CONODONTS of the LOWER CARBONIFEROUS Abstract

Geoinformatics, Vol. 4. No. 3, p.309-312, 1993 309

INDEX CONODONTS OF THE LOWER CARBONIFEROUS

Sonny Baxter Department of Geology, University of Cincinnati Cincinnati Ohio, 45271-0013, USA Ohio Uni- versity at Lancaster

Abstract : A comparison of conodont collections from the Lower Carboniferous indicates that many conodont elements are excellent index fossils. Some of these have a world-wide distribution and always represent the same stratigraphic interval. These, the type A index conodonts, include species of Eotaphrus, Pseudopolygnathus, Siphonodella, and Taphrognathus and are good inter-and intra-regional markers. Type B index conodonts are restricted to specific regions and are good stratigraphic markers within these. Bactrognathus distortus is present in most samples from the middle of the Burlington Formation from that formation's erosional edge in east-central Iowa to just south of St. Louis Missouri, in a few samples from the Floyds Knob Member of the Borden Formation in eastern Kentucky (Goodman, 1975, Bax- ter 1993a), and in samples from the Andrecito Member of the Lake Valley Formation of New Mexico (Burton, 1964). This whole region was just south of the equator during the Carbonifer- ous (Baxter, 1993b). Another type B index conodont is Mestognathus. This taxon is common in collections from Europe, but has also been recovered from exotic, accretionary terrains in Atlantic Canada and Western North America. Type C index conodonts occur word-wide, and although they are good markers within a region, they have different ranges in different regions. Gnathodus texanus and several species of Cavusgnathus range throughout the Visean of North America. In the Midcontinent Region G. texanus occurs throughout this interval but is restricted to the Chester in Western Canada. Cavusgnathus is present throughout the Visean of Western Canada but does not occur below the upper part of the St. Louis Formation in the Midcontinent Region. Type A index conodonts are essential for word-wide correlation. The other two types are good for correlation within specific regions, but may also serve other purposes. The type B index conodonts may reflect fixed barriers such as restrictive currents or limiting salinity or temperature gradients. This type may also serve to identify exotic terrains and plate movements. Type C index conodonts may be indicative of fluctuations in environmental conditions or may reflect the lateral migration of a taxon. key words : Correlation, Exotic terrains, Environmental indicators, Index fossils

INTRODUCTION recognition of the other two types. The type B index Certain elements from the apparatus of a number conodonts have the same characteristics, but are re- of conodont species are excellent index fossils for the stricted to a specific geographic region. The type C Lower Carboniferous. These, often the P-or plat- index conodonts also have a world - wide distribution form element, are easily recognized and make good and generally satisfy the same criteria, but their time-markers. Many conodont workers have noted, stratigraphic range varies from region to region. however, that although many conodonts had a world- The type A index conodonts include many of the wide distribution, some had different ranges in dif- species of Eotaphrus Pierce & Langenheim, Polyg- ferent regions while others were restricted to particu- nathus Hinde, Pseudopolygnathus Branson & Mehl, lar areas. Siphonodella Branson & Mehl, Synclydognathus Rexroad & Varker, and Taphrognathus Branson & INDEX CONODONTS Mehl. Together these have made it possible to estab Some conodont elements have a distinct and char- lish a sequence of concurrent range zones. Notable acteristic shape, a characteristic stratigraphic range, amongst type B index conodonts are Bactrognathus and a world-wide or regional distribution: these are distortus Branson & Mehl, and several of species of termed index conodonts (Fig.1). When we study their Mestognathus Bischoff. Bactrognathus distortus oc- stratigraphic distribution within the concept of con- curs in most samples from the lowest ten feet of the current range zones, we find that many of these have Haight Creek Member of the Burlington Formation a world-wide distribution, always have the same rel- and in some samples from the Floyds Knob Member ative position with respect to each other, and always of the Borden Formation and the Andrecito Member have the same geologic range. These are the type A of the Lake Valley Formation. Its geographic limits index conodonts. They form the backbone for con- are the erosional edge of the Burlington Formation in odont correlations and supply the framework for the Iowa to the north, parts of the Floyds Knob near 310 Sonny Baxter

Fig.1. Stratigraphic Ranges of Index Conodonts of the Lower Carboniferous (Mississippian) of North America. Unless indi- cated, ranges shown are those of the genus as a whole. Dashed lines indicate that a taxon ranges through that interval in some regions but not in others.

Moorhead, Kentucky to the east, sections just south of the Chester. In Western Canada several species of St. Louis, Missouri, and the Lake Valley Formation Cavusgnathus coexist with Taphrognathus through- in New Mexico to the west. As shown by Bambach et out the latter's range and continue upward through- al. 1980, that region was just south of the Equator dur- out the Chesterian. In the Midcontinent Region, the ing the Carboniferous. Most Mestognathus are from first occurrence of Cavusgnathus is above the range of the Lower Carboniferous of Europe; those reported Taphrognathus. An interesting relationship, outlined from North America occur in exotic, accretionary in Baxter (1981), is the distribution of Doliognathus terrains in Atlantic Canada to the east and in a few latus Branson & Mehl, and Gnathodus cuneif ormis isolated sections out West. Typical type C index Mehl & Thomas. Doliognathus latus occurs within conodonts are Gnathodus texanus Roundy, and a the range of G. cuneiformis, but never in the same number of species of Cavusgnathus Harris & sample. Osagean sections near St. Louis, Missouri, Hollingsworth. In the Midcontinent Region G. tex- have G. cuneiformis restricted to intervals above and anus first appears, together with Taphrognathus, in below the range of D. latus. But in sections further the uppermost part (Unit 5 of Baxter, 1988) of the north G. cuneiformis occurs throughout its range, Burlington Formation and ranges throughout the up- and D. latus is absent. per Mississippian; out west, this form is restricted to Index conodonts of the Lower Carboniferous 311

DISCUSSION The type A index conodonts are typical guide fossils and also provide the framework necessary for other applications. The other two types of index conodonts not only serve a function in biostratigraphy, but also have other uses. The peculiarities of their occurrences give information about the conditions and changes in their environments and, in some cases , may identify plate movements. The type B index conodonts were probably restricted to a region by some fixed barrier, perhaps a restricting current or limiting temperature or salinity gradients. The distribution of Bactrognathus distortus may have been restricted by a strong equatorial current, low salinities due to heavy precipitation, or some limiting temperature. Isolated occurrences of Mestognathus may ulti- mately shed some light on past plate movements by identifying exotic terrains. The distribution of class C may either reflect the opening of migration routes during the Lower Carboniferous, as in the case of Cavusgnathus or Gnathodus texanus, or lateral changes in temperature or salinity as in the case of Doliogrwxthus latus and Gnathodus cuneif ormis. The preferred environment of one was probably intolera- ble to the other. Of some significance is an interesting study by Tsukamoto (1992) who reports on leptocephali, eel larvae, and on spawning sites of eels in the Atlantic and Pacific oceans. The presence of a salinity •h front•h in the Pacific appears to trigger spawning behavior in the adults, and the North Equatorial and Kuroshio currents return the leptocephali to the coastal waters of Asia. In the Atlantic, however, a temperature •gfront•h is the trigger, and the spawn is returned by the Gulf Stream. It is not difficult to see how a shift in the salinity gradient would change the spawning sites of the Pacific eels and thus their distribution, whereas a constant temperature distribution would maintain a constant, but restricted, distribution for the Atlantic eels. It is not difficult to envi- sion analogous situations that would control distribu tions for the conodont animals. In such a scenario, the type C conodonts would correspond to the Pacific eels, the type B to those of the Atlantic, and the type A index conodont would be those unaffected by either temperature of salinity.

CONCERNS AND RESTRICTIONS For a concept like this to be meaningful and utili- tarian, it is necessary that sampling be sufficiently detailed, not only that all of the taxa are found and documented, but also that there is a reasonable de- gree of certainty that absent taxa were really absent. For example, the suggestion that Mestognathus is a cosmopolitan form (von Bitter et al., 1986) is rejected. The explanation for the absence of this taxon from most of North America -- insufficient sampling -- is not valid. Over a thousand samples collected throughout the Mississippian succession in Western Canada (Baxter, 1972; Baxter and von Bitter, 1984) and numerous samples from Illinois, Indiana, Iowa, 312 Sonny Baxter

Zonation for the Osagean. Geol. Soc. Amer., Ab- Floyds Knob Bed and Edwardsville Member of stracts with Programs, vol. 13, p. 271. the Muldraugh Formation (Valmeyeran) South- Baxter, Sonny (1984) The Eotaphrus-Bactrognathus ern Indiana and North-Central Kentucky. Uni- Zone, a New Name for a Conodont Zone from the versity of Illinois at Champaign-Urbana , unpub- Type Burlington Formation. Ninth Intern. Congr. lished M.Sc. Thesis. of Carboniferous Stratigraphy and Geology, Comptes Rendu, vol. 2, pp. 247-252. Baxter, Sonny (1988) Burlington Conodonts and Tournaisian-Visean Boundary, Hannibal, Mis- souri, and Burlington Iowa. in Association Round Table, Amer. Assoc. Petr. Geol. Bull., vol. 72, p. 865. Baxter, Sonny (1991) Conodonts as Markers in Sedi- ment Cycles of the Burlington Formation, Iowa and Missouri. Eleventh Intern. Congr. of Car- bonif erous Stratigraphy and Geology, Comptes Rendu, vol. 3, pp. 6-19(1989). Baxter, Sonny (1993a) Bactrognathus distortus and Taphrognathus varians from the Floyds Knob Member of the Borden Formation: Diachronism or Miscorrelation. Geol. Soc. Amer., 1993 Ab- stracts with Programs, vol. 25/3, p. 6. Baxter, Sonny (1993b) Index Conodonts: A new Con- cept in Biostratigraphy with Examples from the Lower Carboniferous. Geol. Soc. Amer., 1993 Abstracts with Programs, vol. 25/3, p. 6. Baxter, Sonny and von Bitter, P.H. (1984) Conodont Succession in the Mississippian of Southern Canada. Ninth Intern. Congr. of Carboniferous Stratigraphy and Geology, Comptes Rendu, vol. 2, pp.253-264. Burton, R.C. (1964) A preliminary range chart of Lake Valley Formation (Osage) conodonts in the southern Sacramento Mountains, New Mexico. New Mexico Geol. Soc. Fifteenth Field Conf., pp . 73-75. Collinson, C., Rexroad, C.B., and Thompson, T.L. (1972) Conodont zonation of the North American Mississippian. Sweet, W.C., and Bergstrum, S. M., (eds.) Symposium on Conodont biostratigraphy. Geol. Soc. AmericaMem., no. 127,pp.353-394 Goodman, W.R. (1975) The Conodont Fauna of the Floyds Knob Member, Borden Formation. Uni- versity of Cincinnati, unpublished M.Sc. Thesis, Thompson T.L. and Fellows, L.D. (1970) Stratigra- phy and Conodont biostratigraphy of Kinder- hookian and Osagean (Lower Mississippian) rocks of southwestern Missouri and adjacent areas. Missouri Geol. Surv. and Water Resources Rept. Inv. 45. Tsukamoto, Katsumi (1992) Discovery of the spawning area for Japanese eel. Nature vol. 356, pp. 789 -791. von Bitter, P.H., Sandberg, C.A., and Orchard, M.C. (1986) Phylogeny, Speciation, and Paleoecology of the Early Carboniferous (Mississippian) Con- odont Genus Mestognathus. Royal Ontario Mu- seum, Life Sciences Contributions no. 143, 114 pp. Whitehead, N.H., III (1976) The Stratigraphy, Sedi- mentology, and Conodont Paleontology of the