S. J. MAZZULLO Department of Geology, Brooklyn College of the City University of New York, Brooklyn, New York 11210

Length of the Year during the Silurian and Devonian Periods: New Values

ABSTRACT were directly counted on each specimen after it was etched for two minutes in a 5 percent Daily growth increments and monthly dilute hydrochloric acid bath. On poorly pre- markings on Silurian and Devonian corals served specimens, however, the number of and brachiopods were counted using a maxi- days per month was determined by dividing mum count method. Early and Middle Silur- the observed number of days per year by the ian fossils indicate that the number of days observed number of months per year. per year during these periods was 421 and The results of this study indicate that the 419, respectively; the number of days per number of days per year during the Early year in the early Middle Devonian Period Silurian Period was 421; that it was 419 days was 410. The number of days per month has during the Middle Silurian Period, and 410 decreased from 32.4 in the Early Silurian to days during the early Middle Devonian 31.5 in the early Middle Devonian. The Period. Coincident with this decrease in the values obtained for these periods are con- number of days per year from the Silurian to sidered more accurate than the values ob- the Devonian is the decrease in the number tained using average count methods. of days per month from 32.4 to 31.5; a decrease in the number of months per year GEOCHRONOLOGY from 13 in the Silurian to 12 at the present The realization that various fossil organisms time has also occurred (Table l). Scrutton could be useful as geochronological tools (1964) observed monthly bands on the epi- has led to major advances in the field of theca of several Devonian corals. A count of geochronology. Wells (1963) was the first to these bands, in what he assumed to be an suggest that the minute lines observed on annual zone, indicated a Devonian year con- the epitheca of various fossil corallites were sisting of 13.04 months. By dividing 399 daily growth increments, and that the most (the number of days in a Devonian year as prominent ridges observed on the corallites computed geophysically) by 13.04 months, were annual increments. A count of the Scrutton concluded that there were 30.6 days number of daily increments contained per month in the Devonian year. Berry and between two successive annual ridges indi- Barker (1968), using Late Cretaceous bivalves, cates the number of days per year for the determined that there were 370.3 days, 12.5 geological period during which the organism synodic months, and 29.6 days per month in lived. Numerous other workers have since the Late Cretaceous year. Wells (1963) deter- contributed greatly to our knowledge of the mined that the average lengths of the years number of days and months per year in the during the Middle Devonian and Pennsyl- geologic past with the aid of paleontological vanian Periods were 400 and 390 days, re- evidence. The present study was undertaken spectively. MacClintock and others (1968) in order to redetermine Silurian and Devonian also presented paleontological evidence that geochronological values from such evidence, the number of days per synodic month and employing a maximum count method of the the number of synodic months per year has daily growth increments observed. decreased throughout geologic time. Silurian and Devonian corals and brachio- Previously reported geochronological pods were examined with the binocular micro- values have been accepted by a great number scope under a magnification of 15 X. Daily of geologists. These values were based upon growth increments and monthly markings average counts of the daily growth incre-

Geologkal Society of America Bulletin, v. 82, p. 1085-1086, April 1971 1085

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TABLE 1. VALUES FROM THIS STUDY REPRESENT AVERAGES OF MAXIMA FOR A SET OF 50 SPECIMENS*

Fossil Age Days/Year Days/Month Author Holophragma calceloides Early Silurian 421 32.4 Cyrtia radians Middle Silurian 419 32.25 Eospirifer radiatus do- 419 32.25 Spinocyrtia sp. early Middle Devonian 409 31.5 Brachyspirifer audaculus do. 403 31.0 this study Brachyspirifer audaculus do. 409 31.5 Brachyspirifer audaculus do. 410 31.5 Craspedophyllum archacii do. 410 31.5 Heliophyllum halli do. 410 31.5 , various corals Middle Devonian 400 Wells (1963) various corals Pennsylvanian 390 Wells (1963) various corals Devonian 399t 30.6 Scrutton (1964) various bivalves Late Cretaceous 370.3 29.6 Berry and Barker (1968) mollusks and stromatolites /Upper Cambrian 31.56 \ MacClintock and others \Middle Devonian 30.53 / (1968) *Values obtained as a result of previous studies and those of the author. The number of observed monthly markings on all specimens was 13. fFrom geophysical computations.

ments rather than a maximum count, as sug- Dr. W. G. Heaslip, of the Department of gested by Clarke (1968). It must be empha- Geology at Cortland, New York, for critic- sized that the formation of the daily growth ally reading the early drafts of the manuscript increments is attributed to the diurnal secre- and offering many helpful suggestions. I tion of calcium carbonate by the organism also acknowledge the encouragement and (Goreau, 1959). Therefore, the average count aid from Dr. E. L. Savage, Mr. P. Agostino, method falls prey to two factors: under ad- and Mr. M. Kane, of Brooklyn College. verse conditions dutin g the life of the organism involved, only very limited secretion may REFERENCES CITED occur; the resulting growth lines may be Berry, W.; and Barker, R. Fossil bivalve shells obscure, and they may be totally absent if no indicate longer month and year in Creta- diurnal growth takes place. Hence, there is an ceous than present: Nature, Vol. 217, p. inherent disagreement between the observed 938-939, 1968. number of daily growth increments contained Clarke, G. R. Mollusk shell: Daily growth within an annual zone on a particular fossil lines: Science, Vol. 161, p. 800-802, 1968. and the actual number of days per year in the Goreau, T. F. The physiology of skeleton for- period during which the organism lived. If mation in corals. I. A method for measur- used, the average count method would yield ing the rate of calcium deposition by corals a minimum value for the number of days per under different conditions: Biol. Bull., Vol. 116, p. 59-75, 1959. month or year, while the maximum count MacClintock, C.; Pannella, G.; and Thomp- method would overcome this systematic error son, M. Paleontological evidence of varia- and yield a higher, more accurate value. tions in length of synodic month since In summation, the use of a maximum count Late Cambrian: Science, Vol. 162, p. 792- method yields more precise geochronological 796, 1968. values than does the average count method, Scrutton, C. T. Periodicity in Devonian coral and it (maximum count method) should be growth: Palaeontology, Vol. 7, p. 552-558, used for future determinations. The discrep- 1964. ancy between previously published values Wells, J. W. Coral growth and geochronometry: and those established by this investigation Nature, Vol. 197, p. 948-950, 1963. are likely to be due to the different "count" methods used. MANUSCRIPT RECEIVED BY THE SOCIETY DECEMBER 16, 1970 ACKNOWLEDGMENTS I wish to thank Dr. N. D. Newell, of the American Museum of Natural History, and PRINTED IN U.S-A.

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