Quick Search:    advanced search

GSW Home   GeoRef Home   My GSW Alerts   Contact GSW   About GSW   Journals List   Help

This Article Figures Only Full Text Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Similar articles in Web of Science Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via Web of Science (5) Citing Articles via Google Scholar Google Scholar Articles by Kim, K. Articles by Mitchell, C. E. Search for Related Content GeoRef GeoRef Citation

Morphometric analysis of ontogeny and allometry of the Middle Ordovician trilobite Triarthrus becki

¹ Keonho Kim. Department of Geology, 876 Natural Sciences Complex, State University of New York at Buffalo, Buffalo, New York 14260. keonhokim{at}hotmail.com
² H. David Sheets. Department of Physics, Canisius College, 2001 Main Street, Buffalo, New York 14208. sheets{at}canisius.edu
³ Robert A. Haney, and Charles E. Mitchell. Department of Geology, 876 Natural Sciences Complex, State University of New York at Buffalo, Buffalo, New York 14260. Robert\|[lowbar]\|Haney{at}brown.edu, cem{at}acsu.buffalo.edu

Traditionally, the distinction between meraspis and holaspis among trilobites has been based on the achievement of the full adult complement of thoracic segments. Using a large sample (over 700 specimens collected from a single bed) we explore the utility of employing the ontogenetic trajectory of the cranidium as an alternative means to differentiate trilobite growth stages. This method is particularly useful for species represented solely by exuviae and disarticulated individuals. We use geometric morphometrics to examine shape change among cranidia ranging in size from 0.9 mm to 11.6 mm in cephalic length. The 114 measured specimens exhibit a rather continuous gradation in size in which no distinct instars are evident.

The meraspid and holaspid specimens exhibit allometry when partial warp scores and uniform components of shape derived from thin-plate spline analysis are regressed onto log centroid size. To describe allometric shape change, deformation vectors from the smallest to the largest specimen in both ontogenetic stages are presented in three different superimposition settings by using a new software program. We have concluded that a new superimposition method (the Sliding Baseline Registration) is a useful tool for visualizing allometry in organisms that contain an axis of symmetry. As a result, we conclude that allometry is evident in meraspides and holaspides, but the degree of allometry in holaspides is very small relative to that in meraspides. The boundary between meraspis and holaspis in Triarthrus becki appears to correspond to a large change in the rate of ontogenetic change, but neither to a change in the direction of that trajectory nor to a cessation of ontogenetic change. This boundary also corresponds to a cranidium centroid size that matches well previous determinations that holaspis begins at about 2.8 mm in cephalic length.

This article has been cited by other articles:

				M. J. HOPKINS and M. WEBSTER ONTOGENY AND GEOGRAPHIC VARIATION OF A NEW SPECIES OF THE CORYNEXOCHINE TRILOBITE ZACANTHOPSIS (DYERAN, CAMBRIAN) Journal of Paleontology, July 1, 2009; 83(4): 524 - 547. [Abstract] [Full Text] [PDF]

				M. WEBSTER ONTOGENY AND EVOLUTION OF THE EARLY CAMBRIAN TRILOBITE GENUS NEPHROLENELLUS (OLENELLOIDEA) Journal of Paleontology, November 1, 2007; 81(6): 1168 - 1193. [Abstract] [Full Text] [PDF]

				N. C. Hughes, A. Minelli, and G. Fusco The ontogeny of trilobite segmentation: a comparative approach Paleobiology, December 1, 2006; 32(4): 602 - 627. [Abstract] [Full Text] [PDF]

				TRIARTHRINID TRILOBITES (OLENIDAE) FROM THE MIDDLE AND UPPER ORDOVICIAN, PRECORDILLERA OF ARGENTINA Journal of Paleontology, January 1, 2005; 79(1): 89 - 109.