Hostname: page-component-586b7cd67f-t8hqh Total loading time: 0 Render date: 2024-11-25T18:21:34.508Z Has data issue: false hasContentIssue false
  • English
  • Français

Ontogeny and shape change of the phacopid trilobite Calyptaulax

Published online by Cambridge University Press:  10 July 2019

Gabriel S. Jacobs  and
Jesse R. Carlucci
Gabriel S. Jacobs
Affiliation:
University of Missouri, Geological Sciences, Columbia, MO 65211
Jesse R. Carlucci
Affiliation:
Midwestern State University, Kimbell School of Geosciences, Wichita Falls, TX 76308
Get access Rights & Permissions [Opens in a new window]

Abstract

Major transitions in trilobite ontogeny have historically been defined based on the number and distribution of trunk segments, and articulation between the trunk and cephalon. This study documents additional morphological change across the meraspid-holaspid transition on the Ordovician phacopid trilobite Calyptaulax strasburgensis. An extensive dataset of silicified cranidia and pygidia collected from the mid-Ordovician Edinburg Formation of Virginia was subjected to a series of multivariate analyses, with a primary focus on the intersections and termini of furrows. Multivariate regression of partial warp scores demonstrates statistically significant change in allometric growth patterns over the course of development. These changes are concentrated in earlier instars, but are coincident in cranidia and pygidia. This sharp decrease in the rate of allometry, present in both tagmata, is expressed as significant breakpoints derived from a segmented regression, with the largest portion of allometric change found in the pre-breakpoint individuals. The term holeidos is proposed to describe the completion of form during trilobite development, independent of the completion of the thorax. The most dramatic change in shape during this period of ontogeny includes lateral glabellar expansion through deflection of the axial and palpebral furrows, possibly reflecting a change in the feeding habit during later development. Other morphological changes include the development of a more angular appearance to the anterior portion of the glabella, and anterior migration of the pygidial anterior margin. The appearance of these growth patterns in Calyptaulax extends the temporal range of these changes, some of which have only been documented in Devonian phacopids.

Type
Articles
Information
Journal of Paleontology , Volume 93 , Issue 6 , November 2019 , pp. 1105 - 1125
Copyright
Copyright © 2019, The Paleontological Society 

Access options

Get access to the full version of this content by using one of the access options below. (Log in options will check for institutional or personal access. Content may require purchase if you do not have access.)

References

Adams, D.C., Rohlf, F.J., and Slice, D.E., 2004, Geometric morphometrics: ten years of progress following the ‘revolution’: Italian Journal of Zoology, v. 71, p. 5–16, doi:10.1080/11250000409356545.Google Scholar
Akaike, H., 1974, A new look at the statistical model identification: IEEE Transactions on Automatic Control, v. 19, 716–723.Google Scholar
Akam, M., Averof, M., Castelli-Gair, J., Dawes, R., Falciani, F., and Ferrier, D., 1994, The evolving role of Hox genes in arthropods: Development, v. 1994, Supplement, p. 209–215.Google Scholar
Amati, L., 2004, Systematics and Paleoecology of Trilobites from the Late Ordovician Viola Group, South-Central Oklahoma: Oklahoma City, University of Oklahoma, 527 p.Google Scholar
Barrande, J., 1846, Notice préliminaire sur le systême Silurien et les trilobites de Bohême: Leipzig, C.L. Hirschfeld, 97 p.Google Scholar
Beecher, C.E., 1895, The larval stages of trilobites: The American Geologist, v. 16, p. 166–196.Google Scholar
Bookstein, F.L., 1991, Morphometric Tools for Landmark Data: Geometry and Biology: Cambridge/New York, Cambridge University Press, 456 p.Google Scholar
Brett, C.E., Goodman, W.M., and LoDuca, S.T., 1990, Sequences, cycles, and basin dynamics in the Silurian of the Appalachian Foreland Basin: Sedimentary Geology, v. 69, p. 191–244, doi:10.1016/0037-0738(90)90051-T.Google Scholar
Butts, C., 1941, Geology of the Appalachian Valley in Virginia, part 2, fossil plates and explanations: Virginia Geological Survey Bulletin 52, pt. 2, 271 p.Google Scholar
Campbell, H.D., 1905, The Cambro-Ordovician limestones of the middle portion of the Valley of Virginia: American Journal of Science, ser. 4, v. 20, p. 445–447.Google Scholar
Carlucci, J.R., and Westrop, S.R., 2012, Trilobite biofacies along an Ordovician (Sandbian) carbonate buildup to basin gradient, southwestern Virginia: Palaios, v. 27, p. 19–34, doi:10.2110/palo.2011.p11–069r.Google Scholar
Carlucci, J.R., and Westrop, S.R., 2015, Trilobite biofacies and sequence stratigraphy: an example from the Upper Ordovician of Oklahoma: Lethaia v. 48, p. 309–325.Google Scholar
Carlucci, J.R., Westrop, S.R., Brett, C.E. and Burkhalter, R., 2014, Facies architecture and sequence stratigraphy of the Ordovician Bromide Formation (Oklahoma): a new perspective on a mixed carbonate-siliciclastic ramp: Facies, v. 60, p. 987–1012, doi:10.1007/s10347-014-0412-6.Google Scholar
Chatterton, B.D., 1971, Taxonomy and ontogeny of Siluro-Devonian trilobites from near Yass, New South Wales: Palaeontographica abteilung A, v. 137, p. 1–108.Google Scholar
Chatterton, B.D., 1994, Ordovician proetide trilobite Dimeropyge, with a new species from northwestern Canada: Journal of Paleontology, v. 68, p. 541–556.Google Scholar
Chatterton, B.D., and Speyer, S.E., 1997, Ontogeny, in Kaesler, R.L., ed., Treatise on Invertebrate Paleontology, Pt. O, Arthropoda I: Trilobita (Revised): Lawrence, Kansas, Geological Society of America and University of Kansas, p. O173–O247.Google Scholar
Chatterton, B.D., Edgecombe, G.D., Speyer, S.E., Hunt, A.S., and Fortey, R.A., 1994, Ontogeny and relationships of Trinucleoidea (Trilobita): Journal of Paleontology, v. 68, p. 523–540, doi:10.1017/S0022336000025907.Google Scholar
Clarkson, E.N., and Ahlberg, P., 2002, Ontogeny and structure of a new, miniaturised and spiny olenid trilobite from southern Sweden: Palaeontology, v. 45, p. 1–22, doi:10.1111/1475-4983.00224.Google Scholar
Clarkson, E.N., Ahlgren, J., and Taylor, C.M., 2003, Structure, ontogeny, and moulting of the olenid trilobite Ctenopyge (Eoctenopyge) angusta Westergård, 1922 from the Upper Cambrian of Västergötland, Sweden: Palaeontology, v. 46, p. 1–27, doi:10.1111/1475-4983.00284.Google Scholar
Cooper, B.N., 1953, Trilobites from the lower Champlainian formations of the Appalachian Valley: Geological Society of America Memoir 55, 69 p.Google Scholar
Cooper, B.N., and Cooper, G.A., 1946, Lower Middle Ordovician stratigraphy of the Shenandoah Valley, Virginia: Geological Society of America Bulletin, v. 57, p. 35–114.Google Scholar
Cooper, G.A., 1930, Upper Ordovician and Lower Devonian stratigraphy and paleontology of Percé, Quebec, Part II, New species from the Upper Ordovician of Percé: American Journal of Science, v. 20, p. 365–392, doi:10.2475/ajs.s5-20.119.365.Google Scholar
Crônier, C., 2007, Larval morphology and ontogeny of an Upper Devonian phacopid: Nephranops from Thuringia, Germany: Journal of Paleontology, v. 81, p. 684–700, doi:10.1666/pleo0022-3360(2007)081[0684:LMAOOA]2.0.CO;2.Google Scholar
Crônier, C., 2013, Morphological disparity and developmental patterning: contribution of phacopid trilobites: Palaeontology, v. 56, p. 1263–1271, doi:10.1111/pala.12024.Google Scholar
Crônier, C., and Feist, R., 1997, Morphologie et évolution ontogénétique de Trimerocephalus lelievrei nov. sp., premier trilobite phacopidé aveugle du Famennien nord-africain: Geobios, v. 30, p. 161–170.Google Scholar
Crônier, C., and Fortey, R.A., 2006, Morphology and ontogeny of an Early Devonian phacopid trilobite with reduced sight from southern Thailand: Journal of Paleontology, v. 80, p. 529–536, doi: 10.1666/0022–3360(2006)80[529:MAOOAE]2.0.CO;2.Google Scholar
Crônier, C., Renaud, S., Feist, R., and Auffray, J.C., 1998, Ontogeny of Trimerocephalus lelievrei (Trilobita, Phacopida), a representative of the Late Devonian phacopine paedomorphocline: a morphometric approach: Paleobiology, v. 24, p. 359–370, doi:10.1666/0094-8373(1998)024[0359:OOTLPA]2.3.CO;2.Google Scholar
Crônier, C., Auffray, J.C., and Courville, P., 2005, A quantitative comparison of the ontogeny of two closely related Upper Devonian phacopid trilobites: Lethaia, v. 38, p. 123–135, doi:10.1080/00241160510013240.Google Scholar
Delabroye, A., and Crônier, C., 2008, Ontogeny of an Ordovician trinucleid (Trilobita) from Armorica, France: a morphometric approach: Journal of Paleontology, v. 82, p. 800–810, doi:10.1666/07-084.1.Google Scholar
Dempster, A.P., Laird, N.M. and Rubin, D.B., 1977, Maximum likelihood from incomplete data via the EM algorithm: Journal of the Royal Statistical Society, v. 39, p. 1–38.Google Scholar
Dyar, H.G., 1890, The number of molts of lepidopterous larvae: Psyche: A Journal of Entomology, v. 5, p. 420–422.Google Scholar
Edgecombe, G.D., Chatterton, B.D., Vaccari, N.E., and Waisfeld, B.G., 1997, Ontogeny of the proetoid trilobite Stenoblepharum, and relationships of a new species from the Upper Ordovician of Argentina: Journal of Paleontology, v. 71, p. 419–433, doi:10.1017/S0022336000039445.Google Scholar
Ettensohn, F.R., 2008, The Appalachian foreland basin in eastern United States, in Miall, A., ed., Sedimentary Basins of the World, Volume 5. The Sedimentary Basins of the United States and Canada: Oxford/Boston, Elsevier Science, p. 105–179.Google Scholar
Feist, R., McNamara, K.J., Crônier, C., Lerosey-Aubril, R., 2009, Patterns of extinction and recovery of phacopid trilobites during the Frasnian-Famennian (Late Devonian) mass extinction event, Canning Basin Western Australia: Geological Magazine, v. 146, p. 12–33.Google Scholar
Fortey, R.A., and Morris, S.F., 1978, Discovery of nauplius-like trilobite larvae: Palaeontology, v. 21, p. 823–833.Google Scholar
Forty, R.A., and Owens, R.M., 1999, Feeding habits in trilobites: Palaeontology, v. 42, p. 429–465.Google Scholar
Fusco, G., Hughes, N.C., Webster, M., and Minelli, A., 2003, Exploring developmental modes in a fossil arthropod: growth and trunk segmentation of the trilobite Aulacopleura koninckii: The American Naturalist, v. 163, p. 167–183, doi:10.1086/381042.Google Scholar
Fusco, G., Garland, T., Hunt, G. and Hughes, N.C., 2012, Developmental trait evolution in trilobites: Evolution, v. 66, p. 314–329.Google Scholar
Gellon, G., and McGinnis, W., 1998, Shaping animal body plans in development and evolution by modulation of Hox expression patterns: BioEssays, v. 20, p. 116–125, doi:10.1002/(SICI)1521-1878(199802)20:2<116::AID-BIES4>3.0.CO;2-R.3.0.CO;2-R.>Google Scholar
Goodall, C., 1991, Procrustes methods in the statistical analysis of shape: Journal of the Royal Statistical Society, ser. B, v. 53, p. 285–339.Google Scholar
Green, J., 1832, A Monograph of the Trilobites of North America with coloured models of the species: Philadelphia, J. Brano, 93 p.Google Scholar
Hall, J., 1847, The Paleontology of New York: Albany, C. Van Benthuysen, v. 1, 247 p.Google Scholar
Hammer, O., Harper, D.A.T, and Ryan, P. 2001: PAST: Paleontological statistics software for education and data analysis: Palaeontologia Electronica 4, 9, http://palaeo-electronica.org/2001_1/past/issue1_01.htm.Google Scholar
Hartnoll, R.G., 1983, Strategies of crustacean growth: Memoirs of the Australian Museum, no. 18, p. 121–131.Google Scholar
Holland, S.M., and Patzkowsky, M.E., 1996, Sequence stratigraphy and long-term paleoceanographic change in the Middle and Upper Ordovician of the eastern United States, in Witzke, B.J., Ludvigson, G.A., and Day, J., eds., Paleozoic Sequence Stratigraphy; Views from the North American Craton: Geological Society of America, Special Paper 306, p. 117–130, doi:10.1130/SPE306.Google Scholar
Hopkins, M.J., and Pearson, J.K., 2016, Non-linear ontogenetic shape change in Cryptolithus tesselatus (Trilobita) using three-dimensional geometric morphometrics: Palaeontologia Electronica, v. 19, art. 42A, p. 1–54, doi:10.26879/665.Google Scholar
Hopkins, M.J., and Webster, M., 2009, Ontogeny and geographic variation of a new species of the corynexochine trilobite Zacanthopsis (Dyeran, Cambrian): Journal of Paleontology, v. 83, p. 524–547, doi:10.1666/08-102R.1.Google Scholar
Hughes, N.C., 2007, The evolution of trilobite body patterning: Annual Review of Earth and Planetary Sciences, v. 35, p. 401–434, doi:10.1146/annurev.earth.35.031306.140258.Google Scholar
Hughes, N.C., and Chapman, R.E., 1995, Growth and variation in the Silurian proetid trilobite Aulacopleura koninckii and its implications for trilobite palaeobiology: Lethaia, v. 28, p. 333–353, doi:10.1111/j.1502-3931.1995.tb01824.x.Google Scholar
Hughes, N.C., Minelli, A., and Fusco, G., 2006, The ontogeny of trilobite segmentation: a comparative approach: Paleobiology, v. 32, p. 602–627, doi:10.1666/06017.1.Google Scholar
Hunt, G., and Chapman, R.E., 2001, Evaluating hypotheses of instar-grouping in arthropods: a maximum likelihood approach: Paleobiology, v. 27, p. 466–484.Google Scholar
Kim, K., Sheets, H.D., Haney, R.A., and Mitchell, C.E., 2002, Morphometric analysis of ontogeny and allometry of the Middle Ordovician trilobite Triarthrus becki: Paleobiology, v. 28, p. 364–377, doi:10.1666/0094-8373(2002)028<0364:MAOOAA>2.0.CO;2.2.0.CO;2.>Google Scholar
Klingenberg, C.P., and Zimmermann, M., 1992. Dyar's rule and multivariate allometric growth in nine species of waterstriders (Heteroptera:Gerridae): Journal of Zoology, v. 227, p. 453–464.Google Scholar
Kobayashi, T., 1933, Upper Cambrian of the Wuhutsui Basin, Liaotung, with special reference to the limit of the Chaumitian (or Upper Cambrian) of eastern Asia, and its subdivision: Japanese Journal of Geology and Geography, v. 11, p. 55–155.Google Scholar
Kobayashi, T., and Hamada, T., 1968, A Devonian phacopid recently discovered by Mr. Charan Poothai in peninsular Thailand: Geology and Palaeontology of Southeast Asia, v. 4, p. 22–28.Google Scholar
Kozub, D., Khmelik, V., Shapoval, Y., Chentsov, S., Yatsenko, B., Litovchenko, B., and Starykh, V., 2008, Helicon Focus v.5.3x64: HeliconSoft, http://www.heliconsoft.com.Google Scholar
McLaughlin, P.I., and Brett, C.E., 2007, Signatures of sea-level rise on the carbonate margin of a Late Ordovician foreland basin: a case study from the Cincinnati Arch, USA: Palaios, v. 22, p. 245–267, doi:10.2110/palo.2006.p06-106.Google Scholar
McNamara, K.J., 1986, The role of heterochrony in the evolution of Cambrian trilobites: Biological Reviews, v. 61, p. 121–156, doi:10.1111/j.1469-185X.1986.tb00464.x.Google Scholar
Meek, F.B., 1870, Descriptions of fossils collected by the US Geological Survey under the charge of Clerence King, Esq.: Proceedings of the Academy of Natural Sciences of Philadelphia, v. 22, p. 56–64.Google Scholar
Mitchell, C., Adhya, S., Bergstrom, S., Joy, M., and Delano, J., 2004, Discovery of the Ordovician Millbrig K-bentonite bed in the Trenton Group of New York State: implications for regional correlation and sequence stratigraphy in eastern North America: Palaeogeography, Palaeoclimatology, Palaeocology, v. 210, p. 331–346, doi:10.1016/j.palaeo.2004.02.037.Google Scholar
Muggeo, V.M.R., 2008, Segmented: an R package to fit regression models with broken-line relationships: R News, v. 8/1, p. 20–25.Google Scholar
Oehlert, D.P., 1895, Sur les Trinucleus de l'Ouest de la France: Bulletins de la Société Géologique de France, v. 23, p. 299–335.Google Scholar
Oosterbaan, R.J., 2011, SegReg: segmented linear regression with breakpoint and confidence intervals: https://www.waterlog.info/segreg.htm.Google Scholar
Oosterbann, R., Sharma, D.P., Singh, K.N., and Rao, K.V.G.K, 1990, Crop production and soil salinity: evaluation of field data from India by segmented linear regression with breakpoint: Proceedings for the Symposium on Land Drainage for Salinity Control in Arid and Semi-Arid Regions, v. 3, p. 373–383.Google Scholar
Palmer, A.R., 1957, Ontogenetic development of two olenellid trilobites: Journal of Paleontology, v. 31, p. 105–128.Google Scholar
Palmer, A.R., 1962, Comparative ontogeny of some opisthoparian, gonatoparian and proparian Upper Cambrian trilobites: Journal of Paleontology, v. 36, p. 87–96.Google Scholar
Park, T.Y., and Choi, D.K., 2010, Ontogeny of the Furongian (late Cambrian) remopleuridioid trilobite Haniwa quadrata Kobayashi, 1933 from Korea: implications for trilobite taxonomy: Geological Magazine, v. 148, p. 288–303, doi:10.1017/S0016756810000701.Google Scholar
Raw, F., 1925, The Development of Leptoplastus salteri (Callaway), and of other Trilobites (Olenidae, Ptychoparidae, Conocoryphidae, Paradoxidae, Phacopidae, and Mesonacidae): Quarterly Journal of the Geological Society, v. 81, p. 223–324.Google Scholar
Raymond, P.E., 1905, Trilobites of the Chazy Limestone: Annals of the Carnegie Museum, v. 3, p. 328–396.Google Scholar
Read, J.F., 1980, Carbonate ramp-to-basin transitions and foreland basin evolution, Middle Ordovician, Virginia Appalachians: AAPG Bulletin, v. 64, p. 1575–1612.Google Scholar
Read, J.F., 1982, Geometry, facies, and development of Middle Ordovician carbonate buildups, Virginia Appalachians: AAPG Bulletin, v. 66, p. 189–209.Google Scholar
Reed, F.R.C., 1905, The classification of the Phacopidae: Geological Magazine, v. 2, p. 172–178.Google Scholar
Richter, R., and Richter, E., 1926, Die Trilobiten des Oberdevons, Beiträge Kenntnis devonischer Trilobiten IV: Abhandlungen der preussischen geologischen Landesanstalt, v. 99, p. 1–314.Google Scholar
Robison, R.A., 1964, Late middle Cambrian faunas from western Utah: Journal of Paleontology, v. 38, p. 510–566.Google Scholar
Robison, R.A., 1967, Ontogeny of Bathyuriscus fimbriatus and its bearing on affinities of corynexochid trilobites: Journal of Paleontology, v. 41, p. 213–221.Google Scholar
Rohlf, F.J., 1993, Relative warp analysis and an example of its application to mosquito wings, in Marcus, L.F., Bello, E., and García-Valdecasas, E., eds., Contributions to Morphometrics: Museo Nacional de Ciencias Naturales, Madrid, Monografias, p. 131–159.Google Scholar
Rohlf, F.J., 2013, tpsDig: Digitize coordinates of landmarks and capture outlines: Stony Brook, N.Y., Department of Ecology and Evolution, Stony Brook University, http://life.bio.sunysb.edu/morph/.Google Scholar
Rohlf, F.J., and Marcus, L.F., 1993, A revolution in morphometrics: Trends in Ecology & Evolution, v. 8, p. 129–132, doi:10.1016/0169-5347(93)90024-J.Google Scholar
Salter, J.W., 1864, On some new fossils from the Lingula-flags of Wales: Quarterly Journal of the Geological Society, v. 20, p. 233–241.Google Scholar
Sebens, K.P., 1987, The ecology of indeterminate growth in animals: Annual Review of Ecology and Systematics, v. 18, p. 371–407, doi:10.1146/annurev.es.18.110187.002103Google Scholar
Shaw, A.B., 1957, Quantitative trilobite studies II. Measurement of the dorsal shell of non-agnostidean trilobites: Journal of Paleontology, v. 31, p. 193–207.Google Scholar
Shaw, F.C., 1974, Simpson Group (Middle Ordovician) trilobites of Oklahoma: The Paleontological Society Memoir 6, 54 p.Google Scholar
Sheets, H.D., 2001, Standard6beta: Buffalo, New York, Department of Physics, Canisius College, http://www.canisius.edu/sheets/morphsoft.html.Google Scholar
Sheets, H.D., 2010, TwoGroup for integrated morphometrics package, Version 7: Buffalo, N.Y., Department of Physics, Canisius College, https://www3.canisius.edu/~sheets/imp7.htm.Google Scholar
Sheets, H.D., 2011a, CoordGen v.7a: Coordinate generating utility: Buffalo, N.Y., Department of Physics, Canisius College, https://www3.canisius.edu/~sheets/imp7.htm.Google Scholar
Sheets, H.D., 2011b, PCAGen v.7a: PCA of shape data: Buffalo, N.Y., Department of Physics, Canisius College, https://www3.canisius.edu/~sheets/imp7.htm.Google Scholar
Sheets, H.D., 2011c, Regress v.7a: Thin plate spline and regression utility: Buffalo, N.Y., Department of Physics, Canisius College, https://www3.canisius.edu/~sheets/imp7.htm.Google Scholar
Sheets, H.D., Kim, K., and Mitchell, C.E., 2004, A combined landmark and outline-based approach to ontogenetic shape change in the Ordovician trilobite Triarthrus becki, in Elewa, A.M.T., ed., Morphometrics: Berlin, Heidelberg, Springer-Verlag, p. 67–82, doi:10.1007/978-3-662-08865-4_6.Google Scholar
Simpson, A.G., Hughes, N.C., Kopaska-Merkel, D.C., and Ludvigsen, R., 2005, Development of the caudal exoskeleton of the pliomerid trilobite Hintzeia plicamarginis new species: Evolution & Development, v. 7, p. 528–541.Google Scholar
Struve, W., 1959, Suborder Phacopina, in Moore, R.C., ed., Treatise on Invertebrate Paleontology, Pt. O, Arthropoda 1: Lawrence, KS, University of Kansas, p. O461–O462.Google Scholar
Ulrich, E.O., and Delo, D.M., 1940, Phacopid trilobites of North America: Geological Society of America Special Paper 29, p. 1–135.Google Scholar
Wagner, A.K., Soumerai, S.B., Zhang, F., and Rose-Degnan, D., 2002, Segmented regression analysis of interrupted time series studies in medication use research: Journal of Clinical Pharmacy and Therapeutics, v. 27, p. 299–309, doi:10.1046/j.1365-2710.2002.00430.x.Google Scholar
Walcott, C.D., 1879, Fossils of the Utica slate and metamorphoses of Triarthrus becki: Transactions of the Albany Institute, no. 10, p. 18–38.Google Scholar
Webster, M., 2011, The structure of cranidial shape variation in three early ptychoparioid trilobite species from the Dyeran-Delamaran (traditional “lower-middle” Cambrian) boundary interval of Nevada, USA: Journal of Paleontology, v. 85, p. 179–225, doi:10.1666/10-075.1.Google Scholar
Webster, M., 2015, Ontogeny and intraspecific variation of the early Cambrian trilobite Olenellus gilberti, with implications for olenelline phylogeny and macroevolutionary trends in phenotypic canalization: Journal of Systematic Palaentology, v. 13, p. 1–74, doi:10.1080/14772019.2013.852903.Google Scholar
Webster, M., 2019, Morphological homeostasis in the fossil record: Seminars in Cell & Developmental Biology, v. 88, p. 91–104, doi:10.1016/j.semcdb.2018.05.016Google Scholar
Webster, M., and Sheets, H.D., 2010, A practical introduction to landmark-based geometric morphometrics, in Alroy, J., and Hunt, G., eds., Quantitative Methods in Paleobiology: The Paleontological Society Papers, v. 16, p. 168–188.Google Scholar
Webster, M., and Zelditch, M.L., 2005, Evolutionary modifications of ontogeny: heterochrony and beyond: Paleobiology, v. 31, p. 354–372, doi:10.1666/0094-8373(2005)031[0354:EMOOHA]2.0.CO;2.Google Scholar
Westergård, A.H., 1922, Sveriges olenidskiffer: Sveriges Geologiska Undersökning Avhandlingar och uppsatser series Ca, v. 18, 269 p.Google Scholar
Whittington, H.B., 1957, The ontogeny of trilobites: Biological Reviews, v. 32, p. 421–467, doi:10.1111/j.1469-185X.1957.tb00779.x.Google Scholar
Whittington, H.B., and Evitt, W.R., 1954, Silicified Middle Ordovician trilobites (Virginia): Geological Society of America Memoir 59, 137 p.Google Scholar
Zelditch, M.L., Swiderski, D.L., and Sheets, H.D., 2012a, Geometric Morphometrics for Biologists: A Primer: New York and London, Elsevier Academic Press, 437 p.Google Scholar
Zelditch, M.L., Swiderski, D.L., Sheets, H.D., 2012b, A Practical Companion to Geometric Morphometrics for Biologists: Running analyses in freely-available software: http://booksite.elsevier.com/9780123869036/content/Workbook.pdf.Google Scholar