characteristics and potential problems of these zones.

Yang & Yancey (2000) discussed the Guadalupian fusulinids of the Glass Mountains and described several new species. They recognized seven Guadalupian fusulinid zones (Fig. 2). In the lower Guadalupian Roadian Stage they recognized three zones, which were, in ascending order, the Parafusulina boesei Zone, the Parafusulina rothi Zone, and the Parafusulina trumpyi Zone. In the middle Guadalupian Wordian Stage, they recognized the lower Wordian Parafusulina sellardsi Zone and the upper Wordian Parafusulina antimonioensis Zone. Sixteen holotypes of Roadian/lower Wordian Parafusulina species from the Permian Basin region are illustrated in figure 5 of Nestell et al. (2019). In the upper Guadalupian Capitanian Stage, Yang & Yancey (2000) recognized, in ascending order, the Polydiexodina shumardi Zone, the Reichelina lamarensis Zone, and the Lantschichites spendens Zone. The fusulinid zonations proposed by Wilde (1990) and Yang & Yancey (2000) are compared to each other and to the currently accepted conodont-based chronostratigraphy in Figure 2 herein.

Prior to the 1990s, Guadalupian stage boundaries in the Permian Basin were most often defined using fusulinid biostratigraphy (as done in the above-mentioned studies on the Glass Mountains), but as the pelagic conodonts became the favored global Late Paleozoic index fossils, Guadalupian stage boundaries were redefined and shifted based on conodont species ranges (Glenister et al., 1992, 1999; Wardlaw et al., 2004; Henderson et al., 2012; Henderson & Shen, 2020; Fig. 2). Nestell & Nestell (2006, 2009), and Nestell et al. (2006, 2015) published several papers on the integrated biostratigraphy of Guadalupian strata in the Permian Basin region, and Nestell et al. (2019) recently reviewed and correlated Guadalupian conodont, fusulinid, and radiolarian biostratigraphy of the Permian Basin region. Significantly, the stratigraphic positions of the new conodont-based Guadalupian stage boundaries are different from the former fusulinid-based stage boundaries, which creates some confusion in correlations around the Permian Basin region, as discussed briefly by Wahlman et al. (2020; Fig. 2).

Todd (1976) showed that the San Andres Formation is a diachronous lithofacies. In the updip shelf areas, such as the San Andres stratotype section in the San Andres Mountains of south-central New Mexico, the San Andres strata have been age-dated as Leonardian based on ammonoids (Kottlowski et al., 1956, p. 61). In the Guadalupe Mountains, 100 miles (160 km) downdip to the southeast and located along the margin of the Delaware Basin, Skinner (1946, p. 1865) identified Parafusulina rothi Dunbar & Skinner, P. deliciasensis, P. sellardsi, and P. lineata from San Andres strata, an assemblage that age-dated the strata as middle Guadalupian (Wordian) according to the former fusulinid-based Guadalupian stadial zonation. Wilde & Todd (1968, p. 16) agreed that all the San Andres strata in the Guadalupe Mountains were Guadalupian in age. Wilde (1990) assigned the interval with that fusulinid assemblage to his PG-3 Zone (lower “Cherryan”), which is the equivalent of lower Wordian in the former fusulinid-based chronostratigraphic zonation (e.g., Yang & Yancey, 2000), but that interval is now considered upper Roadian in the currently accepted conodont-based zonation (Nestell et al., 2019; Fig. 2).

Upper San Andres strata on the shelves of the Permian Basin were previously age-dated in the fusulinid-based chronostratigraphy as early Wordian, but based on the current conodont-based chronostratigraphy are currently age-dated as late Roadian (Nestell et al., 2019, fig. 1; Fig. 2). As pointed out above, the Midland Basin lowstand carbonate unit discussed in this study correlates to the post-Upper San Andres unconformity and therefore is considered correlative with the lower Grayburg Formation and the G10 HFS of Kerans et al. (2013) and Kerans & Ruppel (2020). The Grayburg Formation was previously age-dated with fusulinids as being entirely Wordian in age, but in the new conodont-based chronostratigraphy it is now age-dated as the late Roadian and earliest Wordian (Fig. 2).

As discussed above, Todd (1976) cited fusulinid data from the Midland Basin lowstand carbonate unit generated by John W. Skinner, who identified Parafusulina deliciasensis Dunbar & Skinner 1936 and P. lineataDunbar & Skinner 1937 from the cores they studied, and Skinner age-dated those fusulinids as middle Guadalupian. Unfortunately, none of the fusulinids identified from the cores studied by Todd and Skinner were documented with illustrations. Some years later, Garner Wilde (1990) presented a fusulinid zonation for the Permian Basin, and his Zone of Parafusulina lineata–P. deliciasensis (Zone PG-3) was age-dated as early Middle Guadalupian (early Wordian) and was correlated with shelfal carbonate strata of the Grayburg Formation and with basinal siliciclastic strata of the Cherry Canyon Formation in the northern Delaware Basin (Fig. 2). As previously noted, Kerans & Ruppel (2020, figs. 3, 30) have interpreted the lowstand carbonate unit in the central part of the Midland Basin as representing a forced regression during their Guadalupian G10 HFS sequence, and they suggested that the lowstand carbonate unit should be assigned to the Grayburg Formation. As discussed below, the fusulinids from the lowstand carbonate unit in Well No. 1 confirm that the age of the Midland Basin lowstand carbonate unit is Guadalupian and correlative with the Grayburg Formation (Fig. 2). According to the traditional fusulinid-based chronostratigraphy, the Grayburg Formation was assigned to the middle Wordian stage, but based on the currently accepted conodont-based chronostratigraphy, the Grayburg is considered to be upper Roadian and lowermost Wordian (Fig. 2).

In the Well No. 1 core, fusulinids were preserved in partly dolomitized fusulinid-skeletal packstone facies from 4769 ft to 4789 ft (1453.1–1459.7 m; Figs. 4, 5), which are interpreted to be an outermost carbonate ramp facies. Brief lithologic descriptions of the fusulinid samples studied are given below in descending stratigraphic order and include lists of the fusulinids identified from each sample. The fusulinid species are discussed in a later section and are illustrated on Figures 6–15.

Lithology. Dolomitic limestone, fusulinid-skeletal packstone in a pelleted mud matrix that is partially dolomitized; some dark gray to brown, probable argillaceous material in matrix; slightly micropyritic; tightly packed. Abundant fusulinids, moderately common crinoid ossicles, sparse recrystallized thin shell fragments (probably brachiopods); very sparse fenestrate bryozoan fragments and ostracods; rare palaeotextulariid foraminifers; and possible rare trilobite fragments (Figs. 6.1–6.5, 7.1–7.4).

Remarks. Fusulinids are dark and somewhat recrystallized and are commonly slightly distorted as a result of grain overpacking and the dolomitization. It was difficult to get good thin-sections across the proloculi, which were commonly recrystallized or plucked areas of specimens in the thin-sections. The walls of the fusulinids are very dark, and the keriothecal wall structure is generally not very well preserved.

Fusulinids. Parafusulina deliciasensis, P. sellardsi, and Parafusulina cf. P. lineata.

Lithology and Remarks. Partly dolomitized fusulinid-skeletal packstone, similar to above, but more mud-rich and with less dolomitization of the mud matrix; bioclast content is similar to sample above (Figs. 7.5–7.6, 8.1–8.4).

Fusulinids. Parafusulina sellardsi and P. deliciasensis.

Lithology and Remarks. Partly dolomitized and partly silicified fusulinid-skeletal packstone, grains are tightly packed; fusulinids are largely recrystallized by dolomitization but some are still identifiable (Figs. 9.1, 9.2).

Fusulinids. Parafusulina rothi and P. lineata.

Lithology and Remarks. Partly dolomitized fusulinid-skeletal packstone, similar to above, tightly packed to overpacked; matrix darkened by brownish argillaceous material, and mostly dolomitized with both fine and coarse dolomite crystals; sparse quartz silt grains. Bioclast content similar to above, with moderately common megalospheric Parafusulina (Figs. 9.3–9.7, 10.1–10.4).

Fusulinids. Parafusulina antimonioensis, P. rothi, P. sellardsi, Parafusulina cf. P. deliciasensis, and possibly Parafusulina aff. P. wordensis(?).

Lithology. Silty, slightly dolomitic crinoid-fusulinid packstone; pelletal mud matrix mostly recrystallized to dolomitic microspar; bioclasts are mostly finer grained than above samples, quartz silt to fine-grained sand grains scattered throughout; grains tightly packed. Abundant crinoid ossicles. Moderately common microspheric fusulinids and sparse megalospheric fusulinids (Figs. 11.1–11.7, 12.1–12.4, 13.1–13.8, 14.1–14.3, 15.1–15.5).

Remarks. Twenty oriented fusulinid thin-sections were made from this one foot of core (4788–4789 ft = 1459.4–1459.7 m) at the base of the outermost ramp packstone facies. The fusulinid assemblage was a mixture of megalospheric and microspheric morphotypes, and the fusulinids in this sample are mostly slightly distorted by compaction and partial dolomitization.

Fusulinids. Megalospheric Parafusulina include P. marathonensis, Parafusulina cf. P. sellardsi, and Parafusulina cf. P. lineata, P. delicasensis, P. rothi. There are also many elongate and thin-walled microspheric Parafusulina sp. that cannot be confidently matched up with the associated megalospheric Parafusulina species.

Lithology. Silty, slightly dolomitic crinoid-fusulinid packstone; similar lithology to above sample, but in this sample all the fusulinids are microspheric morphotypes. This sample was an unoriented petrographic thin-section from a core plug end (Fig. 15.5).

Remarks. This sample was a single petrographic thin-section of a core plug from the original reservoir study. It is thought to be significant that all the fusulinids in this deepest thin-section are microspheric morphotypes because in previously described occurrences of microspheric Parafusulina, they are associated with dominant numbers of megalospheric morphotypes.

Fusulinids. This thin-section contained only microspheric morphotypes of Parafusulina sp. that cannot be confidently matched with associated megalospheric Parafusulina species.

The species of Parafusulina identified from the lowermost 20 ft (6.1 m) of outer ramp facies in the Well No. 1 cored section (4769–4789 ft, 1453.1–1459.7 m) are all clearly part of the same shallowing-upward carbonate depositional sequence and are considered to represent the same chronostratigraphic age (Figs. 4, 5). The purpose of this paper is primarily biostratigraphic and so only brief morphological comments are given on each species identified and their previously reported stratigraphic occurrences. The age-dates of occurrences cited from pre-2019 publications are based on the former fusulinid-based stage boundaries and can be compared to the new conodont-based stages in Figure 2.

Because of the partial dolomitization, the preservation of the fusulinids is quite variable, and so many of the species identifications are tentative. Tentatively identified species (e.g., Parafusulina cf. P. sellardsi or Parafusulina aff. P. sellardsi) of poorly preserved or poorly oriented specimens are included in the figure citations for each of the following species discussions. The species are discussed below in alphabetical order of species names.

Phylum FORAMINIFERA d’Orbigny, 1826

Superorder FUSULINOIDEA Fursenko, 1958

Order SCHWAGERINIDA Solovieva in Epshtein et al., 1985

Family POLYDIEXODINIDAE Miklukho-Maklay, 1953

Subfamily PARAFUSULININAE Bensh in Rauser-Chernousova et al., 1996

Genus ParafusulinaDunbar & Skinner, 1931 

Type species: Parafusulina wordensisDunbar & Skinner, 1931. Middle Permian, Wordian, Glass Mountains, West Texas, USA

• Parafusulina antimonioensisDunbar, 1953 Figs. 9.3–9.6 

Discussion. This species differs from other large species of Parafusulina of the Word Formation by its very elongate and slender shell shape, its relatively heavy axial deposits, and its low septal folds. It somewhat resembles P. deliciasensis, but the test of P. antimonioensis is larger, more slender and more elongate, and it has more numerous tightly coiled volutions.

Occurrence. Specimens of this species were identified in this study only from the 4781 ft (1457.2 m) sample. This species was originally described by Dunbar (1953) from Permian strata in Sonora, Mexico. Ross (1963) reported the species from the Fourth Word Limestone (Apple Ranch Member) of the Glass Mountains of West Texas, where it was associated with Parafusulina deliciasensis. Yang & Yancey (2000) also reported the species from the Apple Ranch Member, the uppermost limestone member of the Word Formation in the Glass Mountains. Wahlman et al. (2020) reported the species from the slope debris flow beds of the Word Formation in the west wall of Gilleland Canyon of the Glass Mountains. In the earlier fusulinid-based zonation, this species was considered to be an index fossil for the upper part of the Wordian Stage, but considering the associated fauna herein, and based on the new conodont-based chronostratigraphy, the species ranges down into the upper part of the Roadian Stage.

• Parafusulina deliciasensis Dunbar & Skinner, 1936 Figs. 6.1–6.5, 7.2, 8.1–8.4, 10.3, 13.1–13.2 

Discussion. Wahlman et al. (2020) discussed the taxonomic disagreement regarding the differentiation of Parafusulina deliciasensis, P. maleyi, and P. maleyi var. deferta, which were described from the Permian Basin area by Dunbar & Skinner (1937). Ross (1963) synonymized all three morphotypes and claimed that P. deliciasensis is one of the most common and long-ranging (Roadian and Wordian) species of Guadalupian Parafusulina in the Glass Mountains section. Wilde (1990) and Yang & Yancey (2000) also synonymized P. deliciasensis and P. maleyi var. deferta but maintained that P. maleyi was a distinct species. Yang & Yancey (2000) did not report any P. deliciasensis in their Glass Mountains samples but did recognize P. maleyi throughout the upper Roadian and middle Wordian. Wahlman et al. (2020) recognized P. deliciasensis in all three of their slope debris flow units in the Word Formation of Gilleland Canyon in the Glass Mountains.

Occurrence. Dunbar & Skinner (1936) originally described this species from Guadalupian strata near Las Delicias, Mexico (Dunbar et al., 1936). Specimens were identified in this study from samples at 4773 ft (1454.8 m), 4776 ft (1455.7 m), 4781 ft (1457.2 m), and 4788 ft (1459.4 m).

• Parafusulina lineataDunbar & Skinner, 1937 Figs. 7.3–7.4, 9.2, 12.1–12.3, 12.4–12.5?

Discussion. This species is distinguished mainly by its relatively small elongate and slender shell (L = 14 mm and W = 2.0-2.4 mm in 7–8 volutions), compact close coiling throughout, slightly curved axis, a small proloculus, a distinct tunnel, numerous intensely fluted septa, and minor axial filling (Dunbar & Skinner, 1937). The identified specimens in the present samples agree well with that description, but the proloculi appear smaller than the types.

Occurrence. Dunbar & Skinner (1937) described the species from the lower part of the Delaware Mountain Formation in the Guadalupe Mountains region, where it was associated with Parafusulina maleyi Dunbar & Skinner and P. rothi. Ross (1963) reported the species from the First Word Limestone (Road Canyon Formation, lowest Roadian) in the Glass Mountains. Wilde (1990), however, cited an error in the sample numbers of Dunbar & Skinner (1937) and claimed that the type specimens were actually from the lower part of the Cherry Canyon Formation and were thus early Wordian in age. Yang & Yancey (2000) cited the species as ranging from the middle Roadian to the lower Wordian in the Glass Mountains (i.e., through their P. rothi, P. trumpyi Thompson & Miller, and P. sellardsi Zones). Nestell et al. (2019, fig. 11) showed P. lineata as ranging throughout the conodont-based Roadian and Wordian Stages. Specimens identified in this study are from Core No. 1 samples at 4773 ft (1454.8 m), 4780 ft (1457 m), and 4788 ft (1459.4 m).

The near axial specimen shown in Figures 12.4–12.5 is tentatively identified as Parafusulina cf. P. lineata but differs from the typical species by being somewhat more elongate and narrow throughout growth, having thinner test walls, and lower and more spaced septal folds. Note that the specimen apparently has a tiny proloculus, shows no evidence of an eccentric juvenarium, and has a weak tunnel. It could be a paleoenvironmental microspheric form of the species.

• Parafusulina marathonensisYang & Yancey, 2000 Figs. 11.1–11.3 

Discussion. This species is large, has an elongate fusiform shell shape (L = 18 mm, W = 3.6 mm), and has strong low regular septal fluting, numerous subcircular foramina, and axial filling mostly in the juvenile volutions.

Occurrence. Yang & Yancey (2000) described this species from their Parafusulina trumpyi Zone (middle Roadian of conodont-based zonation) of the Glass Mountains. There are no other known reported occurrences. The identification of this species in Well No. 1 cored section would extend the range of the species into the upper Roadian of the conodont-based chronostratigraphy. Specimens were identified in this study in the sample at 4788 ft (1459.4 m).

• Parafusulina rothi Dunbar & Skinner, 1936 Figs. 9.1, 9.7, 15.1–15.4 

Discussion. This species has a subcylindrical test with rounded poles, is moderate in size (L = 15–16 mm, W = 3.7–3.9 mm), and is moderate in most other morphological characteristics.

Occurrence. Dunbar & Skinner (1937) cited the species from the lower third of the Delaware Mountain Group in the Guadalupe Mountains, where it was associated with Parafusulina maleyi and P. lineata, and from the upper half of the Word Formation in the Glass Mountains (Dunbar et al., 1936). Ross (1963) did not report the species in his Glass Mountains collections. Wilde (1990) cited the species as occurring throughout the Roadian, but being particularly characteristic of the upper Roadian. Yang & Yancey (2000) recognized the species as ranging from the middle part of the Roadian through the lower part of the Wordian. Nestell et al. (2019, fig. 11) showed the species to range throughout the Roadian and Wordian. Wahlman et al. (2020) reported the species from the two upper debris flow beds in the Word Formation of Gilleland Canyon in the Glass Mountains. Specimens were identified in this study from samples at 4780 ft (1457 m) and 4781 ft (1457.2 m).

• Parafusulina sellardsiDunbar & Skinner, 1937 Figs. 6.5, 7.1–7.2, 7.5–7.6, 10.1–10.2, 11.4–11.7, 12.1–12.2 

Discussion. This is a large species with a stout subcylindrical shell, a thick middle, and bluntly rounded poles (L = 16–18 mm and W = 4–5 mm). Specimens tentatively identified as Parafusulina cf. P. sellardsi are also illustrated on plate 6, figures 3–6.

Occurrence. Dunbar & Skinner (1937) described this species from the Third Word Limestone (Willis Ranch Member of the Word Formation) in the Glass Mountains. Ross (1963) reported the species from the Second and Third Word Limestones, but more abundant in the latter. Wilde (1990) cited the species as ranging from the upper Roadian (his “Brushyan” stage, Parafusulina rothi-P. maleyi, PG2 Zone) and becoming more common in the Wordian (his “Cherryan” stage, PG-3 Zone). Yang & Yancey (2000) reported the species from the Willis Ranch Member (Third Word limestone). Nestell et al. (2019, fig. 11) showed the species as ranging only through the lower part of the Wordian. Wahlman et al. (2020) reported the species from the Word Formation debris flow units of Gilliland Canyon. Specimens were identified in this study from samples at 4773 ft (1454.8 m), 4776 ft (1455.7 m), 4781 ft (1457.2 m), and 4788 ft (1459.6 m).

• Parafusulina cf. P. wordensisDunbar & Skinner, 1931 Fig. 10.4 

Discussion. Dunbar & Skinner (1937) distinguished this species primarily as being the largest species of Parafusulina in their collections, attaining a length of 30 mm and a diameter of 5.5–6.0 mm. One slightly oblique, parallel sagittal specimen of a megalospheric Parafusulina in the present samples has a diameter of 6–8 mm and is tentatively identified as P. cf. P. wordensis based mainly on the huge shell size.

Occurrence. Dunbar & Skinner (1931) described this species from the Third Word Limestone (Willis Ranch Member of the Word Formation) of the Glass Mountains, where it was associated with Parafusulina sellardsi. Similarly, the single large specimen in the 4781 ft (1457.2 m) sample from the Well No. 1 core is associated with P. sellardsi. Nestell et al. (2019) showed this large species ranging through the middle and upper parts of the Wordian Stage.

• Parafusulina sp. (Microspheric forms) Figs. 11.4, 12.4–12.5, 13.3–13.8, 14.1–14.3, 15.5 

Discussion. The microspheric Parafusulina specimens illustrated herein are not microspheric “giants”, as described by Stevens (1995) and other authors. As is typical of microspheric forms of Parafusulina, these specimens have tiny microspheric proloculi, very narrow elongate subcylindrical shell shapes, and very thin keriothecal walls, but their shell dimensions are typically only 13–15 mm in length and 1.5–1.8 mm in width (Form Ratio = 7.7–8.6). In other words, they have similar or only slightly greater length dimensions to several Parafusulina megalospheric shells in the Well No. 1 core samples, but they have much smaller diameters, and therefore have more elongate and narrow test shapes. They have relatively low, irregular to regular, intense septal fluting and thin axial filling, which make them similar to the megalospheric specimens identified herein as Parafusulina marathonensis, which occurs in the upper part of the interval containing the microspheric forms. Therefore, it is possible that the microspheric morphotypes could be questionably assigned to that species.

The specimens shown in Figures 12.4–12.5 might be “paleoenvironmental microspheric” forms of Parafusulina cf. P. lineata, having a similar curved elongate axial shell form, but having more elongate and narrow juvenile and adult whorls, thinner test walls, and somewhat more evenly spaced septal fluting. Significantly, that specimen shows no evidence of having an eccentric juvenarium and has a weak tunnel.

Occurrence. Microspheric Parafusulina specimens were seen only in the basal 1 ft (0.3 m) (4788–4789 ft = 1459.4–1459.7 m) of the outermost ramp fusulinid-skeletal packstone facies, immediately above the contact with the upper slope facies composed of dark-gray quartz siltstone.

Dunbar et al. (1936) first described sexual dimorphism in the fusulinid genus Parafusulina based on microspheric specimens of Parafusulina deliciasensis Dunbar & Skinner from Middle Permian (Guadalupian) limestone strata near Las Delicias, Coahuila State, Mexico; on specimens of Parafusulina rothi Dunbar & Skinner from the Delaware Sandstone and Getaway Limestone (Middle Permian) in the Guadalupe Mountains of West Texas; and on specimens of Parafusulina kingorumDunbar & Skinner 1937 from the uppermost part of the Word Formation (Middle Permian) in the Glass Mountains of West Texas. They noted that in their collections, smaller sized individuals with large proloculi (macrospheric or megalospheric forms) greatly outnumbered sparse specimens that were much larger “giant” individuals with tiny proloculi and eccentric coiled juvenaria (microspheric forms). They related these two morphotypes to studies on modern foraminifers and Eocene nummulitid larger foraminifers, in which the megalospheric forms were interpreted to represent the products of asexual reproduction and the microspheric forms to represent the products of sexual reproduction. They noted four major morphological differences between the megalospheric and microspheric shells: (1) proloculus size, with megalospheric forms having large, often irregularly-shaped proloculi, and microspheric forms having tiny spherical proloculi; (2) microspheric forms have more numerous juvenile volutions; (3) megalospheric morphotypes have bilaterally symmetrical shells throughout growth and fewer volutions, but the larger microspheric forms have eccentric juvenaria and more numerous volutions; and (4) megalospheric forms usually have a well-defined central tunnel in axial view, but microspheric forms lack a distinct central tunnel in their shells.

Dunbar (1963, 1969) further discussed fusulinid dimorphism and gigantism. He pointed out that in Pennsylvanian and Early Permian (Wolfcampian-Leonardian) fusulinids, microspheric and megalospheric forms of a species are alike in shell size and shape, and the distinction can only be determined in thin-sectioned specimens. In addition, microspheric forms are relatively rare in a sample with abundant megalospheric forms (e.g., 1 in 100 or more), and in some genera, such as the Late Pennsylvanian Triticites, microspheric forms are almost never seen.

Gigantism appeared rather suddenly in Middle Permian (Guadalupian) Parafusulina microspheric forms, with some microspheric specimens developing shells several times as large as the associated megalospheric shells. Such microspheric giants became more common in some species such as Parafusulina antimonioensis and P. rothi. Interestingly, Dunbar noted that such microspheric giants are not known in Leonardian species of Parafusulina, and he considered the rather sudden appearance of microspheric giants in mid-range Guadalupian species of Parafusulina to be an unsolved problem.

Kobayashi (2016) stated that microspheric forms of advanced schwagerinid fusulinids are uncommon from the Tethyan regions, except for Monodiexodina kattaensis (Schwager, 1887) from the Lower Permian of Pakistan (Douglass, 1970), Eopolydiexodina from the Middle Permian of Iran (Kobayashi & Ishii, 2003), and Parafusulina japonica (Gumbel, 1874) from the Middle Permian (upper Wordian) of Japan. He further confirmed that microspheric giants of Guadalupian Parafusulina are typically more than twice the size of megalospheric forms, but in Leonardian species of Parafusulina and the closely related Skinnerella microspheric and megalospheric forms are about the same size.

Stevens (1995) discussed 15 North American species of Parafusulina that have megalospheric morphotypes with test lengths over 2 cm and associated microspheric giants with lengths that reach 10 cm length or more. Five of those species occur in strata deposited on the Late Paleozoic North American craton in present-day West Texas-New Mexico and adjacent Mexico (Coahuila). Stevens (1995) cited dimorphic specimens of Parafusulina antimonioensis, P. deliciasensis, P. rothi, and P. kingorum (and possibly P. wordensis). According to Stevens (1995), other North American species of Parafusulina with known specimens of microspheric giants occur in western North American tectonostratigraphic exotic terranes that were transported and attached to the North American continent at locations from present-day Mexico to Alaska. He thought that all the allochthonous terranes containing microspheric giant fusulinids actually originated in low paleolatitudes.

Stevens (1995) noted that the Texas-Coahuila species of Parafusulina known to display dimorphism differed from the exotic terrane species in a few morphological features. The cratonic Texas/Coahuila megalospheric forms generally have form ratios (axial shell L/W) from 5.0 to 5.8, except for P. antimonioensis which has form ratios up to 6.7. On the other hand, known megalospheric forms from western exotic terranes mostly have notably larger form ratios than the Texas/Coahuila forms. The proloculi of megalospheric forms are large in most species, but often display intraspecific variability in the proloculus diameters and shapes. The amounts of secondary axial filling vary greatly between both cratonic and exotic terrane megalospheric species. In the Texas/Coahuila megalospheric forms, except for specimens of Texas P. antimonioensis, the septal folds are commonly high and somewhat irregular, similar to most other Parafusulina species. Low, isolated folds may appear after about the fourth volution, but generally they are subordinate to the higher, more irregular types of septal folds.

Stevens (1995) described the morphological features of the microspheric morphotypes of Parafusulina. Illustrations of complete microspheric specimens of only seven species of Parafusulina from five localities have been published, those being: P. antimonioensis from Sonora, Mexico; P. deliciasensis from Coahuila, Mexico; P. deliciasensis from Texas; P. rothi from Texas; P. kingorum from Texas; P.? californica (Staff 1908) from California; P.? virga Thompson & Wheeler 1946 from California; and P.? sp. A Stevens from Alaska. He noted that with so few complete microspheric giants known, consistent similarities and differences are difficult to describe. The microspheric morphotypes tend to have lower and more regular septal fluting than their megalospheric counterparts. From what is known, the Texas microspheric specimens have form ratios of 8 or less, whereas most other microspheric specimens have form ratios greater than 10. Therefore, Stevens concluded that the Texas microspheric forms appear to be slightly different from the microspheric forms of the western exotic terrane species.

Interestingly, Stevens (1995, p. 805) noted that the microspheric giant fusulinids mostly “occur in well-sorted, commonly laminated, fine- to medium-grained calcarenites and calcareous sandstone facies that are generally dark gray in color.” He did not attempt to interpret the paleoenvironmental significance of that consistent lithofacies association.

The microspheric specimens of Parafusulina described herein are not typical of most of the described Guadalupian microspheric “giants”. The Well No. 1 core microspheric specimens are approximately the same length, or slightly longer, as many of the megalospheric Parafusulina morphotypes in the associated strata. They differ from the associated megalospheric forms by having (1) tiny proloculi; (2) more elongate, narrow, cylindrical shell shapes (much higher form ratios); (3) more irregular coiling; (4) more irregular septal fluting; and (5) very thin test walls and septa.

It should be noted that even though 20 thin-sections were examined from the thin 1-ft (0.3 m) dolomitic limestone interval containing the microspheric Parafusulina, no microspheric proloculi were identified with absolute certainty, but clearly the proloculi of these specimens are tiny. Recrystallization might have hindered recognition of the tiny initial chambers, but it is also considered significant that no evidence of eccentric juvenaria was seen. The apparent absence of eccentric juvenaria, and the fact that the microspheric forms are only slightly larger than associated megalospheric specimens, suggests that these microspheric specimens do not represent typical products of sexual dimorphism as seen in other occurrences of North American Guadalupian microspheric Parafusulina.

It should be further noted that whereas previously described microspheric Parafusulina are generally rare in samples dominated by their megalospheric counterparts, the microspheric specimens from Well No. 1 dominate the assemblage in the thin 1-ft (0.3 m) interval in which they occur. It appears that all preserved fusulinids in overlying strata are megalospheric morphotypes. The microspheric morphotypes occur only in the deepest outer ramp skeletal packstone facies that directly overlies dark-gray, slightly calcareous, quartz siltstone slope facies with no fusulinids and only rare small crinoid ossicles and other bioclasts. It is suggested here that the microspheric Parafusulina morphotypes from the outermost ramp facies in the Well No. 1 core could actually be the product of their outermost ramp paleoenvironment in the lower part of photic zone, rather than the product of sexual dimorphism.

As recently reviewed and discussed by Hallock & Seddighi (2021), modern larger foraminifera have algal symbionts and are dependent on sunlight for photosynthesis, and so they thrive mostly in shallow clean-water carbonate environments. Based on recent studies of modern larger foraminifers (Haller et al., 2019; Hallock & Reymond, 2022), the larger foraminifers inhabiting low-light outer ramp environments tend to be smaller and have thinner test walls. Oron et al. (2018) analyzed the growth trends of specimens of the modern larger foraminifer Operculina ammonoides (Gronovius, 1781). They placed specimens at 15 m, 30 m, and 45 m depths in the Gulf of Aqaba-Eilat for over 20 days, and the results showed that the larger foraminifers constructed thinner chambers after relocation to lower-light conditions.

Photomicrographs of the microspheric specimens from the Well No. 1 core were sent to modern larger foraminifer expert Pamela Hallock Muller (email communication, 2022), and she agreed with the proposed suggestion that if the Parafusulina inhabited a lower photic zone paleoenvironment near the base of their depth range, where light intensities were relatively low, then they might not have had enough energy to produce thick tests, and possibly would not have been able to accumulate sufficient energy to even reproduce or achieve “gigantic” size.

It should be noted that oxygen levels have also been related to fusulinid morphologies. Payne et al. (2012) attempted to demonstrate a relation between Late Paleozoic fusulinid gigantism and atmospheric hyperoxia. Of greater potential significance here are studies that those authors cited on modern foraminifers inhabiting low oxygen settings that have smaller, less spherical shells with thinner shell walls (Kaiho, 1994; Gooday et al., 2000, 2009). Considering that the microspheric Parafusulina described herein occur in strata transitioning from well-oxygenated, light-colored, outer ramp carbonates to dark-gray, upper slope siltstones, it is possible that decreasing oxygen levels might have also had a role in the development of the elongate, thin-walled, microspheric Parafusulina specimens.

It is noteworthy to recall here the observation by Stevens (1995) that most microspheric “giant” Parafusulina that he studied occurred in dark-colored sedimentary rocks that were commonly laminated, which appears to be a consistent lithofacies characteristic that could represent deeper-water low-light depositional settings. That observation presents the possibility that at least some Middle Permian microspheric Parafusulina are the product of deeper-water low-light paleoenvironments rather than a sexual dimorphic reproductive stage. Further studies are needed to determine the depositional settings of Middle Permian (Guadalupian) facies containing microspheric forms of Parafusulina, and to determine if there are other potential occurrences of paleoenvironmental microspheric Parafusulina.

Three studied subsurface cores from the central area of the Midland Basin contain a Middle Permian (Guadalupian) carbonate stratigraphic unit composed of prograding shallow-water carbonate facies deposited on the sloping basin floor during a sea level lowstand (Figs. 1–3). The cores from Well No. 2 and Well No. 3 are composed of inner to middle ramp dolomitized carbonate facies. The most downdip Well No. 1 core partially consists of interfingering middle ramp, outer ramp, and slope facies, and one interval of outermost ramp facies in that core contains moderately well-preserved fusulinids (Figs. 4–5).

The shallowing-upward facies sequence comprising the interval in the Well No. 1 core discussed herein consists of an upper slope dark-gray quartz siltstone facies that is overlain by outermost ramp facies composed of tan fusulinid-skeletal packstone, which grades upward into oolitic foreshoal and shoal grainstone facies (Figs. 3–5). The shoal-foreshoal grainstone facies and the mid-to-outer ramp fusulinid-skeletal packstone facies above 4769 ft (1453.6 m) in the Well No. 1 core are completely dolomitized and fusulinids are common but are skelmoldic or completely recrystallized. The outermost ramp facies interval from 4769 ft to 4789 ft (1453.6–1459.7 m) directly overlies the dark slope facies and is composed of partly dolomitized fusulinid-skeletal packstone with moderately well-preserved fusulinids. The identifiable fusulinids from that interval are all species of Middle Permian (Guadalupian) Parafusulina (Figs. 6–15).

In lithostratigraphic correlation terms, the Midland Basin carbonate unit was deposited on the basin floor during a sea level lowstand and is thought to correlate with the erosional unconformity capping Upper San Andres shelfal facies on the adjacent Central Basin Platform. According to lithostratigraphic, seismic stratigraphic, and sequence stratigraphic analyses, Kerans et al. (2013) and Kerans & Ruppel (2020, figs. 3 and 30) assigned this Midland Basin lowstand carbonate unit to the Grayburg Formation and to the G10 high-frequency sequence (HFS) of the CS11 composite sequence (Fig. 2). That assignment correlates the lowstand carbonate unit with the lower Wordian Stage of the earlier fusulinid-based chronostratigraphy, and with the upper Roadian of the currently accepted conodont-based chronostratigraphy (Fig. 2).

The North American Guadalupian fusulinid database is based mainly on outcropping limestone sections in the Guadalupe, Apache, and Glass Mountains of West Texas (Fig. 1). Guadalupian carbonate strata in the subsurface of the Permian Basin are mostly dolomitized and so fusulinids are not usually well-preserved. The stratigraphic ranges of early and middle Guadalupian fusulinid species of Parafusulina reported in the literature are somewhat inconsistent, partly because of the rather limited database, and partly as a result of the conodont-based redefinition of the Guadalupian stage boundaries (Fig. 2). Prior to the 1990’s, Guadalupian chronostratigraphic stages in the Permian Basin were based mostly on fusulinid biostratigraphy, but since the Guadalupian Series was formally established as the stratotype for Middle Permian stages, Guadalupian stage boundaries have been based primarily on conodont biostratigraphy, and the horizons of stage boundaries were significantly changed. Figure 2 herein is an attempt to provide correlations of Guadalupian lithostratigraphy and sequence stratigraphy with the earlier fusulinid-based chronostratigraphic units and the currently accepted conodont-based chronostratigraphy.

Essentially all the Permian Basin fusulinid biostratigraphic data published before the recent comprehensive paper by Nestell et al. (2019) used the fusulinid-based chronostratigraphic stage boundaries. The earlier published fusulinid-based stage designations and the currently accepted conodont-based stages are both used in the following discussions and can be approximately correlated using Figure 2.

The fusulinids identified in the outermost ramp facies in the Well No. 1 core (4769–4789 ft = 1453.6–1459.7 m) are, in species alphabetical order: P. antimonioensisDunbar, 1953; Parafusulina deliciasensis Dunbar & Skinner, 1936; P. lineataDunbar & Skinner, 1937; P. marathonensisYang & Yancey, 2000; P. rothi Dunbar & Skinner, 1936; P. sellardsiDunbar & Skinner, 1937; and one oblique sagittal specimen large enough to possibly be Parafusulina cf. P. wordensisDunbar & Skinner, 1937. This fusulinid assemblage is clearly part of the same shallowing-upward outermost ramp depositional sequence (see Figs. 4–5) and therefore is considered to represent the same age. The original published ranges of all those species are based on the earlier fusulinid-based chronostratigraphic stages and those ranges need to be adapted to the current conodont-based chronostratigraphy (Fig. 2), as attempted by Nestell et al. (2019).

Todd (1976) cited fusulinids from the Midland Basin lowstand carbonate unit as Parafusulina deliciasensis and P. lineata, which were interpreted by J.W. Skinner to indicate an early middle Guadalupian (early Wordian) age. Ross (1963, text-fig.4) reported P. deliciasensis as ranging through the upper Roadian and Wordian Stages in the Glass Mountains section, but Wilde (1990) reported the species only from his lower Wordian PG-3 Zone. Ross (1963, text-fig. 4) reported P. lineata only from the lower Roadian in the Glass Mountains, but Wilde (1990) cited the species as characteristic of his lower Wordian PG-3 Zone. In their Glass Mountains study, Yang & Yancey (2000) did not report P. deliciasensis, and cited P. lineata as ranging from the middle Roadian through the lower Wordian. According to the currently accepted conodont-based chronostratigraphic stages, Nestell et al. (2019, fig. 11) showed both P. delicasensis and P. lineata as ranging throughout the Roadian and Wordian Stages.

Similarly, Wilde (1990) and Yang & Yancey (2000) reported P. rothi as ranging through the fusulinid-based upper Roadian and lower Wordian Stages, and Nestell et al. (2019) reported the species to range throughout the conodont-based Roadian and Wordian Stages.

Parafusulina marathonensis was described by Yang & Yancey (2000) from their upper Roadian Parafusulina trumpyi Zone in the Glass Mountains (Fig. 2).

Parafusulina sellardsi is one of the most common fusulinids in the Well No. 1 samples. According to Ross (1963) and Wilde (1990), P. sellardsi appears in the upper Roadian and becomes more common in the overlying lower Wordian. In the Glass Mountains section, the species appears in the Second Word Limestone (China Tank Member) but increases in abundance higher in the section and is characteristic of the Third Word Limestone (Willis Ranch Member of the Word Formation; Ross, 1963; Yang & Yancey, 2000; Fig. 2).

The elongate specimens tentatively identified herein as Parafusulina antimonioensis are important because in earlier fusulinid studies that species was considered characteristic of the upper Wordian. Ross (1963) and Yang & Yancey (2000) reported P. antimonioensis from the upper Wordian Fourth Word Limestone (Apple Ranch Member) in the Glass Mountains, Wilde (1990) cited the species in his upper Wordian PG-4 Zone, and Nestell et al. (2019) cited the species from the upper Wordian of the Guadalupe and Apache Mountains. Wahlman et al. (2020) recently identified the species from the Willis Ranch Member (Third Word Limestone) debris flow strata of Gilleland Canyon in the Glass Mountains (upper Roadian-lower Wordian), and the occurrence in this study is thought to represent a similar horizon.

The oblique sagittal specimen tentatively identified as Parafusulina cf. P. wordensis based mainly on its very large size, is another species usually considered to be characteristic of the upper Wordian Stage (Wilde, 1990, PG-4 Zone), but Nestell et al. (2019, fig. 11) showed the range of that species to overlap with P. sellardsi in the conodont-based middle part of the Wordian Stage.

The assemblage containing P. sellardsi and P. antimonioensis, and a possible Parafusulina cf. P. wordensis, suggest that the 4769–4789-ft (1453.6–1459.7 m) interval in the Well No. 1 core correlates with Grayburg Formation of the northern Permian Basin section and with the Willis Ranch Member of the Word Formation (Third Word Limestone) in the Glass Mountains section. Those species are characteristic of the upper part of the range of the genus Parafusulina and could indicate a later Roadian to late Wordian age according to the earlier fusulinid-based chronostratigraphy, or a late Roadian to early Wordian age according to the current conodont-based chronostratigraphy.

As discussed above, apparent microspheric forms of Parafusulina occur in the basal 1 ft (0.3 m; 4788–4789 ft = 1459.4–1459.7 m) of the outermost ramp fusulinid-skeletal packstone facies of the Well No. 1 core, immediately above upper slope dark-gray siltstone facies (Figs. 3–5). Those microspheric forms differ from normal megalospheric Parafusulina by having a tiny proloculus, more narrow and elongate shell forms, more irregular coiling, more irregular septal fluting, and very thin test walls and septa. It is noteworthy that these microspheric morphotypes of Parafusulina are not “giants”, as most Middle Permian microspheric Parafusulina have been described (e.g., Stevens, 1995), and they do not show any evidence of having eccentric juvenaria. It is thought to be significant that these microspheric forms are restricted to the basal 1 ft (0.3 m) of the outermost ramp carbonate facies that is transitional into the underlying dark-gray siltstone upper slope facies. This occurrence suggests the possibility that these microspheric forms are not the product of sexual dimorphism, but rather are the result of inhabiting a paleoenvironment in the low-light conditions of the lower part of the photic zone, where their symbiotic algae could not provide enough energy to produce normal megalospheric morphotypes. Furthermore, previously reported microspheric forms of Parafusulina are relatively rare in strata dominated by megalospheric forms, but in the present occurrence the microspheric forms are the most common morphotype in the basal 1 ft (0.3 m) of the outermost ramp facies. The observation by Stevens (1995) that many of the samples of microspheric Parafusulina that he examined were in dark-gray laminated host rocks is noteworthy and bears further consideration. It is possible that some other occurrences of microspheric Parafusulina are from outer ramp lower photic zone facies and should be examined to determine if any might also actually represent paleoenvironmental microspheric morphotypes.

We thank the petroleum exploration company who drilled the cored wells studied herein for approving and facilitating the sampling of the lowstand carbonate unit in their Well No. 1 core for fusulinid biostratigraphic analyses, and for authorizing the publication of this study. Thanks to Pamela Hallock-Muller (University of South Florida) for her helpful comments and for providing a few bibliographic references on modern larger foraminifers from low-light outer ramp settings. Thanks to Charles Kerans (University of Texas at Austin) for sharing his seismic and sequence stratigraphic interpretations of the Midland Basin Guadalupian lowstand carbonate unit discussed in this study. Thanks also to Charles Kerans and Xavier Janson (Texas Bureau of Economic Geology) for discussions on the Guadalupian stratigraphy of the Guadalupe and Glass Mountains. Finally, we greatly appreciate the reviewers Michael Read (Stephen F. Austin University) and Lowell Waite (University of Texas at Dallas) for their many helpful comments and suggestions.