Podocyrtis (Lampterium) mitra Ehrenberg

Podocyrtis mitra Ehrenberg, 1854, pl.36, fig.B20; 1873, p.251; non Ehrenberg, 1875, pl.15, fig.4; Riedel and Sanfilippo, 1970, p.534, pl.11, figs.5-6; 1978a, text-fig.3

DESCRIPTION

Cephalis [three-lobed], hemispherical, with few to many small pores, bearing a bladed horn of about the same length. Thorax thick-walled, conical to campanulate, with usually rather large subcircular pores and rough surface. Collar and lumbar strictures distinct. Abdomen thick-walled, expanding distally and contracting abruptly near its end, with large subcircular pores longitudinally aligned and in some specimens with the rows separated by ribs. Abdominal pores in many specimens have delicate spines projecting into them from their periphery. Abdominal surface is usually smooth, sometimes rough. Three short feet and flat, spathulate or triangular, or absent (Riedel and Sanfilippo, 1970).

DIMENSIONS

Length of thorax in early forms 60-80 m, in late forms 55-60 m. Abdominal length (excluding feet) in early forms 115-160 m, in late forms 185-205 m. The number of longitudinal rows of abdominal pores ranges from 26 in early specimens to 13 in late ones (Sanfilippo et al., 1985).

DISTINGUISHING CHARACTERS

P. (L.) mitra is distinguished from P. (L.) trachodes by having a smooth rather than a rough outline, from P. (L.) sinuosa by having its abdomen widest distally rather than medially, and from P. (L.) chalara by having more than 13 pores on the circumference of the abdomen at its widest part. The abdomen has a thicker wall, and larger, more regularly arranged pores, than those of P. (L.) helenae (Sanfilippo et al., 1985).

VARIABILITY

P. (L.) mitra, being a member of the evolutionary lineage from P. (P.) papalis to P. (L.) goetheana, shows a continuous reduction in the size of the thorax and feet, and an increase in abdominal length and the size of the abdominal pores, with time (Sanfilippo et al., 1985).

DISTRIBUTION

This middle middle Eocene species is common and widely distributed in low-latitude assemblages. Sometimes it is overwhelmed by abundant P. (L.) fasciolata during a brief part of its range, and by abundant P. (L.) trachodes later in its range. Its evolutionary transition from Podocyrtis sinuosa defines the base of the Podocyrtis mitra Zone. Its evolutionary transition to Podocyrtis chalara defines the base of the Podocyrtis chalara Zone.

PHYLOGENY

This species is descended directly from P. (L.) sinuosa, and is ancestral to P. (L.) chalara.

REMARKS

Additional illustrations can be found in Nigrini, 1974, pl.lL, fig.5, non fig.6; Riedel and Sanfilippo, 1981, figs.12-18, 5-8.

For generic level taxonomy see Sanfilippo and Riedel, 1992.

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