Recognition of Annular Growth on Centra of Teleostei with Application to Hiodontidae of the Cretaceous Dinosaur Park Formation

The Campanian of Alberta and Saskatchewan produce the oldest hiodontid fossils from North America, which are represented only by isolated centra. As such, very little is known about the taxonomic affinities, morphology, life history characteristics, and paleoecology of the taxon or taxa represented by the Cretaceous centra. To study the age and growth characteristics of Hiodontidae from the Cretaceous Dinosaur Park Formation we examined the annular bone deposition on the fossil centra and compared it to extant specimens. We used annuli on opercula and centra to determine the age of dried museum skeletons of extant hiodontids and to describe criteria for identification of annuli on centra. Using the criteria from extant specimens, we then examined the Dinosaur Park Formation centra for annuli and determined the age and growth characteristics of the fossil individuals. Ages obtained from opercula and centra of extant Hiodon individuals ranged from 5-11 years. Annuli on extant centra had repeated broad opaque bands coinciding with raised, papilose surfaces and were followed by thin translucent bands. Nine centra from the Dinosaur Park Formation were also examined for similar yearly markings (i.e., raised/papilose band followed by a thinner band) using the methodology applied to extant centra. Ages obtained from the Dinosaur Park Formation centra ranged from 2-6 years. The Dinosaur Park Formation hiodontid centra were 48% smaller than those of extant Hiodon and the longevity of Cretaceous hiodontids appears to have been shorter than modern Hiodon. Future research will include the examination of approximately 80 hiodontid centra from the Dinosaur Park Formation to better describe the age and growth characteristics of Cretaceous hiodontids. Figure 2. Operculum and centrum of extant Hiodon alosoides (KU 12730): A, left operculum exhibiting 10 annuli and points of measurement, medial view, posterior oriented toward the top; B, an abdominal centrum, showing 10 annuli as translucent bands (arrows) and points of measurement (filled circles). INTRODUCTION The Campanian of Alberta and Saskatchewan produce the oldest known hiodontid fossils from North America, which are represented only by isolated centra (Brinkman and Neuman 2002). As such, very little is known about the taxonomic affinities, morphology, life history characteristics, and paleoecology of the taxon or taxa represented by the Cretaceous centra. Here we describe criteria for defining annular bone deposition on the fossil centra, which can be used to study the age and growth characteristics of Hiodontidae from the Cretaceous Dinosaur Park Formation. The objectives of this research are to: 1) use annuli on opercula to determine the age of dried museum skeletons of extant hiodontids; 2) examine centra from the same museum specimens to describe criteria for identification of annuli; 3) test the criteria by comparing age estimates based on various structures; and 4) use the new criteria to examine the Dinosaur Park Formation centra for annuli and determine the age and growth of the fossil individuals. MATERIALS AND METHODS Extant Taxa Extant Hiodon is only found in North America and is represented by two freshwater species, H. alosoides Rafinesque and H. tergisus Lesueur (Fig. 1A, B). The genus ranges in latitude from the Arctic (68°N) to the Gulf Coast of Louisiana (30°N) (Lee et al. 1980)(Fig. 1A, B). Hiodon can live to at least 16 years of age (Donald et al. 1992) and exhibit sexual size dimorphism, with females attaining larger sizes (Scott and Crossman 1973). Material Examined Opercula and centra from 12 dried skeletons of H. alosoides were examined: two skeletons from the Natural History Museum and Biodiversity Research Center (KU), University of Kansas, 12728 and 12730; and 10 skeletons from the Bell Museum of Natural History (JFBM), University of Minnesota, 38800, 40126, 41718, 43306, 43308, 43310, 43311, 43312, 43313, and 43314. Opercula and centra from four skeletons of H. tergisus were also examined (JFBM 41120, 43320, 43331, 43332). Nine fossil centra from the Dinosaur Park Formation (Brinkman and Neuman 2002) were examined in the Royal Tyrrell Museum (TMP) collection: 86.53.81, 86.179.6, 86.198.42, 87.153.39, 90.113.57b, 90.115.50a, 93.93.94, 95.163.44b, and 2000.6.3. Meristic Data and Measurements Opercula are commonly used to determine age of extant hiodontids. Opercula have been found to produce the most reliable age assignments (Donald et al. 1992) and thus we examined the effectiveness of ageing centra by comparing ages obtained from opercula and centra from various individuals. We identified opercular annuli as thin translucent bands that were separated by broad, more opaque bands, and then recorded the total number of annuli to determine age following methods described in Donald et al. (1992). Pairs of opaque and translucent bands were sometimes noted on opercula of individuals greater than three years of age; these bands combined dorsally and ventrally and were counted as a single annulus. To assess growth, the diagonal distance to the nearest 0.01 mm was recorded from the point just posterior to the hyomandibular articulation facet to each annulus toward the posteroventral margin (Fig. 2A). The validity of opercular age assignments was tested using the Von Bertalanffy (1938) growth equation in SYSTAT® version 10.0 (2002); valid age assignments typically produce realistic and significant maximum size (i.e., L∞ and DD∞) and K growth parameters at the 95% confidence interval (Burnham-Curtis and Bronte 1996): where: DDt = opercular diagonal distance (mm) at t (age in years); DD∞ = maximum diagonal distance; K = the Brody growth coefficient; t = time (i.e., age in years); t0 = time at age zero (time at theoretical zero distance). RECOGNITION OF ANNULAR GROWTH ON CENTRA OF TELEOSTEI WITH APPLICATION TO HIODONTIDAE OF THE CRETACEOUS DINOSAUR PARK FORMATION Michael G. Newbrey, Departments of Biological Sciences and Geosciences, Fargo, North Dakota 58105, USA; Mark V.H. Wilson, Department of Biological Sciences, University of Alberta, Edmonton, Alberta T6G 2E9, Canada Sixteen centra from extant Hiodon were examined for repeating patterns in circuli. To examine for annular features, light was angled at 45° across the surface to highlight relief. A preliminary assessment showed that broad opaque bands coinciding with raised, papilose surfaces often preceded thin translucent bands in a repeating pattern (Fig. 2B). Because growth during summer (i.e., raised bands) is presumed to be greater than that during winter in extant Hiodon, the thin translucent bands are hypothesized to be annuli. To assess growth, the center of the notochord foramen was marked using an ocular grid and radial distance (mm) to each translucent band toward the lateral margin was determined to the nearest 0.01 mm using a digital micrometer under a binocular dissecting microscope. To verify the validity of age estimates based on centra, we compared age estimates of centra to those based on opercula. To assess the feasibility of an age and growth study of fossil specimens, nine centra from the Dinosaur Park Formation were also examined for similar yearly markings (i.e., raised/papilose band followed by a thinner band) using the methodology applied to extant centra.