Examining Enameloid Chemistry in Lower Vertebrates to Determine Conodont Taxonomic Affinity

11th International GeoRaman Conference (2014) 5090.pdf

EXAMINING ENAMELOID CHEMISTRY IN LOWER VERTEBRATES TO DETERMINE CONODONT TAXONOMIC AFFINITY. C. P. Marshall1 and A. Olcott Marshall1, 1Department of Geology, Lindley Hall, 1475 Jayhawk Blvd, University of Kansas, Lawrence, KS, 66045, [email protected] and [email protected]

Introduction: Conodonts are an extinct group of the ν1 P-O totally symmetric stretching mode (Ag marine organisms, which were soft bodied apart from symmetry), is observed to shift between 960 to 965 the phosphatic mineralized tooth-like elements that cm-1. This shift in the observed Raman frequencies is 3- superficially resemble small fish teeth ranging in di- due to constriction or dilation of the tetrahedral PO4 mensions from 0.1 to 5 mm [1]. These organisms oc- polyatomic anion within the apatite lattice as oxygen cupied marine environments with a temporal range atoms of neighboring tetrahedra are either drawn near from the Early Cambrian (495 Ma) through to the Late or pushed apart on substitution of cations (e.g., Na+, + 2+ - 2- - - Triassic (199 Ma) [1]. Well-preserved conodont ele- K , and Mg ) and anions (e.g., OH , CO3 , Cl , and F ) ments are divided into two portions, commonly called of different size. Therefore, the shift in Raman fre- the crown and basal filling, which resemble enamel quency of the dominant ν1 P-O totally symmetric and dentine of teeth [2,3]. Despite over one hundred stretching mode (Ag symmetry) is characteristic for and fifty seven years of study, the phylogenetic posi- different apatite minerals and hence is a powerful tion of conodonts remains controversial. There have technique for identifying apatite mineralogy. been many vertebrate and invertebrate groups pro- Results: Five Raman spectra were acquired on posed as the closest living analogues of the conodont individual conodonts isolated from the eight geological group, including chaetognaths [4], primitive chordates formations of various age, thermal alteration, and [5], protochordates [6], and agnathans [7]. The aim of lithology, in the frequency region 400 to 1200 cm-1. this research is to determine the phylogenetic position Figure 1 shows a representative spectrum from the 75 of conodonts through inorganic tooth chemistry. collected spectra. The dominant band, which can be -1 Conodonts are often considered to represent the observed at 965 cm , is assigned to the ν1 P-O totally 3- earliest vertebrate mineralized tissue [8], and, as such, symmetric stretching mode (Ag) of the PO4 tetrahe- considered to be the precursor to all teeth. The evolu- dron which is the characteristic Raman shift for that of tion of mineralized tissues, including teeth, was a cru- fluorapatite. Despite the difference in host rock lithol- cial step in the radiation of vertebrates, as these tissues ogy, geological age and thermal alteration, Raman allowed for predation, protection, and locomotion. spectroscopy demonstrates that these conodonts are However, despite the importance of these structures, composed of fluorapatite. their evolution is not very well understood. Vertebrate teeth consist of two mineralized layers, a harder surface layer and a softer internal layer. In lower vertebrates, the harder surface layer is referred to as enamaloid, while in tetrapods it is referred to as enamel, while both higher and lower vertebrates have dentin in their internal layer [9]. Dentine, enamel, and enameloids are composed of bioapatite, inorganic calcium phosphate compounds with the general formula Ca5(PO4)3X, where X is typically F (fluorapatite – francolite), OH (hydroxyapatite), or CO (carbonated 3 hydroxyapatite – dahllite). Lower chordates and lower Figure 1: Representative Raman spectrum showing the vertebrates biomineralize various bioapatite com- diagnostic ν1 P-O totally symmetric stretching mode pounds, such as apatite Ca (PO ) , whitlockite - -1 5 4 3 β (Ag) at 965 cm . Ca3(PO4)2, hydroxylapatite Ca10(PO4)6(OH)2, dahllite Ca5(PO4, CO3)3(OH), and fluorapatite Ca10(PO4)6F2, Discussion: Although studies elucidating the inorganic depending on their phylogeny. chemical composition of teeth and mineralized Raman spectroscopy of calcium phosphate salts: enameloids from lower chordates and lower vertebrate There are four internal phonon modes of the tetrahe- groups are lacking, it has been shown that modern fish 3- dral PO4 polyatomic anion that constitute the funda- enameloids are composed of fluorapatite, hydroxyapa- mental phonon modes of apatite minerals. The strong- tite, and carbonated hydroxyapatite; modern and fossil est intensity band and hence the most diagnostic band, sharks, skates, stingrays enameloids are composed of fluorapatite; modern hagfish enameloids are composed

Abstract for 11th GeoRaman International Conference, June 15-19, 2014, St. Louis, Missouri, USA 11th International GeoRaman Conference (2014) 5090.pdf

of carbonate; modern lungfish enameloids are composed of hydroxyapatite. Further work will be under- taken to elucidate enameloid composition of other modern and fossil lower chordate and lower vertebrate teeth to better constrain our phylogenetic placement of these enigmatic microfossils. From our preliminary work thus far, the enameloid composition of conodont points to a primative vertebrate affinity.

References: [1] Clark, D. L., W. C. Sweet, S. M. Bergström, G. Klapper, R. L. Austin, F. H. T. Rhodes, K. J. Müller, W. Ziegler, M. Lindström, J. F. Miller, and A. G. Harris. (1981) Treatise on invertebrate pale- ontology. [2] Müller, K. J., Nogami, Y.U. (1971) Memoirs of the Faculty of Science, Kyoto University, Series of Geology and Mineralogy 38, 1–87. [3] Katvala, E.C., Henderson, C.M. (2012) Paleobiol- ogy, 38, 447-458. [4] Kasatkina, A. P. & Buryi, G. I. (1997) Doklady Biological Sciences 356, 503-505. [5] Briggs, D.E.G., Clarkson, E.N.K., Aldridge, R.J. (1983) Lethaia 16, 1–14. [6] Kemp, A., Nicoll, R.S. (1997) Modern Geology 21, 197–213. [7] Krejsa, R. J., Bringas Jr., P., Slavkin, H. C. (1990) Lethaia 23, 359– 378. [8] Janvier, P. (2013) Nature 502, 457–458. [9] Kawasaki, K., Suzuki, T., Weiss, K.M. (2005) Pro- ceedings of the National Academy of Sciences 102, 18063–18068.

Abstract for 11th GeoRaman International Conference, June 15-19, 2014, St. Louis, Missouri, USA