Conodonts and the Devonian–Carboniferous transition in the Dyer Formation, Colorado

Conodont assemblages from shallow platform carbonate strata of the Dyer Formation in northwestern Col‐ orado indicate that the unit spans the Devonian–Carboniferous transition, and include four new taxa, described herein. The underlying Parting Formation and most of the Broken Rib Member of the Dyer Formation are in the Palmatolepis expansa Zone. The upper portion of the Broken Rib Member and the lower part of the Coffee Pot Member of the Dyer Formation are in the Bispathodus aculeatus Zone to the Bispathodus ultimus Zone, and associ‐ ated with an ~6‰ positive δ 13 C shift identified as the end-Devonian Hangenberg excursion. Upper Coffee Pot Mem‐ ber conodonts are equivocally Tournaisian, and cap the δ 13 C excursion. Collectively, these observations suggest that the Devonian – Carboniferous boundary is in the upper Coffee Pot Member of the Dyer Formation, and thus the overlying Gilman Sandstone is Carboniferous.


INTRODUCTION
The end-Devonian-earliest Carboniferous (D-C) was a time of significant global change marked by glaciation in the Americas-southern Laurentia and western Gondwana (Isaacson et al., 1999(Isaacson et al., , 2008Brezinski et al., 2010;Ettensohn et al., 2020), and sea level fluctuation (Myrow et al., 2013). These changes were associated with the multiphasic Hangenberg Crisis interval that includes a major mass extinction (Walliser, 1984;Kaiser et al., 2016). Notable changes occur in conodont, brachiopod, goniatite, palynomorph, trilobite, and vertebrate faunas, as well as a carbon isotope excursion of up to +7‰ (Caplan and Bustin, 1999;Kaiser, 2009;Filipiak and Racki, 2010;Sallan et al., 2010;Cole et al., 2015;Stolfus et al., 2020). The D-C boundary in the United States lies often within a disconformable interval (Over, 2020). In the Dyer Formation, conodonts from 24 productive horizons spread across five measured sections provide biostratigraphic constraint for a measured carbon isotope excursion consistent with the Hangenberg event. The conodonts and carbon isotope shifts allow placement of the Devonian-Carboniferous boundary within a shallowwater carbonate platform setting.

Sections of the Dyer Formation at Bear Scat
Creek, Bison Lake, Crane Park, and Monument Lake (Appendix 1) were measured, using a steel tape and Jacob's staff. Bulk 2+ kg samples were collected for conodonts and δ 13 C carb chemostratigraphy. To replicate the Broken Rib Member-Coffee Pot Member interval at Bear Scat Creek, a supplementary section was logged and collected for biostratigraphy nearby, at Deep Creek Side Canyon. Data from Sandberg (1976), Sandberg and Poole (1977), and Myrow et al. (2011) from Glenwood Canyon were also incorporated into this study (Figs. 1 and 2). Conodont samples were crushed to 1 cm maximum dimension, and dissolved in buffered Creek Side Canyon; 2, Bison Lake; 3, Monument Lake; 4, Glenwood Canyon location of Sandberg and Poole (1977) and Myrow et al. (2011Myrow et al. ( , 2013; and 5, Crane Park Quarry. Dotsero is an unincorporated community near where the Eagle River joins the Colorado River and an exit on Interstate 70. Base map from R. Blakey, Colorado Plateau Geosystems Inc., used with permission. See Appendix 1 for detailed locality information. 10% formic acid following the method of Jeppsson and Anehus (1995). The acid digestion residues were collected on a 120 µm screen, density separated using 2.78 sg lithium metatungstate, and the heavy fraction scanned for microfossils. Selected conodonts were coated with Au-Pd, and im-aged using secondary electron or backscatter detectors on a ZEISS EVO scanning electron microscope.
Hand samples for chemostratigraphy were collected in each of the study sections, thin-sectioned, stained, and petrographically screened. The least altered phases (principally  Hauck (1954), Bass and Northrop (1963), Campbell (1966), and Strauss (2006). One sample (4*) came from the Deep Creek Side Canyon section, which is adjacent and parallel to the Bear Scat Creek section. Rather than plot this supplementary section separately, we have indicated the equivalent stratigraphic position of this sample within the Bear Scat Creek section. Biostratigraphy samples are keyed to Tables 1-5; arrowheads indicate samples that only contained unidentifiable conodont elements and fragments; samples with numbered arrowheads contained identifiable taxa; samples 16-19 are from Sandberg (1976). micrite, dolomicrite, and oolite) were micro-drilled, avoiding late-stage spar, fossils, and allochems. Stable isotope ratios (δ 18 O and δ 13 C) were measured using a Thermo Fisher Scientific Kiel III automated carbonate preparation device coupled to a Finnigan MAT 252 gas-ratio mass spectrometer. Powdered samples were reacted with dehydrated phosphoric acid under vacuum at 70°C. The isotope ratio measurement was calibrated based on repeated measurements of NBS-19, NBS-18, and CAR2, an in-house carbonate standard. The following isotope ratios were assigned to these standards (δ 18 O followed by δ 13 C): +1.95;; CAR2, -1.41, +2.03. Analytical precision was ± 0.10‰ for δ 18 O, and ±0.08‰ for δ 13 C (1σ). Chemostratigraphic analysis of the Dyer succession, including δ 18 O and 87 Sr/ 86 Sr work on carbonate phases and selected conodont separates, will be treated in greater detail in a separate paper.

RESULTS
Twenty-six of 46 samples from the Dyer Formation yielded conodonts ( Fig. 2; Tables 1-5). Of these, 17 samples, as well as four horizons from Glenwood Canyon collected and described by Sandberg (1976, Sandberg and Poole (1977, fig. 15), Sandberg and Dreesen (1984, fig. 5), and some illustrated in Sandberg and Zielger (1979) and Sandberg and Dreesen (1984), had recognizable taxa representing eight genera and 19 species, of which one new species and three new subspecies are described. Eleven of the 20 barren samples came from the Coffee Pot Member of the Dyer Formation, four samples were from carbonates within the underlying Parting Formation, and five samples were from sandy carbonates within the overlying Gilman Sandstone. Carbon isotope data show a 6‰ positive excur-sion starting in the upper Broken Rib Member of the Dyer Formation or lower Coffee Pot Member (Fig. 3).

DISCUSSION: IMPLICATIONS AND IMPORTANCE OF THE CONODONTS
Conodonts are generally well preserved in the Dyer (Figures 4-7), although commonly marred by dolomite crystal impressions and overgrowths (e.g., Figs. 4.9 and 4.12). Conodont color alteration index (CAI) of 2 to 3 indicates thermal heating between 60 and 200 degrees Celsius (Epstein et al., 1977).
Only two samples in the upper Coffee Pot yielded conodonts. A sample at 50.5 m in our measured section at Bison Lake contained Icriodus costatus ssp., a Devonian taxon, and three small fragments. A sample from 59.9 m at Bison Lake contained conodonts in fused clusters of sand and fragments, possibly coprolitic in origin. The conodonts include Polygnathus sp., Pseudopolygnathus?, and Siphonodella?, which have affinities to Tournaisian forms (see Systematic Paleontology, Appendix 2).
These conodonts suggest the Siphonodella duplicata Zone and indicate that the Devonian-Carboniferous (D-C) boundary is between 50.5 m and 60 m in the section at Bison Lake. This interval also corresponds to the decline in the positive δ 13 C shift at the end of the Hangenberg isotope excursion and the D-C boundary (Myrow et al., 2011;Kaiser et al., 2016;Fig. 3

this paper).
Numerous conodont taxa in the Dyer are similar to species described elsewhere from the high Famennian of Laurentia Mehl, 1934a, 1934b;Klapper, 1958Klapper, , 1966Sandberg and Ziegler, 1979;Sandberg and Dreesen, 1984;Johnston and Chatterton, 2001;Hogancamp and Rodriguez, 2020). Their presence indicates faunal  Figure 2. Gray band denotes stratigraphic interval bearing the acme of the Hangenberg excursion. The Hangenberg appears to be condensed in some sections. In the Bear Scat Creek, Bison Lake, and Monument Lake sections, the Hangenberg excursion is either interrupted by a suite of negative excursions, or there exists a two-pronged suite of positive excursions in the aculeatus to ultimus Zone. The majority of data from Glenwood Canyon are from Strauss (2006) and Myrow et al. (2011). The section at Crane Park Quarry has been moved upward 5% relative to other sections to accommodate spacing, and align the prominent pre-Hangenberg nadir observed in each succession.  , Crane Park Quarry 31.5 m (24 on Figure 2), DMNH EPV.136317. 8, 12, Bispathodus cf. Bi. aculeatus plumulus Rhodes, Austin, and Druce, 1969, Bear Scat Creek 19.7 m (3 on Figure 2), DMNH EPV.136318, EPV.136319. 9, 10, Pandorinellina cf. P. insita (Stauffer, 1940) of Sandberg and Ziegler (1979); 9, Bear Scat Creek 19.7 m (3 on Figure 2), DMNH EPV.136320; 10, Crane Park Quarry 25.5 m (22 on Figure 2), DMNH EPV.136321. 11, Bispathodus aculeatus aculeatus , Crane Park Quarry 25.5 m (22 on Figure 2), DMNH EPV.136322.    Figure 2), DMNH EPV.1363156. 7-8, Pseudopolygnathus? sp., fused clusters, Bison Lake 59.9 m (13 on Figure 2); 7, specimen processed for Sr/Sr analysis; 8, DMNH EPV.136350. 9-10, Polygnathus sp., fused cluster, Bison Lake 59.9 m (13 on Figure 2), DMNH EPV.136357. communication with the interior basins of Laurentia, or migration and exchange around the periphery of the paleocontinent. Notable is the occurrence of Patrognathus donbassicus from 31.5 m at Crane Park Quarry (lower Coffee Pot Member). This taxon is also known from Ukraine (Kozitskaya et al., 1978), which at this time was on the eastern margin of Laurentia-Laurussia, where it is associated with similar conodont taxa. This indicates global migration around the continental margin, at least some of the time, even into the more restricted shallow platform environments represented by the Dyer Formation. This would be much more likely during times of higher sea level before the onset of extensive glaciation in the Bispathodus ultimus Zone. Such communication is also supported by the presence of pterygoid tooth plates, holocephalian tooth plates, and Orodus teeth from the Dyer that are most similar to forms from the Famennian of Turkey, Western Australia/ Poland, and Ireland/England, respectively (Schultze et al., in press). Other taxa seem to be endemic to the Dyer or western Laurentia, such as Polygnathus communis pahvantensis, which occurs in the upper Broken Rib and lower Coffee Pot, which was also reported by Sandberg and Ziegler (1979) from the Pinyon Peak Limestone in Utah, illustrated as P. com. carina Hass, 1959. It is unclear if this taxon occurs anywhere else. Pseudopolygnathus primus craneparkensis and Icriodus costatus coffeepotensis may also be endemic to the Dyer Formation.

CONCLUSIONS
The Dyer Formation conodonts, while not the most diverse or prolific for this interval, are important, because they provide a connection between biostratigraphically better constrained offshore to deeper-water D-C successions and shallower successions that are typically more depauperate in conodonts, yet diverse with macroscopic fossils. As an example, the Dyer lies geographically between thin exposures of the Ouray, Williams Canyon, and Temple Butte formations of Colorado and Arizona, and thicker coeval units to the west in the Pinyon Peak Limestone and Leatham Formation of Utah, and to the north in the Sappington and Bakken formations. Our new Dyer biostratigraphy offers opportunities to compare and correlate between these strata.
The occurrence of biostratigraphically relevant conodonts in suspected coprolites also highlights how fused clusters are a useful search image for paleontologists seeking biostratigraphic or ecologic control in otherwise "barren" stratigraphic successions like the intermittently lagoonal to peritidal facies of the Coffee Pot Member. Such records are relatively common in the American West and midcontinent, and may house a more complete record of these biotic events than previously recognized.

Patrognathus donbassicus
Holotype: 68/481 IGS AS UkrSSR, borehole 932, near the town of Dokuchaevsk, Upper Devonian, Famennian (C 1 t a Zone), the Donets Basin-reillustrated in Figure 4.4. Original description (translated by T. Nemirovskaya from Kozitskaya et al., 1978): Platform is straight and narrow in small specimens; it is wider and strongly curved in large specimens, with maximum width in the ventral part. The dorsal end is pointed. The upper surface of the platform is ornamented by low transverse ridges (ribs), which can grow together in the axial part and form a thin low ridge. The length of the blade is one-third of the total length of the element or slightly longer. The blade is formed by several denticles of almost equal height and the very large last denticle (cusp). This denticle is usually shifted inward. The basal cavity of small specimens is almost symmetrical and elongated. It is strongly asymmetrical in large specimens. It is very wide in the ventral third, with protruding edges. In the dorsal part, the basal cavity is narrower than the platform. Remarks: No other elements in the apparatus can be unequivocally assigned to the taxon. Other features noted in the P 1 element are that the large posterior cusp of the free blade is always on the right side of the element and the small ventral-most denticle disappears in larger specimens. In juvenile specimens a thin, axial ridge connects the denticles on the platform. This ridge persists in the dorsal portion of the platform in larger specimens. An axial groove may be present between the marginal middle and ventral platform denticles where the transverse ridges are not completely developed in larger specimens. Patrognathus donbassicus differs from the morphologically similar Late Devonian taxon P. ourayensis Sandberg and Ziegler, 1979, in that the basal cavity of P. donbassicus is tapered dorsally, more smoothly convex, and not as widely flared under the cusp; the cusp is reclined dorsally, and the dorsal platform has an axial ridge connecting the denticles and transverse ridges. In the Donets Basin, P. donbassicus occurs above the first occurrence of Bispathodus costatus and Bi. ziegleri, yet before P. andersoni and Siphonodella aff. S. sulcata. Figures 5.1 -5.4, 5.8 Polygnathus communis carina Hass, 1959, of Sandberg andZiegler, 1979, p. 188, pl. 5, figs. 12 and13. Holotype: USNM 257660 SEP 77/1174, LDD-2, Pinyon Peak Limestone,~10 m above the base of the unit, Pahvant Range, Utah, plate 5, fig. 12 of Sandberg and Ziegler (1979). Diagnosis: Subspecies of Polygnathus (Neopolygnathus) communis Branson and Mehl, 1934b-P 1 element distinguished by a moderately curved dorsal carina and platform, raised margins of the platform in small specimens form a ventral rostrum in larger specimens where the platform out-side the rostral ridges may have raised margins as well, especially on the caudal side, resulting in three to four short longitudinal ridges. Description: P 1 element, carminiplanate, long free blade equal to length of platform in small specimens, two thirds to half the length in larger specimens; the ovoid platform, widest about mid-length, and carina are moderately curved with deep ventral adcarinal troughs. The platform margins are raised in the ventral portion, forming a rostrum in large specimens where the platform outside the rostral ridges may also have raised margins, especially on the caudal side, resulting in three to four short longitudinal ridges. The carina consists of fused denticles in the ventral portion that become more discrete dorsally, and they terminate before the tip of the platform. On the abaxial side the basal pit is small and symmetrical, with raised lips, starting at the end of the free blade in the ventral portion of the platform; dorsal from the pit is an elongate shallow depression; a narrow keel extends from the pit to the dorsal tip, widening slightly. Etymology: The subspecies is named for the Pahvant Range, Utah, where the holotype was collected by Sandberg (Sandberg and Ziegler, 1979). Remarks: This taxon was initially identified as Polygnathus communis carina Hass, 1959, a Tournaisian taxon and zone indicator quite distinct from this form. Hartenfels and Becker (2018, p. 616) and Hogancamp and Rodriguez (2020, p. 92) gave a thorough review of subspecies and species similar to Polygnathus (Neopolygnathus) communis ssp.

Polygnathus communis pahvantensis new subspecies
Polygnathus sp. in P. inornatus group of Hogancamp and Rodriguez, 2020 Figure 5.11 Diagnosis: Species of Polygnathus cf. P. inornatus Branson and Mehl, 1934b, that has a moderately curved dorsal carina composed of discrete denticles that reach the tip of the platform, the caudal margin is straight with short transform ridges on the raised rim of the platform; the rostral platform is wider and uniformly curved, the raised margin has short transform ridges. Description: P 1 element, carminiplanate with lanceolate platform outline, free blade highest ventrally, less than half the length of the entire element. At mid-length of the platform the carina and outer platform margin curve moderately in the caudal direction; at this point and dorsally, the carina consists of discrete denticles that reach the pointed dorsal tip. Adcarinal troughs are well developed, and wider on the rostral side; the platform margins are raised and ornamented with short transverse ridges. The ventral platform margins merge abruptly at approximately the same position at the start of the free blade. On the abaxial side, the basal cavity is large with a raised rim beneath the ventral third of the platform, and is asymmetrical, grading into a keel with a central groove that extends to the ventral tip. Remarks: The illustrated specimen is relatively small and similar to Polygnathus sp. C of Hogancamp and Rodriguez (2020, p. 100, pl. 5, figs. N, O, Q, R) in the curved nature of the ventral carina, differing in that the platform is widest at the dorsal end. The P. inornatus group of Hogancamp and Rodriguez (2020) includes several latest Devonian and Tournaisian taxa. Figures 6.1-6.4 Holotype: Figure 6.3, EPV.136340 from the upper Broken Rib Member, Crane Park Quarry, DMNH 4258, 25.5 m. Diagnosis: Subspecies of Icriodus costatus Thomas, 1949-P 1 element moderately arched, slightly curved, distinguished by a narrow and parallel-sided platform with a wide, asymmetrical basal cavity. Description: P 1 element, segminiscaphate, moderately arched, slightly curved, narrow and parallel sided, cusp low and directed dorsally, medial denticles are low and narrow, marginal nodes are low, rounded, and aligned with the medial denticles. A thin longitudinal ridge connects the medial denticles; aboral surface is characterized by a wide, asymmetrical basal cavity, widest in the dorsal half, continuously convex on the rostral side, concave and narrowing ventrally on the caudal side. Etymology: The subspecies is named for Coffee Pot Springs, Colorado, for which the Coffee Pot Member of Campbell (1970) is presumably named. Figure 6.10 ? Pelekysgnathus cf. P. brevis Sandberg and Dreesen of Johnston and Chatterton, 2001, pl. 4, fig. 15 (only). Holotype: Figure 6.10, EPV.136347 from the lower Coffee Pot Member, Crane Park Quarry, DMNH 4258, 31.5 m. Diagnosis: Species of Pelekysgnathus-P 1 element distinguished by a short, high, gently curved blade, reclined dorsal cusp, and wide basal cavity that extends beneath the ventral denticles. Description: P 1 element, segminate, short gently curved blade, slightly arched, large reclined dorsal cusp, several laterally compressed denticles, blade highest mid-length. Basal cavity extends the length of element, widest mid-length to the dorsal tip. Etymology: The subspecies is named in honor of Linda Soar for her exemplary work in the field and contributions to the Denver Museum of Nature & Science. Remarks: No coniform elements were found associated with the P 1 elements. Pelekysgnathus soarae has a short blade that is similar to the Givetian taxon P. brevis Sandberg and Dreesen, 1984, but the former is characterized by a reclined cusp, more denticles, and a widening of the basal cavity that extends beneath the ventral portion of the blade. Johnston and Chatterton (2001) illustrated several specimens as P. cf. P. brevis from Famennian carbonate strata in western Canada characterized by only one or two denticles, as well as one similar to P. soarae. Pseudopolygnathus primus craneparkensis new subspecies Figures 7.1-7.5 Holotype: Figure 7.3, EVP.136353, upper Broken Rib Member, Crane Park Quarry, DMNH 4258, 27 m. Diagnosis: Subspecies of Pseudopolygnathus primus Branson and Mehl 1934b-P 1 element distinguished by a gently curved free blade, carina, and platform, more obvious in smaller specimens, absence of a distinct caudal lobe, and larger nodes on the right side of the platform, regardless of the direction of curvature. Description: P 1 element, carminiplanate, free blade less than one-third the length of the element, widening dorsally where it meets the platform. Platform outline is lanceolate in small specimens, irregular in large specimens. The free blade, carina, and platform are gently curved. Denticles on the free blade number 5 to 7. Denticles on the ventral carina are fused and become discrete in the dorsal carina, where they extend to the tip of the platform. Adcarinal troughs are well developed. The rostral platform margin is continuous, or has a sinus in the dorsal half that results in a ventral lobe; nodes on the margin are evenly spaced, larger in the ventral portion. On larger specimens the platform margin bears ridges and elongate nodes that extend perpendicular to the margin; the caudal platform margin is continuous, or there is a small lobe developed over the expanded basal cavity; larger specimens may have an additional dorsal lobe. Nodes on the platform margin, or in larger specimens ridges and elongate nodes that extend perpendicular to the margin, are smaller or absent. Some elongate nodes or ridges may connect to the carina. On the abaxial side the basal cavity is large, asymmetrical, extending from where the free blade widens to the dorsal tip; the cavity is widest below the ventral platform, with a central longitudinal slit and pit. Etymology: The subspecies is named for Crane Park Quarry along the Coffee Pot Road in the White River National Forest, Colorado, where this taxon was collected.