Eocene sepioid cephalopods - Belosaepia

This web page prepared by T. E. Yancey
Eocene age sediments along the northern Gulf of Mexico, especially in Mississippi and Texas, contain some very well preserved specimens of sepioid cephalopods (relatives of cuttlefish). These are often considered to be an oddity, because they produced a skeleton with a tooth-like projection at the end that has a minor resemblance to the jaw parts of a squid or octopus. Consequently the fossils are often called "squid beaks" by collectors. However, the internal part of the skeleton contains septa, showing that they are chambered cephalopods. A typical example of the most common species of these fossils is shown below.

Click on pictures to magnify

   

Figures 1,2: A nice specimen of Belosaepia ungula Gabb, 1860. The scale bar is 1 cm long.

The main parts of the belosaepiid skeleton are the prong (the tooth-like part), the flat basal collar-like corona and the heavy knobby callus on the top. The open space containing septa is rarely preserved because it is weak and crushes easily when deposited in sediment. Usually only the posterior end of the skeleton is preserved and more complete skeletons are very rare.

The specimen shown above is better than average for Belosaepia, but nearly all Belosaepia skeletons preserve only 5% to 10% of the skeleton secreted by the animal and this one preserves only about 10%. The specimen below (#3) is an unusually complete skeleton (about 90% complete), revealing that the whole skeleton is much different then the normal fossil. This is one of 4 nearly complete skeletons known for the genus. Most of the skeleton is weak and breaks apart, leaving only the heavy, solid tooth-like part of the skeleton as a recognizable fossil. A reconstruction of the living animal (#4) shows the solid tooth-like prong is at the back (posterior) end to the skeleton.

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Figures 3,4: A nearly complete skeleton of B. ungula from Brazos County, Texas and a reconstruction of the animal with skeleton. The dashed line indicates that the outer surface of the body tissue and shows that the skeleton is internal. Some of the lower margin is reconstructed.

Although different in appearance, the belosaepiid fossil is most similar to the cuttlebone of the common living cuttlefish, a cephalopod that is common from Europe and Africa to east Asia. In contrast to the chambered condition of a cuttlebone, Belosaepia has a large amount of solid skeleton, but there are similar features in both Belosaepia and Sepia. The figure below shows some comparable characters on each skeleton. Belosaepia is an ancestor of the modern Sepia.

Figure 5: The prong of Belosaepia is equivalent to the spine of Sepia; the callus of Belosaepia is an over-developed equivalent of the rough upper surface of the cuttlebone. Both features are more developed on Belosaepia. The arrows point to an odd feature of sepioid skeletons: the prong/spine on these skeleton is easily split along a median plane. It is not known why this plane of weakness develops in the sepioid skeleton. It is apparently a surface with a higher content of non-mineral tissue.

Belosaepia skeletons were secreted as an internal skeleton and were entirely covered with tissue; the animal secreting regular layers one on top of the other. Like the rings in wood grown by a tree. One of the surprising aspects of the belosaepiid skeleton is that late in life, the animal started to dissolve much of the skeleton mass, leaving a reduced and etched remnant of the well developed skeleton. Photos below (#6) show an etched skeleton created by resorbing part of the skeleton mass. This occurred late in life and presumably was related to reproduction. (Sepiids reproduce only once, then die.) The occurrence of etching during life can be recognized by looking at the sides of a skeleton. If the edges of growth layers in the skeleton are visible (#7), it means that skeleton was dissolved by the animal. An abraded surface or a broken surface looks different.

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Figures 6,7: Appearance of belosaepiid skeletons resorbed late in life by the animal. Compare with the skeleton shown at top of the page. Resorbtion produces a smooth, shiny surface and thinning. A closeup view of the surface reveals the edges of growth lines, indicating truncation by dissolution.

A detailed study of Belosaepia ungula, the most common belosaepid fossil in North America, was published in the 2010 March-April issue of the Journal of Paleontology. This report documents the large and small features of the skeleton and some of the microstructure of the skeleton. Belosaepiid fossils from the northern Gulf of Mexico region are among the best preserved fossils of belosaepiids known. A companion study of the sepioid genus Anomalosaepia is presented in the Journal of Paleontology, vol. 85, p. 904-915 (see below).

The right to distribute digital copies of these articles was arranged with the Paleontological Society.
[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.
[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.

Eocene sepioid cephalopods - Anomalosaepia: Click Here

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