Eocene sepioid cephalopods - Anomalosaepia
This web page prepared by T. E. Yancey
Knowledge of Eocene sepioids expanded with recent description of the genus Anomalosaepia, a companion of the common genus Belosaepia in the Eocene sepioid assemblage. This genus is anomalous because it has a skeleton containing large amounts of the mineral calcite, a rarity among sepioids, and the skeleton has a distinctive shell microstructure. It was first described using fragmentary fossils, prompting a redescription when much better specimens were recovered from sediments in Mississippi and Texas. The redescription is presented in the 2011 September-October issue of the Journal of Paleontology. This is a companion publication to the redescription of the common species Belosaepia ungula, published during the previous year in Journal of Paleontology (Yancey et al., 2010).
Anomalosaepia is smaller than the more common Belosaepia, and has a more tightly coiled skeleton. Drawings and photographs of Anomalosaepia are shown below. (#1,#2) Despite its smaller size and more compact form, Anomalosaepia has many characters in common with Belosaepia, showing that the two genera are related. What stands out as most distinctive is the large amount of calcite forming the skeleton. This calcite occurs a solid, dense skeleton that is more easily preserved than the less dense skeleton of Belosaepia. Therefore, the sturdy skeletons of Anomalosaepia can be found in sediments where Belosaepia has been lost to dissolution.
Click on figures to magnify
Figure 1: Drawings of Anomalosaepia Weaver and Ciampaglio, 2003. The scale bar is 1 cm long.
Figure 2: Varied views of the skeleton of A. parmula Yancey and Garvie, 2011, from Mississippi and Texas. These were internal skeletons, covered with living tissue during life of the animal. The dark brown areas are parts of the skeleton composed of coarse crystalline calcite while white areas are composed of microcrystalline aragonite. The smooth surfaces of most specimens is the result of skeleton dissolution during late maturity. The ventral corona structure is preserved only on the holotype.
Despite their differences in secreting a skeleton, the two genera show similar growth histories in having a late maturity growth stage when skeleton is resorbed, presumably related to reproduction. By comparison with living sepiids and coleoids, it is apparent that this coincides with times of mating and reproduction, followed by death of the animal. This late stage modification of the skeleton results in much variation in fossils of sepioids, making it necessary to understand life history to make reasonable species identifications.
The skeleton grew by accretion and it has a thin inner layer of aragonite covered with a later-formed thick layer of calcite. The secretion of a late-formed calcite layer appeared later in side and bottom (ventral) areas than on the upper (dorsal) surface. (#3) The calcite was secreted as a dense layer of intergrown large crystal calcite. These large calcite crystals have a dark appearance that contrasts with the whitish aragonite, producing a two-tone coloration for the skeleton. Both the inner layer aragonite and the outer layer calcite grew outward in a fan-radial pattern, a distinctive shell microstructure. (#4) In some areas the boundary between inner aragonite and outer calcite skeleton contains voids that probably were filled with organic secretions that have decayed to leave empty spaces. (#5)
Figure 3: Cross-section of the upper (dorsal) part of Anomalosaepia, showing how a growth surface passes from calcite shell on the upper part to an area of aragonite secretion. Closely spaced fine growth lines can be seen in the aragonite shell, which grew outward in plumes composed of micro-crystalline aragonite prisms.
Figure 4: Skeletal microstructure of the aragonite shell layer of Anomalosaepia. The later-formed layers are composed of larger crystals that grow in a fan-radial pattern. The scale bar is 100 µm on 1 and 20 µm on 2-4.
Figure 5: Cross-sections of the upper (dorsal) part of Anomalosaepia, showing the thin inner aragonite (A,white) layer with outward projecting plumes and the coarsely crystalline outer calcite (C,dark) layer. Both photos show void spaces between the layers, marked by a 3-cornered star in the first photo. The 5-pointed star indicates a places where calcite has certainly grown directly on the surface of an aragonite plume. Each large unit of calcite is a single crystal.
The right to distribute digital copies of these articles was arranged with the Paleontological Society.
[pdf] Yancey, T.E., and Garvie, C.L., 2011, Redescription of Anomalosaepia (Cephalopoda: Coleoida): a sepioid with a bimineralic calcite and aragonite skeleton; Journal of Paleontology, v. 85, no. 5, p. 904-915.
See also the 2010 article on Belosaepia:
[pdf] Yancey, T.E., Garvie, C.L. and Wicksten, M., 2010, The Middle Eocene Belosaepia ungula (Cephalopoda: Coleoida) from Texas: Structure, ontogeny and function; Journal of Paleontology, v. 84, no. 2, p. 267-287.
Eocene sepioid cephalopods - Belosaepia: Click Here
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