Didymocyrtis laticonus


Classification: rads_cenozoic -> Coccodiscidae -> Didymocyrtis -> Didymocyrtis laticonus
Sister taxa: D. antepenultima, D. avita, D. bassanii, D. didymus, D. laticonus, D. mammifera, D. penultima, D. prismatica, D. tetrathalamus, D. tubaria, D. violina, D. sp.

Taxonomy

Citation: Didymocyrtis laticonus (Riedel) Sanfilippo and Riedel 1980
Rank: species
Basionym: Cannartus laticonus
Taxonomic discussion: (Riedel) 1959 p.291 pl.1 fig.5 as Cannartus; Sanfilippo & Riedel 1980 p.1010 pl.; Morley1985 pl8 fig5; Sanfilippo et al.1985 p.658 figs.8.5a-b

Catalog entries: Cannartus laticonus

Original description: Cortical twin-shell rather thick-walled, with pores subcircular or circular, ten to thirteen on the half-equator. On the broader parts of the shell, on either side of the equatorial constriction, are pronounced obtuse moundlike protuberances, at which the shell wall is thickened; there is a tendency in some specimens for these protuberances to be so arranged that two girdles of them encircle each half of the twin-shell. At each pole of the shell arises a broadly subconical, densely spongy column, which is almost as broad at its base as the polar surface of the twin-shell. Medullary shells two (or perhaps only one in some specimens), of which the inner is spherical and the outer spherical or lenticularly compressed. This species is distinguished from all others of the genus by the broadly subconical polar columns.

Based on 20 specimens. Length of polar columns 45-70 µ; median breadth 28-40 µ. Length of cortical shell 93-125 µ; maximum breadth 68-113 µ. Breadth of outer medullary shell 30-35 µ. Distinguishing characters (rw): The equatorially constricted cortical shell has a tuberculate surface, and bears wide, spongy polar columns. There are no pronounced caps, but a parallel-sided clear zone, no wider than the height of the tubercules, separates the cortical shell from the columns. (Cannartus laticonus in Riedel and Sanfilippo, 1978a)

Westberg and Riedel (1978) used this name only for specimens "in which the height of the clear zone below the spongy column is less than 0.2 the length of the cortical shell."

D. laticonus is distinguished from its ancestor, D. mammifera, by having "clear zones" parallel to the cortical shell. In addition, the spongy columns tend to be wider than those of D. mammifera. It differs from its descendant D. antepenultima in that the proportion of the length of the clear zone (cap) to the length of the cortical shell is less than 0.20. (Sanfilippo et al., 1985) Phylogeny (rw): D. laticonus evolved from D. mammifera and developed into D. antepenultima.
Remarks on original description: This species is one of a remarkable series of forms which evidently arose in the Upper Oligocene and underwent rapid evolutionary changes during the Miocene. They apparently represent the phyletic connection between the genus Pipettella, in which an ellipsoidal cortical shell without equatorial constriction bears subcylindrical spongy polar columns, and the panartids, in which the equatorially constricted cortical shell bears latticed polar caps. It has previously been shown (Riedel, 1957) that a common late Tertiary panartid, Panarium penultimum, has distinct polar caps surmounted by spongy polar columns, and the author has pointed out that this species probably developed from a form having less conspicuous polar caps and wider terminal spongy columns.

Description

Biogeography and Palaeobiology

Biostratigraphic distribution

Geological Range:
Last occurrence (top): within Neogene Period (2.59-23.03Ma, top in Piacenzian stage). Data source: Lazarus et al. 2015 - "N age group"
First occurrence (base): within Neogene Period (2.59-23.03Ma, base in Aquitanian stage). Data source: Lazarus et al. 2015 - "N age group"

Plot of occurrence data:

References:

Morley, J. J. (1985). Radiolarians from the northwest Pacific, Deep Sea Drilling Project Leg 86. Initial Reports of the Deep Sea Drilling Project. 86: 399-422. gs

Riedel, W. R. (1959). Oligocene and Lower Miocene Radiolaria in tropical Pacific sediments. Micropaleontology. 5(3): 285-302. gs

Sanfilippo, A. & Riedel, W. R. (1980). A revised generic and suprageneric classification of the Artiscins (Radiolaria). Journal of Paleontology. 54(5): 1008-1011. gs

Sanfilippo, A., Westberg-Smith, M. J. & Riedel, W. R. (1985). Cenozoic Radiolaria. In, Bolli, H. M. , Saunders, J. B. & Perch-Nielsen K. (eds) Plankton Stratigraphy. Cambridge University Press, Cambridge 631-712. gs


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Didymocyrtis laticonus compiled by the radiolaria@mikrotax project team viewed: 27-11-2020

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