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Impagidinium arachnion
Impagidinium arachnion De Verteuil and Norris 1996
Holotype. Plate 9, figs. 9-12.
Locus typicus: the Scientists Cliffs South Section, Scientists Cliffs community beach level car park, about 2.5km south of Parker Creek, on the western shore of the Chesapeake Bay, Calvert County, Maryland.
Stratum typicum: Middle Miocene Plum Point Marl Member of the Calvert Formation (Shattuck-bed 13).
Original diagnosis: de Verteuil and Norris, 1996, p. 140: Impagidinium arachnion
A small species of Impagidinium with intergonally moderately to very depressed limbate septa that are distally invaginated and proximally microfenestrate. The ventral surface is characterized by the absence or rudimentary development of septa over specific field boundaries.
Original description: de Verteuil and Norris, 1996, p. 140, 142: Impagidinium arachnion
Proximochorate cysts with an overall ovoidal habit that may present an angular aspect due to the intergonally depressed, limbate septa. The wall comprises homogeneous pedium ca. 0.25µm thick, fine prismatic luxuriae ca. 0.25µm thick and discontinuous tegillum ca.0.25 µm thick. The tegillum arises from the limbi forming peritabular septa with narrow pericoels. The outer surface microstructure of the autoblast in intratabular areas is shagreenate to nannopunctate, the pits being less than 0.2µm (pl. 18, fig.5) and appears smooth under TLM. Septa are characterized by distal invaginations so that shallow canals run along the ridges of the septa forming distally closed triple junctions at gonal positions (pl. 18, fig. 5). Proximally, septa are typically microfenestrate, with holes 0.2 - 1.0µm in diameter forming a lace-like pattern at the base of septa and becoming less densely spaced distally. The occurrence and density of microfenestrations on the septa is variable within and between specimens and may even be absent. Septa are highest gonally, ca. 4 to 6µm or 20% of equatorial diameter, and form symmetrical intergonal depressions that are between 40-60 % of gonal height at their lowest point. Tegillum microstructure of the septa is smooth to scabrate.
Cyst tabulation style is sexiform S-type indicated only by the limbate septa, so that the precise topology of small fields is difficult to determine. The sixth precingular field is planate subtriangular with narrow 6"/1", and the septum adjoining the sulcus absent or rudimentary. The first and fourth apical fields are stenoform and approximately symmetrical about the sulcal plane. Their shared boundary is straight and marked by a low linear cusp or not at all. Other field boundaries consistently lacking septa are 6"1as, 6c/6"", 1c/2"" and 1p/ps; these typically possess a low linear cusp but may lack any peritabular expression. The third precingular and fourth postcingular are mid-dorsal and symmetrical about the sulcal plane but 3c/4c is always offset to the right of this plane (pl. 18, fig. 8). The archeopyle is simple precingular Type P.
Dimensions: Length, excluding septa, 28(35.3)41µm; holotype 38µm. Width at cingulum, excluding septa, 24(28.8)41µm (based on eighteen specimens); holotype 30µm. Breadth at cingulum 28(29.3)33µm (based on six specimens). Septa 2-6µm at gonal positions. Twenty-four specimens measured.
Comparison: Impagidinium arachnion is distinguished from all other species of Impagidinium by the unique combination of character states included in the diagnosis. Among described species of fossil cysts it is most similar to Impagidinium aculeatum, Impagidinium dispertitum, Impagidinium paradoxum, Impagidinium patulum and Impagidinium victorianum. Impagidinium paradoxum, Impagidinium patulum and Impagidinium victorianum all exhibit selective development of peritabular septa on the ventral surface, as does Impagidinium arachnion. For Impagidinium patulum, the pattern of selective development is the same as described for Impagidinium arachnion, while for Impagidinium paradoxum and Impagidinium victorianum it is very similar. Of these three species, Impagidinium patulum and Impagidinium victorianum are larger than Impagidinium arachnion. Moreover, all three have straight septa that are not depressed and are relatively lower that Impagidinium arachnion. Impagidinium aculeatum is also small but has more pronounced intergonal depressions on the septa and lacks basal microfenestration and invagination of septum tops. Ventral tabulation for Impagidinium aculeatum is fully developed except within the sulcus itself. Septum morphology in Impagidinium dispertium is superficially similar to Impagidinium arachnion but the former is larger and has fully expressed ventral tabulation, as is the case with Impagidinium antecarcerem, described above. The species figured by Wrenn and Kokinos (1986; pl. 2, figs.5-7) as Impagidinium sp. D is similar to Impagidinium arachnion in size, tabulation and septum morphology. The former taxon, however, has clearly developed septa at 6c/6"" and lc/2", and does not appear to possess the proximal microfenestrations characteristic of Impagidinium arachnion.
Occurence: Middle through upper Miocene (DN5-DN9) in the Salisbury Embayment. Present to common inthe upper part of the Clavert Formation and in the Choptank Formation. Rare to absent in the Saint Marys Formation.
Holotype. Plate 9, figs. 9-12.
Locus typicus: the Scientists Cliffs South Section, Scientists Cliffs community beach level car park, about 2.5km south of Parker Creek, on the western shore of the Chesapeake Bay, Calvert County, Maryland.
Stratum typicum: Middle Miocene Plum Point Marl Member of the Calvert Formation (Shattuck-bed 13).
Original diagnosis: de Verteuil and Norris, 1996, p. 140: Impagidinium arachnion
A small species of Impagidinium with intergonally moderately to very depressed limbate septa that are distally invaginated and proximally microfenestrate. The ventral surface is characterized by the absence or rudimentary development of septa over specific field boundaries.
Original description: de Verteuil and Norris, 1996, p. 140, 142: Impagidinium arachnion
Proximochorate cysts with an overall ovoidal habit that may present an angular aspect due to the intergonally depressed, limbate septa. The wall comprises homogeneous pedium ca. 0.25µm thick, fine prismatic luxuriae ca. 0.25µm thick and discontinuous tegillum ca.0.25 µm thick. The tegillum arises from the limbi forming peritabular septa with narrow pericoels. The outer surface microstructure of the autoblast in intratabular areas is shagreenate to nannopunctate, the pits being less than 0.2µm (pl. 18, fig.5) and appears smooth under TLM. Septa are characterized by distal invaginations so that shallow canals run along the ridges of the septa forming distally closed triple junctions at gonal positions (pl. 18, fig. 5). Proximally, septa are typically microfenestrate, with holes 0.2 - 1.0µm in diameter forming a lace-like pattern at the base of septa and becoming less densely spaced distally. The occurrence and density of microfenestrations on the septa is variable within and between specimens and may even be absent. Septa are highest gonally, ca. 4 to 6µm or 20% of equatorial diameter, and form symmetrical intergonal depressions that are between 40-60 % of gonal height at their lowest point. Tegillum microstructure of the septa is smooth to scabrate.
Cyst tabulation style is sexiform S-type indicated only by the limbate septa, so that the precise topology of small fields is difficult to determine. The sixth precingular field is planate subtriangular with narrow 6"/1", and the septum adjoining the sulcus absent or rudimentary. The first and fourth apical fields are stenoform and approximately symmetrical about the sulcal plane. Their shared boundary is straight and marked by a low linear cusp or not at all. Other field boundaries consistently lacking septa are 6"1as, 6c/6"", 1c/2"" and 1p/ps; these typically possess a low linear cusp but may lack any peritabular expression. The third precingular and fourth postcingular are mid-dorsal and symmetrical about the sulcal plane but 3c/4c is always offset to the right of this plane (pl. 18, fig. 8). The archeopyle is simple precingular Type P.
Dimensions: Length, excluding septa, 28(35.3)41µm; holotype 38µm. Width at cingulum, excluding septa, 24(28.8)41µm (based on eighteen specimens); holotype 30µm. Breadth at cingulum 28(29.3)33µm (based on six specimens). Septa 2-6µm at gonal positions. Twenty-four specimens measured.
Comparison: Impagidinium arachnion is distinguished from all other species of Impagidinium by the unique combination of character states included in the diagnosis. Among described species of fossil cysts it is most similar to Impagidinium aculeatum, Impagidinium dispertitum, Impagidinium paradoxum, Impagidinium patulum and Impagidinium victorianum. Impagidinium paradoxum, Impagidinium patulum and Impagidinium victorianum all exhibit selective development of peritabular septa on the ventral surface, as does Impagidinium arachnion. For Impagidinium patulum, the pattern of selective development is the same as described for Impagidinium arachnion, while for Impagidinium paradoxum and Impagidinium victorianum it is very similar. Of these three species, Impagidinium patulum and Impagidinium victorianum are larger than Impagidinium arachnion. Moreover, all three have straight septa that are not depressed and are relatively lower that Impagidinium arachnion. Impagidinium aculeatum is also small but has more pronounced intergonal depressions on the septa and lacks basal microfenestration and invagination of septum tops. Ventral tabulation for Impagidinium aculeatum is fully developed except within the sulcus itself. Septum morphology in Impagidinium dispertium is superficially similar to Impagidinium arachnion but the former is larger and has fully expressed ventral tabulation, as is the case with Impagidinium antecarcerem, described above. The species figured by Wrenn and Kokinos (1986; pl. 2, figs.5-7) as Impagidinium sp. D is similar to Impagidinium arachnion in size, tabulation and septum morphology. The former taxon, however, has clearly developed septa at 6c/6"" and lc/2", and does not appear to possess the proximal microfenestrations characteristic of Impagidinium arachnion.
Occurence: Middle through upper Miocene (DN5-DN9) in the Salisbury Embayment. Present to common inthe upper part of the Clavert Formation and in the Choptank Formation. Rare to absent in the Saint Marys Formation.