Entry posted by Rumi
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In this second entry I would like to show well-preserved specimens of two ostracodes: the very long-ranging taxon Amphissites centronotus (Ulrich and Bassler, 1906), and the Permian taxon Cornigella parva Kellett, 1933. The former belongs in the family Amphissitidae, while the latter is placed in the family Drepanellidae.
This specimen is a relatively late instar, but not fully mature, as final instar specimens average about 50% larger. The species is very easy to recognize, the very large and prominent central node being quite distinctive. Additionally, there are two strong ventral flanges, the inner flange curving upward to the anterior cardinal angle. There is a fairly strong dorsal ridge, the ends curving abruptly downward to form anterior and posterior ridges, the former being the longer of the two. The flanges and ridges are considerably weaker on early instars, but the prominent central node is still unmistakable. So far as I am aware, this taxon occurs throughout the Pennsylvanian (and perhaps earlier), and disappears by mid-Permian time, a range in excess of 100 Ma. It has been assumed that this species was a free-swimming benthic form, as the prominent flanges would not be well-suited to an infaunal mode of life.
Betty Kellett described two species of the genus Cornigella from the Fort Riley Limestone of the Chase Group, higher in the Permian section of Kansas: Cornigella parva Kellett 1933, and Cornigella binoda Kellett 1933. They differed in the number of lateral nodes, the former species having a larger number of nodes, while in the latter species only the two prominent dorsal nodes were present. However, Kellett noted that her specimens showed considerable variation, which she attributed to poor preservation and diagenetic crushing. She went so far as to suggest that the two described taxa might actually be the same. Looking at Florena specimens, which are well-preserved complete carapaces, I would agree with her suggestion. The lateral nodes exhibit varying degrees of development; although the two dorsal nodes are always strongly developed, the ventral and anterior nodes may be considerably weaker. The specimen shown here is very well-preserved, and the full (?) complement of lateral nodes is clearly represented. (Note that, since we are looking at a complete carapace, the posterior dorsal node of the right valve is also obvious, as is a hint of the anterior dorsal node.) This specimen is also of interest, in that it shows a lot of the surface sculpturing, not too obvious on other specimens. I have chosen the name C. parva for this taxon, as Kellett's description appears first on the page, and should thus have priority.
I have not seen the description or illustrations of the generotype Cornigella minuta Warthin, 1930, which was described as having eight "prominent spines", one projecting well above the hinge line. Type specimens were from the Pennsylvanian Wetumka Formation of Oklahoma. I would follow Kellett's judgement in deciding that the Permian taxon was not conspecific with that of Warthin.
I had hoped to illustrate a perfect carapace of Ectodemites pinguis (Ulrich and Bassler, 1906) from the Florena, which I had temporarily stored in a small black plastic tray (the lid of a micromount box) on my desktop. Unfortunately, when I went to retrieve it for photography, it had simply disappeared -- even though I thought it to be well covered! Now it's fodder for the vacuum cleaner, one of the hazards of microfossil collecting................!
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