Schizaster variabilis (Slocum, 1990)
Plate: ScVa-10
Series: Upper Cretaceous Group: Selma Formation: Prairie Bluff
Locale: Pontotoc County: Mississippi Stage: Maastrichtian
Length: 28.2 mm Width: 24.3 mm Height: 19.3 mm
U.S.N.M. 108398 Cooke’s Hypotype
Repository: Paleobiology Dept., NMNH
Plate: ScVa-20
Series: Upper Cretaceous Group: Navarro Formation: Corsicana
Locale: San Antonio, Texas County: Bexar, Texas Stage: Maastrichtian
Length: 20.5 mm Width: 18.8 mm Height: 12.6 mm
Collector: Dan Woehr
Repository: Private Collection
HOLOTYPE INFO:
Holotype: Field Museum of Natural History (Chicago) P 10457 (Slocum, 1909).
Locality: One Mile Run, Pontotoc, Pontotoc County, Mississippi.
Stratigraphy: Prairie Bluff Chalk (formerly upper Ripley).
SYNONYMY
SYSTEMATIC PALEONTOLOGY – TAXONOMY
Class: Echinoidea Leske, 1778
Subclass: Euechinoidea Bronn, 1860
Infraclass: Irregularia Latreille, 1825
Superorder: Atelostomata Zittel, 1879
Order: Spatangoida L. Agassiz, 1840
Family: Schizasteridae Lambert, 1905
Genus: SCHIZASTER L. Agassiz, 1836
Species: Schizaster variabilis (Slocum, 1909)
TYPE SPECIES: Schizaster studeri L. Agassiz, 1836
DIAGNOSIS
Test distinctly cordate, inflated, and variable in proportions (length/width). Characterized by deep ambulacral depressions, a deep anterior sulcus, and a sharp posterior keel. Possesses both peripetalous and lateral fascioles. Distinguished from L. tumidula by the petal arrangement and shape (Slocum, 1909).
STRATIGRAPHICAL OCCURRENCE & DISTRIBUTION
Group: Selma (MS); Navarro (TX).
Formations: Prairie Bluff Chalk, Owl Creek Formation (Mississippi); Corsicana Marl (Texas).
Stratigraphic Units: Upper Cretaceous, Maastrichtian.
Counties: Pontotoc, Tippah (Mississippi); Bexar (Texas).
Abundance: Abundant in the Mississippi Embayment (Phillips, 2010); uncommon in Texas.
DESCRIPTION
TOP: Length, width, height: 28.2 mm L × 24.3 mm W × 19.3 mm H (Cooke Hypotype). Top shape: Distinctly cordate; truncated posteriorly. The proportion of length to width varies significantly (Slocum, 1909). Top domed: Dorsal surface elevated, forming a sharp ridge (keel) in the posterior interambulacral area.
SIDE: Side shape: Inflated and sloping to the lateral and anterior margins. Posterior truncation is slightly concave. The angle between the truncation and the base line varies from 77° to 90° (Slocum, 1909).
BOTTOM: Bottom shape: Ventral surface depressed convex (Slocum, 1909); nearly flat (Clark & Twitchell, 1915).
AMB: Amb shape: Straight, petaloid, situated in comparatively deep depressions. Anterior paired petals are about 1.5 times the length of the posterior pair. Amb arrangement: Unpaired anterior ambulacrum (III) is situated in a deep depression, the continuance of which forms a sulcus in the anterior margin.
PORE PAIRS: Shape: Pores in paired petals are elongated and slit-like; connected by a shallow groove. Pores in the unpaired ambulacrum are round, narrow, and situated far apart, with each pair separated by a tubercle (Slocum, 1909).
TUBERCLES: Primary Perforated: Yes. Primary Crenulate: Yes. Distribution: Surface covered with minute tubercles having crenulated bosses. Tubercles increase in size as they approach the peristome.
GRANULES: Distribution: Interspaces filled with small tubercles and microscopic granulations (Slocum, 1909).
FASCIOLE: Peripetalous: Clearly defined; decided bent inward between the anterolateral and posterolateral petals. Lateral: Clearly defined (Slocum, 1909). (Note: The presence of the lateral fasciole technically places this in Linthia according to some classifications, though Schizaster also possesses a lateral fasciole).
APICAL SYSTEM: Location: Central or somewhat anterior to the center (Slocum, 1909). Size: Small and depressed. Genital pores: Four perforated genital plates separated by five small radials.
PERIPROCT (ANUS): Shape: Elongated vertically. Location: Situated near the top of the posterior truncation.
PERISTOME (MOUTH): Shape: Transversely elliptical. Location: Situated near the anterior margin. Lip: Labrum prominent.
SPINES: Unknown.
SPECIMENS STUDIED & CONDITION:
ETYMOLOGY
Named variabilis due to the great individual variation in length/width ratios and apical position observed in the type series (Slocum, 1909).
REMARKS & COMPARISONS
Remarks: Slocum (1909) emphasized the high variability of this species, noting that no well-marked divisions could be made between the broad/flat forms and the high/narrow forms. Cooke (1953) assigned this species to Schizaster due to the deeply sunken petals, although Slocum and Clark & Twitchell (1915) placed it in Linthia. George Phillips (Mississippi Museum of Natural Science) notes this is one of the most common echinoids in the Prairie Bluff Chalk of Mississippi.
Comparisons:
Plate: ScVa-10
Series: Upper Cretaceous Group: Selma Formation: Prairie Bluff
Locale: Pontotoc County: Mississippi Stage: Maastrichtian
Length: 28.2 mm Width: 24.3 mm Height: 19.3 mm
U.S.N.M. 108398 Cooke’s Hypotype
Repository: Paleobiology Dept., NMNH
Plate: ScVa-20
Series: Upper Cretaceous Group: Navarro Formation: Corsicana
Locale: San Antonio, Texas County: Bexar, Texas Stage: Maastrichtian
Length: 20.5 mm Width: 18.8 mm Height: 12.6 mm
Collector: Dan Woehr
Repository: Private Collection
HOLOTYPE INFO:
Holotype: Field Museum of Natural History (Chicago) P 10457 (Slocum, 1909).
Locality: One Mile Run, Pontotoc, Pontotoc County, Mississippi.
Stratigraphy: Prairie Bluff Chalk (formerly upper Ripley).
SYNONYMY
- Linthia variabilis Slocum, 1909
- Schizaster (Linthia) variabilis (Slocum) Cooke, 1953
SYSTEMATIC PALEONTOLOGY – TAXONOMY
Class: Echinoidea Leske, 1778
Subclass: Euechinoidea Bronn, 1860
Infraclass: Irregularia Latreille, 1825
Superorder: Atelostomata Zittel, 1879
Order: Spatangoida L. Agassiz, 1840
Family: Schizasteridae Lambert, 1905
Genus: SCHIZASTER L. Agassiz, 1836
Species: Schizaster variabilis (Slocum, 1909)
TYPE SPECIES: Schizaster studeri L. Agassiz, 1836
DIAGNOSIS
Test distinctly cordate, inflated, and variable in proportions (length/width). Characterized by deep ambulacral depressions, a deep anterior sulcus, and a sharp posterior keel. Possesses both peripetalous and lateral fascioles. Distinguished from L. tumidula by the petal arrangement and shape (Slocum, 1909).
STRATIGRAPHICAL OCCURRENCE & DISTRIBUTION
Group: Selma (MS); Navarro (TX).
Formations: Prairie Bluff Chalk, Owl Creek Formation (Mississippi); Corsicana Marl (Texas).
Stratigraphic Units: Upper Cretaceous, Maastrichtian.
Counties: Pontotoc, Tippah (Mississippi); Bexar (Texas).
Abundance: Abundant in the Mississippi Embayment (Phillips, 2010); uncommon in Texas.
DESCRIPTION
TOP: Length, width, height: 28.2 mm L × 24.3 mm W × 19.3 mm H (Cooke Hypotype). Top shape: Distinctly cordate; truncated posteriorly. The proportion of length to width varies significantly (Slocum, 1909). Top domed: Dorsal surface elevated, forming a sharp ridge (keel) in the posterior interambulacral area.
SIDE: Side shape: Inflated and sloping to the lateral and anterior margins. Posterior truncation is slightly concave. The angle between the truncation and the base line varies from 77° to 90° (Slocum, 1909).
BOTTOM: Bottom shape: Ventral surface depressed convex (Slocum, 1909); nearly flat (Clark & Twitchell, 1915).
AMB: Amb shape: Straight, petaloid, situated in comparatively deep depressions. Anterior paired petals are about 1.5 times the length of the posterior pair. Amb arrangement: Unpaired anterior ambulacrum (III) is situated in a deep depression, the continuance of which forms a sulcus in the anterior margin.
PORE PAIRS: Shape: Pores in paired petals are elongated and slit-like; connected by a shallow groove. Pores in the unpaired ambulacrum are round, narrow, and situated far apart, with each pair separated by a tubercle (Slocum, 1909).
TUBERCLES: Primary Perforated: Yes. Primary Crenulate: Yes. Distribution: Surface covered with minute tubercles having crenulated bosses. Tubercles increase in size as they approach the peristome.
GRANULES: Distribution: Interspaces filled with small tubercles and microscopic granulations (Slocum, 1909).
FASCIOLE: Peripetalous: Clearly defined; decided bent inward between the anterolateral and posterolateral petals. Lateral: Clearly defined (Slocum, 1909). (Note: The presence of the lateral fasciole technically places this in Linthia according to some classifications, though Schizaster also possesses a lateral fasciole).
APICAL SYSTEM: Location: Central or somewhat anterior to the center (Slocum, 1909). Size: Small and depressed. Genital pores: Four perforated genital plates separated by five small radials.
PERIPROCT (ANUS): Shape: Elongated vertically. Location: Situated near the top of the posterior truncation.
PERISTOME (MOUTH): Shape: Transversely elliptical. Location: Situated near the anterior margin. Lip: Labrum prominent.
SPINES: Unknown.
SPECIMENS STUDIED & CONDITION:
- Hypotype USNM 108398: Prairie Bluff Chalk, Mississippi. (Cooke).
- Specimen 2 (ScVa-20): Corsicana Marl, San Antonio, TX. Collected by Dan Woehr.
ETYMOLOGY
Named variabilis due to the great individual variation in length/width ratios and apical position observed in the type series (Slocum, 1909).
REMARKS & COMPARISONS
Remarks: Slocum (1909) emphasized the high variability of this species, noting that no well-marked divisions could be made between the broad/flat forms and the high/narrow forms. Cooke (1953) assigned this species to Schizaster due to the deeply sunken petals, although Slocum and Clark & Twitchell (1915) placed it in Linthia. George Phillips (Mississippi Museum of Natural Science) notes this is one of the most common echinoids in the Prairie Bluff Chalk of Mississippi.
Comparisons:
- Linthia tumidula: The only other species of the genus described from the American Cretaceous at the time (1909), but it is so unlike S. variabilis that detailed comparison was considered unnecessary by Slocum.
