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A new conodont from the uppermost Lamar Limestone of the Delaware Basin of west Texas.

Abstract. -- Clarkina crofti is described from the deepwater pelagic facies of the uppermost Lamar Limestone of the Delaware Basin of west Texas. This new species of conodont is characterized by the presence of a reduced platform. This feature is considered ancestral to the evolutionary line that gave rise to the genus Neospathodus which is characterized by the absence of a platform. Clarkina crofti appears to exhibit a high potential for the correlation of the post-Lamar/pre-Castile level of the Guadalupian type area with deepwater deposits of Tethys origin.

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A new species of Clarkina from the uppermost Lamar Limestone of the Delaware Basin is herein described. It is an immigrant from a deepwater pelagic fauna that has evolved outside the Delaware Basin, so it has high potential for correlation with the Tethyan Permian deepwater pelagic faunas, such as those found in western Sicily. Prior investigations and/or reviews of conodonts from west Texas include those of Druce (1973), Babcock (1976) and Clark & Behnken (1979).

Clarkina Kozur 1990b

Type species. -- Gondolella leveni Kozur & Mostler 1976.

Clarkina leveni (Kozur & Mostler 1976). -- Kozur 1990b.

Remarks. -- Kozur (1990b) established this genus for conodonts exhibiting a gondolellid apparatus in which the platform elements display a subterminal pit, marginal ridges on the otherwise flat keel, and a distinct free blade. With respect to this feature, these forms are similar to Neogondolella Bender & Stoppel and especially to Paragondolella Mosher, which are both believed to have evolved from platform-less ancestors of the genus Neospathodus Mosher. Transitional forms from Neospathodus to Paragondolella are known in the basal Olenekian and from Neospathodus to Neogondolella in the latest Olenekian and basal Anisian (Kozur 1990a; 1990b).

[FIGURE 1 OMITTED]

Clarkina crofti, new species

(Figure 1)

Neogondolella sp. -- Croft 1978 (unpublished), p. 51-52, pl. 5, figs. 1-10. Editor's note. -- The specific epithet proposed by Croft constitutes a nomen nudum and is not published here in compliance with the proposal by Mayr & Ashlock (1991:391).

Clarkina cf. bitteri. -- Kozur 1992d, figs. 1b/17a, b.

Type material. -- Holotype (N 4065) and paratype (N 4066) deposited with the holdings of the Museum of Northern Arizona at Flagstaff, Arizona, U.S.A.

Type locality. -- Outcrop approximately 1 km E of locality H of Ormiston & Babcock (1979), Culberson County, Texas.

Type horizon. -- Sample 243, uppermost Lamar Limestone, 40 cm below its upper surface.

Etymology. -- In honor of Dr. J. S. Croft, who first discovered and reported this new taxon.

Diagnosis. -- Clarkina with long free blade, narrow platform with strongly upturned margins along the posterior two-thirds of the platform element, which is also mostly rudimentary at the posterior end. Carina high, lowest in the posterior part.

Description. -- Platform element small, moderately arched, straight or laterally slightly bent. The platform is strongly reduced and extends only along the posterior two-thirds of the unit. In some specimens the platform is rudimentary throughout its length. The platform is narrow even in its widest part behind the midlength or at the beginning of the posterior third of the unit. Near the posterior end the platform width is distinctly reduced. The free blade is very long and comprises the anterior third of the unit. Along its anterior part no platform rudiments are present, along its posterior part very narrow, ridge-like platform rudiments are present. The platform margins are strongly upturned and display honeycomb microreticulations. No serration is present. Basal cavity subterminal, in forms with strongly reduced platform nearly terminal. Keel narrow. Carina high, with long, unfused denticles. The height of the carina decreases slowly toward the posterior end. All denticles are nearly straight or only slightly inclined.

Occurrence. -- Rare in the topmost Lamar but, according to Croft (1978), dominant in the basal Castile Formation at the very base of the Ochoan below the base of the hypersaline facies. Samples examined during this study of the basal Castile "papery limestone" of the Delaware Basin and of the facially and time-equivalent "papery limestone" of the basal Tessey Formation in the Glass Mountains have not yielded any conodonts. In turn, a first dissolving of a post-Lamar/pre-Castile limestone from the Lamar Cuesta near locality N of Ormiston & Babcock (1979) immediately west of Texas Ranch Road 1108, Culberson County, Texas, has yielded a few broken specimens of Clarkina crofti. This limestone has the same stratigraphic position as the basal "Castile Limestone" sensu Croft (1978) that he sampled near our locality on the Lamar Cuesta at the opposite, eastern side of Texas Ranch Road 1108. The specimens of Croft (1978) are therefore not from the basal Castile, but from a post-Lamar/pre-Castile limestone intercalation (post-Lamar Bell Canyon Formation).

Remarks. -- The material illustrated by Croft (1978) included some specimens with strongly reduced platforms that consist only of a broadened ridge. These specimens are already very similar to Early Triassic through basal Anisian genera in the transitional field from platform-bearing gondolellids to the blade-like Neospathodus and vice versa: Kashmirella Budirov, Sudar & Gupta, the transitional genus from Clarkina to Neospathodus, Chengyuania Kozur (1994), the transitional genus from Neospathodus to Paragondolella and Chiosella Kozur, the transitional genus from Neospathodus to Neogondolella.

DISCUSSION

Conodonts are stratigraphically one of the most important fossil groups in the Permian. They evolved rapidly and, in some stratigraphic intervals, even somewhat more rapidly than Permian ammonoids. Conodonts also exhibit a broader distribution than Permian ammonoids because they are the only stratigraphically important Permian fossil group that is little affected by provincialism. Such provincialism is very strong among all other stratigraphically important Permian fossil groups, especially fusulinids. Boreal and Tethyan ammonoids are also very different from one another.

As with other geological systems, Permian conodont distribution has been influenced by facies differences. Shallow water and pelagic conodont faunas are rather different, but in many places, especially in widespread slope deposits, both shallow water and pelagic conodonts occur in the same rocks. Moreover, some genera, and even some species, occur both in shallow water (except intratidal deposits) and pelagic deposits. Therefore, the correlation of shallow water and pelagic conodont zonation is generally very easy.

The evolutionary rates of shallow-water conodonts of the Lower Permian are considerably more rapid than those of pelagic conodonts. This is considered an exceptional situation in conodont evolution. Lower Permian shallow-water conodont zonation is therefore more detailed than pelagic zonation. During the Upper Permian, the evolutionary rates of both shallow water and pelagic conodonts were rapid; detailed zonations of shallow water and pelagic conodonts have been established. With the exception of the Roadian stage, the shallow-water conodonts of the Middle Permian are currently considered of minor importance relative to conodont biostratigraphy because it is based on the evolution of pelagic conodonts. The development from the serrated Mesogondolella postserrata (Clark & Behnken) through Mesogondolella shannoni Wardlaw into the unserrated Clarkina altudaensis Kozur within the uppermost Altuda Formation of the Glass Mountains at the southern margin of the Delaware Basin (Kozur 1991; 1992b; 1992c; 1992d) can also be found in South China (Jin et al. 1993), allowing the exact correlation of the upper boundary of the Capitanian (and by this of the upper boundary of the Middle Permian Guadalupian Series) with the Tethyan scale as established in South China.

As Kozur (1990b) pointed out, Clarkina evolved from Mesogondolella by slight foreward-shifting of the terminal basal cavity (Mesogondolella) into a subterminal position (Clarkina and most of the stratigraphically younger gondolellids), by a change of the shallow trough-like surface into a flat lower surface of the keel with low narrow marginal ridge, and by the development of a free blade and disappearance of the platform serration (only in serrated Mesogondolella). This development occurred in several lineages within the genus Mesogondolella. The Clarkina features developed within the different lineages at slightly different times during the late Guadalupian or at the end of the Guadalupian (Capitanian/Dzhulfian or Wuchiapingian boundary). Kozur (1990b) noted that Clarkina bitteri (Kozur) is the oldest Clarkina and the forerunner of the main stock within this genus (C. bitteri / C. leveni lineage). A second important lineage leads from serrated Mesogondolella postserrata through serrated M. shannoni to unserrated Clarkina altudaensis. This short-lived species is the forerunner of Clarkina changxingensis (Wang & Wang) that evolved soon after the appearance of C. altudaensis. Clarkina changxingensis is one of the most common conodonts of the Lopingian (Late Permian) Series (Kozur 1992a). A third lineage, important especially for the Boreal realm, leads from serrated Mesogondolella behnkeni (Bando et al.) to unserrated Clarkina rosenkrantzi (Bender & Stoppel).

Jin et al. (1993) expressed a different view, restricting Clarkina to the C. bitteri/C. leveni lineage and leaving other Clarkina, which had not evolved from C. bitteri, within the genus Mesogondolella. By this, no form separation between Mesogondolella and Clarkina is possible. This is best documented by Clarkina changxingensis, which is regarded by many Chinese authors as a subspecies of Clarkina subcarinata (Sweet), a characteristic species of the C. bitteri/C. leveni stock. Clarkina changxingensis, however, evolved in the uppermost Altuda Formation from C. altudaensis. All transitional forms from C. altudaensis to C. changxingensis are present (Kozur 1992a; 1994). The development of one genus from another genus does not necessarily go through one species. As the example of Mesogondolella/Clarkina shows, several species of one genus may evolve the same new features that lead to the separation of a new genus. Examples for this mode of evolution are known also from other fossil groups (e.g., ostracods). Likewise, new families or higher taxonomic categories evolved often by changes of suprageneric features in several genera of the forerunner family. For instance, the characteristic features of the recent Punciacea (Ostracoda), like bracket teeth, narrow mesoplate, calcified inner lamella, and special sculpture elements such as the pseudofrill, evolved independently from each other in different lineages of the latest Permian Kirkbyacea, the forerunners of the Punciacea (Kozur 1993b).

For the first time a strong platform reduction can be observed in Clarkina that lead to the development of the blade-like platform-less Neospathodus during the basal Triassic. The platform reduction is an iterative development within gondolellid conodonts that occurs in different lineages at different times (Kozur 1990b). Despite the strong platform reduction, C. crofti is probably not the direct forerunner of Neospathodus, which begins only in the Gandarian substage (= Dienerian) of the Brahmanian stage (= Induan stage, the original Induan comprises also the lower substage of the Olenekian) (Kozur 1992e). However, it shows for the first time the trend of platform reduction within Clarkina that lead ultimately through transitional forms to Neospathodus in the Lower Triassic.

Clarkina crofti does not belong to the Mesogondolella serrata/M. postserrata lineage of the Delaware Basin and its slopes, but to the C. bitteri group. Clarkina bitteri displays also a distinct free blade with rapid narrowing of the platform at the beginning of the free blade, but the platform is not reduced, but wide, so it has a different outline.

The only similar older species from the Delaware Basin is Mesogondolella (?) denticulata (Clark & Behnken). According to Wardlaw (pers. comm.), "Neogondolella" denticulata is a junior synonym of Mesogondolella postserrata (Behnken). This study agrees with Wardlaw (pers. comm.) in that, with the exception of the holotype, all "N." denticulata illustrated by Clark & Behnken (1979) are typical Mesogondolella postserrata. The holotype, however, is a form with a strongly upturned margin, distinct free blade and very high carina. It appears to be an independent species. The strongly upturned margin argues against a close relationship to the serrated Mesogondolella of the M. nankingensis/ M. postserrata group. The presence or absence of serration in the holotype is not clear from the oblique lateral view. On one side, where the upper surface is seen, no serration of the anterior platform can be observed. On the other side, shallow vertical furrows are present on the lower outer side of the anterior platform. This may indicate the presence of serration on the platform surface, but such shallow vertical furrows on the lower outer side of the platform are also common in unserrated Late Permian and Triassic gondolellids. If the holotype of Mesogondolella (?) denticulata is unserrated, it could be related to Clarkina crofti, but is easily distinguished from this species. With the exception of the anteriormost part, a distinct platform extends along the free blade and the platform width is unreduced near the posterior end of the unit. Moreover, the height of the carina does not decrease toward the posterior end. The sudden appearance of C. crofti in the deep-basin parts of the uppermost Lamar and its dominance in the post-Lamar/pre-Castile Bell Canyon Formation of the same facies indicate immigration from outside the Delaware Basin.

PALEOECOLOGY AND BIOSTRATIGRAPHY

Clarkina crofti occurs only in the deep-basin facies of the Lamar and post-Lamar/pre-Castile Bell Canyon Formation limestones. Its westernmost occurrence is about 1 km E of locality H, about 12 km E of the Capitanian reef margin. The water depth there was below 500 m. In the post-Lamar/pre-Castile limestones it was only found still further in the basin center. It is missing in the contemporaneous shallow pelagic uppermost Altuda Formation. In these beds only C. altudaensis occurs, at first still accompanied by M. shannoni, transitional forms between both species, and Clarkina wilcoxi (Clark & Behnken). In the topmost 20 cm of the Altuda, only Clarkina lanceolata (Ding), a late Wuchiapingian guideform, has been discovered. The fauna of the uppermost Altuda at the investigated localities (Kozur 1992b; 1992c) indicates water depth of about 50-100 m.

In the uppermost Lamar, C. crofti is accompanied by the last very rare Mesogondolella shannoni which is considered transitional to Clarkina altudaensis. In the rather thin (about 2-3 m thick) deep basinal post-Lamar/pre-Castile deposits along the Lamar Cuesta near the Texas Ranch Road 1108 so far only C. crofti and two specimens of C. altudaensis have been found by Croft (1978) and in material examined during this study. Clarkina shannoni is no longer present. Therefore, these beds can be easily correlated with the C. altudaensis zone of the uppermost Altuda in the Glass Mountains. However, the conodont fauna is rather different for facial reasons. Whereas the uppermost Altuda is dominated by C. altudaensis, which in several samples is the only gondolellid conodont, the deep-basin facies equivalents in the post-Lamar/pre-Castile interval are dominated by C. crofti, and C. altudaensis is only very subordinate.

As in the Delaware Basin, in the Tethyan realm a strong radiation of the Clarkina bitteri group can be observed within the Late Permian Lopingian Series. Species of this lineage are the index species of the latest Permian pelagic zones within the Tethys (Teichert et al. 1973: Kozur 1978; 1989; 1990a; 1990b; 1990c; 1993a; Wang & Wang 1979; Nestell & Wardlaw 1987; Clark & Wang 1988; Jin et al. 1993).

Both the shallow pelagic C. altudaensis fauna and the deep pelagic C. crofti fauna are very different from the Mesogondolella faunas of the Middle Permian Guadalupian Series that are characterized by serrated Mesogondolella of the M. nankingensis / M. postserrata group. During the Early and Late Permian, in turn, only smooth gondolellids are present. The base of the Guadalupian Series has been defined by the first appearance of the serrated M. nankingensis. The sudden and world-wide disappearance of this group within phylogenetic lineages (Mesogondolella shannoni / Clarkina altudaensis, Mesogondolella behnkeni / Clarkina rosenkrantzi) is a first order event in conodont evolution and would be an ideal boundary between the Guadalupian and Lopingian Series in conodont stratigraphy, independent from the question of whether C. altudaensis and C. rosenkrantzi are smooth Mesogondolella, as Jin et al. (1993) assumed or Clarkina species as Kozur (1992b, 1992c, 1992d) assumed. Jin et al. (1993) found the same sudden disappearance of serrated Mesogondolella in the M. postserrata/ M. shannoni / Clarkina altudaensis lineage in China as Kozur (1991, 1992b, 1992c, 1992d) found in the Altuda Formation at the margin of the Delaware Basin. Thus, the sudden disappearance of serrated Mesogondolella is not a local event in the Delaware Basin, which could be facies-controlled. For this reason, this study cannot confirm the proposal of Jin et al. (1993) that rapid change from the ribbed Mesogondolella postserrata (in their taxonomy including the similarly ribbed M. shannoni) into the unribbed Clarkina altudaensis should not be regarded as a natural boundary because it does not reflect any evolutionary event in geological and biological development from the Guadalupian to Lopingian.

An evolutionary event must not necessarily reflect the change from one genus to another (as in the interpretation of the lineage by Kozur 1992b; 1992c; 1992d). It can be also defined within a genus (as preferred by Jin et al. 1993). This does not mean, however, that it is not a biological event. The development from the unribbed Mesogondolella idahoensis (Youngquist, Hawley & Miller) to the ribbed M. nankingensis (Ching) is likewise placed within one genus, and there is now a general agreement that this world-wide traceable event is the best marker for the base of the Guadalupian Series (Glenister et al. 1993). Why shouldn't the likewise rapid and world-wide recognizable disappearance of the ribbed Mesogondolella be a biological event from the Guadalupian to the Lopingian Series? This statement would seemingly only "prove" that Clarkina altudaensis is a Mesogondolella, but would not change the fact that the serrated species of Mesogondolella appeared at the base of the Guadalupian and disappeared at the top of the Guadalupian and that this development is not a special, facially-controlled development of the Guadalupian type area, but recognizeable worldwide.

This conodont event is still more important, because it coincides with another lineage, in which the serration disappeared. Mesogondolella behnkeni (Bando et al. 1980), still present in the Lamar, evolved into the unserrated Clarkina rosenkrantzi (Bender & Stoppel). Here, it also is not relevant for the stratigraphic evaluation that his development may be phylomorphogenetic change within a genus (Mesogondolella) or between two genera (Mesogondolella and Clarkina). Clarkina rosenkrantzi is the only gondolellid conodont of the Dzhulfian beds of Greenland and of the Zechstein Limestone in middle and NW Europe. This gondolellid fauna without serrated conodonts is very different from the Guadalupian conodont fauna with serrated conodonts (Kozur 1978; 1992b; 1992c; 1992d).

In the topmost Lamar a very distinct change in the radiolarian fauna can also be observed. Follicucullus ventricosus Ormiston & Babcock and Ishigaconus scholasticus (Ormiston & Babcock), the most common albaillellids of the Late Permian Lopingian Series of the Circum-Pacific area and of the Tethys (toward the west until western Sicily) (Kozur 1993c), begin at this level.

Thus, two of the three important pelagic faunal elements show very distinct changes at the base of the C. altudaensis zone. The third important pelagic group, the ammonoids, are not yet well studied near the Guadalupian-Lopingian boundary. However, Furnish (1973) pointed out that the ammonoid fauna of the (upper) Lamar is post-Guadalupian. Also, the development of all other stratigraphically important fossil groups around the first appearance of C. altudaensis still needs to be studied. The lower Lamar yields Yabeina Deprat, a Tethyan Capitanian fusulinid genus. Above this level, Paraboultonia Skinner & Wilde, Codonofusiella Dunbar & Skinner and Reichelina Erk are common. Unfortunately, Yabeina is an immigrant and it is not clear if its uppermost occurrence in the lower Lamar corresponds to its uppermost occurrence in the Tethys.

The uppermost part of the Kufeng (Gufeng, Kuhfeng) Formation, in which the unserrated C. altudaensis evolved from serrated Mesogondolella, corresponds to the Lengwu Member of the Tinjiashan Formation of Zhejiang Province, China (He 1980). At this stratigraphic level a very interesting brachiopod fauna occurs that contains, in addition to typical elements of the Guadalupian Maokou Formation such as Unisteges maceus (Ching) and Orthotichia nana (Grabau), typical and common elements of the Lopingian Longtan Formation, such as Cathaysia chonetoides (Chao), Haydenella wenganensis (Huang), Leptodus nobilis (Waagen), Neochonetes substrophomenoides (Huang), Tschernyschewia sinensis Chao, and Tyloplecta yangtzeensis (Chao). Therefore, the brachiopod faunas also show distinct changes about at the same level where the serrated Mesogondolella disappeared and C. altudaensis appeared worldwide.

The second argument of Jin et al. (1993), that in the level of the transition from serrated Mesogondolella to the unserrated Clarkina altudaensis no geologic events occur, is rather a phenomenon local to South China. At the level where Clarkina of the C. bitteri / C. leveni group migrated into the depositional area of South China, the event favored by Jin et al. (1993) as the Guadalupian-Lopingian boundary, no pronounced geologic changes can be observed. Moreover, the level of immigration of a certain faunal group always has high potential to be a diachronous boundary, if this level is to be correlated with other occurrences of the immigrants outside South China. Therefore, this level is unsuitable for definition of the Guadalupian-Lopingian boundary. No changes of the radiolarian faunas occur at this level. It is not desireable to choose another level of immigration of Late Permian guideforms than it is to maintain the present base of the Dzhulfian stage at the base of the C. leveni zone, a level where many different Dzhulfian forms immigrated into the Transcaucasian Dzhulfian type area. This boundary can be defined within a cline from C. liangshanensis (Wang) to C. leveni and is therefore better suited as the level of immigration of Clarkina of the C. bitteri / C. leveni group into South China, species with an unknown immediate forerunner.

CONCLUSIONS

The easily recognizable Clarkina crofti, a deep pelagic member of the Clarkina bitteri / C. leveni lineage, has a high potential to become a stratigraphically important form for deep pelagic conodont faunas, like C. altudaensis is for shallow pelagic ones. Further investigations in deep pelagic deposits of the Tethys, especially in the very fossil-rich deposits of western Sicily, are necessary to find this form in the Tethys.

ACKNOWLEDGMENTS

The investigations of H. Kozur in Texas and in the western Tethys have been sponsored by the Deutsche Forschungsgemeinschaft. The New Mexico Museum of Natural History and Science provided additional support. We are grateful to several landowners for access to their land. We wish to thank Bruce Wardlaw and an anonymous reviewer for their helpful comments.

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Heinz Kozur and Spencer G. Lucas

Rezsu u. 83, H-1029 Budapest, Hungary and New Mexico Museum of Natural History and Science, 1801 Mountain Road NW, Albuquerque, New Mexico 87104

SGL at: lucas@darwin.nmmnh-abq.mus.nm.us
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Author:Kozur, Heinz; Lucas, Spencer G.
Publication:The Texas Journal of Science
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Date:May 1, 1996
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