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The taxonomic status of the genus Hubertoceras Spath:
A new light on sexual dimorphism from
the Callovian ammonites of Kutch, India

Rakhi Dutta, Subhendu Bardhan, Shubhabrata Paul and Subhronil Mondal

Plain Language Abstract

Ammonites are strongly dimorphic especially since the Jurassic. Two morphs representing male (microconch) and female (macroconch) are quite dissimilar in shape and size. Microconchs are small and may be equipped with some secondary sexual characters near the mouth margin (for example, lappets). In the fossil record, it is very difficult to establish dimorphic pair within a species. Hubertoceras, an important middle Callovian genus from Kutch belonging to subfamily Sivajiceratinae, remains unpaired since its first description by Spath (1931). Hubertoceras is an evolute genus, small in adult size and has biplicate ribs and lappets. Many coeval macroconchiate genera especially their inner whorls come close to it in shape and ornamentation. We have explored all the possible tests which are considered as prerequisites to establish sexual pair. Finally, we have selected the genus Obtusicostites within the same subfamily Sivajiceratinae, as a plausible candidate to match the sexual partner of Hubertoceras. They are essentially contemporaneous and have similar paleobiogeographic distributions.

Resumen en Español

El estatus taxonómico del género Hubertoceras Spath: nuevas aportaciones sobre el dimorfismo sexual de los amonites del Calloviense de Kutch, India

El género Hubertoceras se describió como perteneciente a la subfamilia Proplanulitinae de los niveles del Calloviense medio a superior (Jurásico Medio) de Kutch. Recientemente revisamos los Proplanulitinae de Kutch y propusimos una nueva subfamilia endémica para ellos, Sivajiceratinae. La nueva subfamilia se compone de los géneros Sivajiceras, Kinkeliniceras, Obtusicostites y Hubertoceras. El desarrollo del dimorfismo sexual fue generalizado en las familias de amonites jurásicos y, al mismo tiempo, hubo un aumento significativo (especialmente desde el Calloviense) de variantes sexuales aisladas, que no podían ser fácilmente emparejadas. En el presente trabajo, hemos explorado el estatus de Hubertoceras a la luz del dimorfismo sexual. El género Hubertoceras se caracteriza por su pequeño tamaño adulto, su concha evoluta con costillas biplicadas y las solapas (lappets) bien conservadas, que inciden en su correspondencia con microconchas. También hemos intentado identificar el probable antidimorfo de Hubertoceras de los grandes géneros contemporáneos con conchas evolutas descritos de Kutch.
Los análisis morfológicos y morfométricos detallados indicaron que Obtusicostites es probablemente la macroconcha de Hubertoceras. El par Hubertoceras-Obtusicostites satisfizo todos los prerrequisitos paleobiológicos y geológicos para ser considerados antidimorfos. Las tendencias evolutivas entre macroconchas y microconchas dentro del linaje Sivajiceratinae también mostraron paralelismo.
La mayoría de los géneros de amonites del Calloviense de Kutch eran fuertemente dimórficos. La competencia intensa por la comida, el hábitat y la reproducción tal vez obligó a estos géneros a sufrir desplazamiento de caracteres. Mientras que las macroconchas permanecieron menos divergentes siendo uniformemente grandes, evolutas y fuertemente adornadas, las microconchas mostraron una amplia variación interespecífica en la forma de las solapas, algunas incluso carecían de modificaciones del peristoma. Diferentes tipos de solapa, o su ausencia, tal vez facilitaron el sistema de reconocimiento de pareja entre otras funciones.

Palabras clave: Hubertoceras (Ammonoidea); Posición sistemática; Dimorfismo sexual; Jurásico Medio; Kutch, India

Traducción: Enrique Peñalver (Sociedad Española de Paleontología)

Résumé en Français

text

Translator: Antoine Souron

Deutsche Zusammenfassung

In progress

Translator: Eva Gebauer

Arabic

Translator: Ashraf M.T. Elewa

 

 

FIGURE 1. Geological map of Kutch showing the major localities (solid squares) from where fossils of the Sivajiceratinae have been collected (modified after Dutta and Bardhan, 2016).

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FIGURE 2. The stratigraphic distribution of the studied antidimorphic pairs under the subfamily Sivajiceratinae of Kutch. Biozonation after Jana et al. (2000, 2005); Bardhan et al. (2012).

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FIGURE 3. Representatives of Hubertoceras described by Spath (1931) from the Callovian of Kutch. 1-2. Holotype of Hubertoceras omphalodes (Waagen), GSI no. 2030, lateral and ventral views; ‘Perisphinctes anceps beds’ of Vanda (=middle Callovian anceps beds). 3-4. Holotype of H. dhosaence (Waagen), GSI no. 2035, lateral and apertural views; ‘Perisphinctes anceps bed’ of Keera (=beds nos. 5-7 of middle Callovian). 5. Holotype of H. hubertus Spath; NHMUK no. C7686, lateral view. 6-7. H. hubertus var. densicostata, GSI no. 16098, lateral and apertural views; ?anceps beds of unknown locality. 8. H? sp. nov.; GSI no. 16099, lateral view; athleta beds of Fakirwadi (=athleta beds of upper Callovian). 9. Holotype of H. arcicosta, GSI no. 2099, lateral view; ‘Golden Oolite’ of Keera (=bed no. 2 of lower Callovian). 10. Holotype of H. mutans; GSI no. 2037, lateral view; ‘Katrol group’ of North Gudjinsir. Beds are following Jana et al. (2000, 2005). Commencement of body chamber and presence of lappets are indicated by ‘x’ and arrows respectively. GSI=Geological Survey of India, NHMUK=Natural History Museum, U.K. Scale bar equals 20 mm.

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FIGURE 4. Different sexual dimorphic ammonites (non-Sivajiceratinae) contemporary to Hubertoceras during the middle Callovian in Kutch. Sexual Dimorphism among these species is well established and the pairs have been shown to be connected by large arrows. 1-2. Reineckia anceps (M, JUM/R/90, bed 6, middle Callovian of Keera and m, JUM/R/44, bed 9, middle Callovian of Jumara, respectively). 3-4.Choffatia cobra (M, JUM 33, bed 7, lower Callovian of Jumara and m, JUM 38, bed 7, lower Callovian of Jumara respectively). 5-6.Idiocycloceras perisphinctoides (M, JUM 450, bed 8, middle Callovian of Jura and m, JUM 270, bed 6, middle Callovian of Keera respectively). 7. J Indosphinctes sp. (M, JUM/IS/01, bed 11, middle Callovian of Jumara). Beds are following Jana et al. (2000, 2005). Note that microconch of Indosphinctes is still unknown. ‘x’ indicates the end of phragmocone. Small arrow indicates presence of lappets. Scale bar equals 20 mm. The sources are: Jana et al., 2005; Kayal, 2009; Bardhan et al., 2012, Dutta and Bardhan, 2016.

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FIGURE 5. Supposed antidimorphic pairs within the subfamily Sivajiceratinae. 1-2. Sivajiceras paramorphum (M, JUM /SP/2, bed 7, lower Callovian of Jumara and m, JUM/SP/22, bed 2, lower Callovian of Keera). 3-4. Kinkeliniceras angygaster (M. JUM/KA/1, locality/stratigraphy unknown; m. JUM/KA/5, Medisar, Jura). 5-6. Obtusicostites obtusicosta (M, JUM/OO/1, bed 11, middle Callovian of Jumara and m, JUM/OO/14, locality/stratigraphy unknown). 'x' indicates the end of phragmocone. Small arrow indicates the presence of lappets. Scale bars=20 mm. The sources are: Jana et al. (2000, 2005) for stratigraphic information and Dutta and Bardhan (2016) for taxonomy.

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FIGURE 6. Bivariate growth graphs between Hubertocerasomphalodes and macroconchs of all contemporary (middle Callovian) species of different genera i.e., Reineckia anceps, Choffatia cobra, Idiocycloceras perisphinctoides and Indosphinctes sp. Cluster of morphological characters of Hubertoceras are separated from those of other genera. D=Diameter of the shell, U=Umbilical diameter, W=Width of the whorl and H=Height of the whorl. Measurements are taken at different ontogenetic stages of the specimens to accommodate intraspecific range of variability of each species. Photos are not to scale.

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FIGURE 7. Bivariate growth graphs between Hubertocerasomphalodes and macroconchs of the subfamily Sivajiceratinae. Maximum homogeneity of points is shown between Obtusicostites obtusicosta and Hubertoceras omphalodes. D=Diameter of the shell, U=Umbilical diameter, W=Width of the whorl and H=Height of the whorl. Measurements are taken at different ontogenetic stages of the specimens to accommodate intraspecific range of variability of each species. Photos are not to scale.

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FIGURE 8. Transverse sections (body chamber hatched) of adult shells. 1.Obtusicostites obtusicosta (M). 2.Hubertoceras omphalodes (m). Note, for 1 and 2, overall similarity of whorl sections and depressed inner whorls with rounded umbilical edge. Septal sutural patterns. 3. Obtusicostites obtusicosta (M) at diameter 150 mm, redrawn from Waagen (1875, plate 38, figure 2). 4.Hubertoceras omphalodes (m) at diameter 48 mm, redrawn from Waagen (1875, plate 38, figure 4c). Scale bar equals 20 mm.

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FIGURE 9. Parallel evolution of median values of different morphological characters for three successive genera (including M and m) within Sivajiceratinae is plotted. Note that both M and m within the lineage show parallel evolutionary trends. D, U, W, and H are the same as in Figure 4. P= Primary rib (per half whorl) and S= Secondary rib (per half whorl). The dark horizontal line in the middle of the each box represents the median values, top frame of the box represents 75th percentile, bottom frame of the box represents 25th percentile and the bars at the end of vertical lines represent the minimum and the maximum data values without outliers (open circles). Sources are: Waagen, 1875; Spath, 1931; Collignon, 1958; Dutta and Bardhan, 2016.

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FIGURE 10. Parallel evolution of the species level for the genus Sivajiceras is plotted. Note that both M and m within the lineage show parallel evolutionary trends. Sources are: Waagen, 1875; Spath, 1931; Collignon, 1958; Callomon, 1993; Dutta and Bardhan, 2016.

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FIGURE 11. Parallel evolution of the species level for the genus Obtusicostites is plotted. Note that both M and m within the lineage show parallel evolutionary trends. Sources are: same as in Figure 10.

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author1Rakhi Dutta, Department of Geological Sciences, Jadavpur University, Kolkata-700032, India, This email address is being protected from spambots. You need JavaScript enabled to view it.

Rakhi Dutta recently completed her Ph.D from Jadavpur University. Kolkata. She is presently working on Jurassic ammonites focusing on intraspecific variability, sexual dimorphism and their evolutionary and biostratigraphic implications.

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author2Subhendu Bardhan, Department of Geological Sciences, Jadavpur University, Kolkata-700032, India, This email address is being protected from spambots. You need JavaScript enabled to view it.

Subhendu Bardhan, received his Ph.D. from Jadavpur University and had been on the faculty there since 1981 from where he has recently retired. He works on various groups of mega-invertebrate fossils and has strong interest in unraveling palaeobiological intrigues, evolution and extinction, and sexual dimorphism in cephalopods. His current research interest involves coastal ecology of molluscs and the prey-predator interaction (drilling and peeling) on molluscs in space and time.

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author3Shubhabrata Paul, Department of Applied Geology, Indian Institute of Technology (Indian School of Mines), Dhanbad-826004, India, This email address is being protected from spambots. You need JavaScript enabled to view it.

Shubhabrata Paul received his PhD degree in 2013 from the University of South Florida, USA. Currently he is an INSPIRE Faculty in the Department of Applied Geology, IIT (ISM) Dhanbad. Focus of his research involves unraveling macroevolutionary trends and their underlying processes. Presently, he is working on ecological and evolutionary significance of background and mass extinctions.

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author4Subhronil Mondal, Department of Geology, University of Calcutta, Kolkata-700019, India, This email address is being protected from spambots. You need JavaScript enabled to view it.

Subhronil Mondal received his PhD degree in 2014 from the University of South Florida, USA. Currently he is an Assistant Professor Department of Geology, University of Calcutta. His works involve long- and short-term macroecological changes and their macroevolutionary impacts in the marine molluscan history of life. The short-term ecological studies include documentation of predation and competition among molluscan communities; while the long-term studies include documentation of interrelationship between macroecology and macroevolution. 

 

APPENDIX 1.

Locality, stratigraphic information and measurements of all specimens (type as well as additional present collection) of Sivajiceratinae within the mainland of Kutch. For detail information of localities see Spath (1931); Waagen (1875); Figure 1. Available in PDF.

APPENDIX 2.

Precise stratigraphy and locality information of the all the specimens of Obtusicostites and Hubertoceras. Available in PDF.

APPENDIX 3.

Stratigraphic information and measurements of all contemporary macroconchiate (middle Callovian) species of different genera along with Hubertoceras (m). Available in PDF.

APPENDIX 4.

Results of Mann-Whitney U test for testing significance of change in morphological traits. Available in PDF.