Leptarbacia arguta (Clark, 1915)
Plate: LeAr-10
Series: Lower Cretaceous Group: Washita Formation: Unknown
Locale: Fort Worth County: Denton Stage: Albian
Length: 19.9 mm Height: 11.8 mm
Specimen in photos: U.S.N.M. 103703 Holotype
Collector: USGS Survey
Repository: Paleobiology Dept., NMNH, Smithsonian Institution
Plate: LeAr-20
Series: Upper Cretaceous Group: Washita Formation: Del Rio
Locale: County: McLellan Stage: Canomanian
Length: 17.2 mm Height: 11.1 mm
Specimen in photo: Collection of Matthew Harrel of Waco
Plate: LeAr-30
Series: Lower Cretaceous Group: Fredericksburg Formation: Walnut
Locale: Austin County: Travis Stage: Albian
Length: 15 mm Height: 7 mm
Specimen in photos: Collection of Ed Elliott
HOLOTYPE INFO:
Holotype: USNM 103703 (Clark, 1915).
Locality: Fort Worth, Denton County, Texas.
Stratigraphy: Washita Group (Formation unspecified, likely Duck Creek or Fort Worth Limestone).
SYNONYMY
SYSTEMATIC PALEONTOLOGY – TAXONOMY
Class: Echinoidea Leske, 1778
Subclass: Euechinoidea Bronn, 1860
Infraclass: Carinacea Kroh & Smith, 2010
Superorder: Stomopneustacea Kroh & Smith, 2010
Order: Stomopneustoida Kroh & Smith, 2010
Family: Stomopneustidae Mortensen, 1903
Genus: LEPTARBACIA Clark, 1912
Species: Leptarbacia arguta (Clark, 1915)
TYPE SPECIES: Leptarbacia arguta Clark, 1915
DIAGNOSIS
Test small, depressed above and flat below. Ambulacra straight and wide at the ambitus; plates compound (typically one primary and three demiplates). Interambulacra feature long bare median areas on the upper surface; lower surface possesses six rows of small perforated and crenulated tubercles (Clark, 1915).
STRATIGRAPHICAL OCCURRENCE & DISTRIBUTION
Group: Washita, Fredericksburg
Formations: Del Rio Formation, Walnut Formation, (Undifferentiated Washita)
Stratigraphic Units: Lower Cretaceous (Albian) to Upper Cretaceous (Cenomanian)
Counties: Denton, McLennan, Travis
Abundance: Rare.
DESCRIPTION
TOP: Length, width, height: 19.9 mm L × 11.8 mm H (Holotype).
Top shape: Circular outline.
Top domed: Depressed upper surface (Clark, 1915).
SIDE: Side shape: Depressed near the peristome. Flat below.
BOTTOM: Bottom shape: Nearly flat (Clark, 1915).
AMB: Amb shape: Straight; narrow adapically, increasing in width to the ambitus, then decreasing toward the peristome (Clark, 1915).
Amb arrangement: Double row of about thirty tubercles in each area; tubercles become indistinct toward the apical disk.
Plating: Compound plates; generally composed of one primary and three demiplates (some with two demiplates).
PORE PAIRS: Arrangement: Irregular simple series in each poriferous zone (Clark, 1915).
TUBERCLES: Primary Perforated: Yes
Primary Crenulate: Yes.
Distribution: Interambulacra contain six rows of tubercles from the ambitus to the peristome. The central row of each half continues to the apical system but becomes less distinct.
Ambulacral Tubercles: ~30 per area.
INTERAMB: Width: About three times the width of the ambulacra.
Features: Characterized by long bare median areas on the upper surface approaching the ambitus (Clark, 1915).
GRANULES: Distribution: Miliary space covered with small granules (Clark, 1915).
FASCIOLE: Absent.
APICAL SYSTEM: Size: Medium (Diagnosis) to Small (Description) (Clark, 1915).
Plating: Genital plates are somewhat larger than the oculars, but rarely project farther than the latter.
PERIPROCT (ANUS): Shape: Nearly circular.
Location: Slightly subcentral.
PERISTOME (MOUTH): Size: Diameter is approximately 1/3 of the test diameter (Book Author).
Lip: Moderate buccal notches.
SPINES: Shape: Lanceolate, evenly tapered.
Ornamentation: Slightly striated on the shaft above the collar.
Base: Rounded and perforated; slightly smaller diameter than the shaft.
SPECIMENS STUDIED & CONDITION:
ETYMOLOGY
From the Latin argutus (clear, bright, or sharp), possibly referring to the distinct ornamentation.
REMARKS & COMPARISONS
Remarks: Clark (1915) originally placed this in Leptarbacia based on the compound plating and general form. Cooke (1946) moved it to Polydiadema due to the perforated tubercles described by Clark. However, modern classification (Kroh & Smith) retains Leptarbacia within the Stomopneustidae.
The observations by the Book Author regarding the spines (perforated base) and peristome size add significant morphological data not present in the original description of the holotype.
Remark (1-3-26): Clark describes the tubercles as perforated. Modern definitions of Leptarbacia (Stomopneustidae) typically require imperforate tubercles. If the tubercles are truly perforated, this species may belong in Polydiadema (Diadematidae), as Cooke suggested.
Plate: LeAr-10
Series: Lower Cretaceous Group: Washita Formation: Unknown
Locale: Fort Worth County: Denton Stage: Albian
Length: 19.9 mm Height: 11.8 mm
Specimen in photos: U.S.N.M. 103703 Holotype
Collector: USGS Survey
Repository: Paleobiology Dept., NMNH, Smithsonian Institution
Plate: LeAr-20
Series: Upper Cretaceous Group: Washita Formation: Del Rio
Locale: County: McLellan Stage: Canomanian
Length: 17.2 mm Height: 11.1 mm
Specimen in photo: Collection of Matthew Harrel of Waco
Plate: LeAr-30
Series: Lower Cretaceous Group: Fredericksburg Formation: Walnut
Locale: Austin County: Travis Stage: Albian
Length: 15 mm Height: 7 mm
Specimen in photos: Collection of Ed Elliott
HOLOTYPE INFO:
Holotype: USNM 103703 (Clark, 1915).
Locality: Fort Worth, Denton County, Texas.
Stratigraphy: Washita Group (Formation unspecified, likely Duck Creek or Fort Worth Limestone).
SYNONYMY
- Leptarbacia argutus Clark, 1915 (Original orthography)
- Polydiadema arguta (Clark, 1915) (fide Cooke, 1946)
SYSTEMATIC PALEONTOLOGY – TAXONOMY
Class: Echinoidea Leske, 1778
Subclass: Euechinoidea Bronn, 1860
Infraclass: Carinacea Kroh & Smith, 2010
Superorder: Stomopneustacea Kroh & Smith, 2010
Order: Stomopneustoida Kroh & Smith, 2010
Family: Stomopneustidae Mortensen, 1903
Genus: LEPTARBACIA Clark, 1912
Species: Leptarbacia arguta (Clark, 1915)
TYPE SPECIES: Leptarbacia arguta Clark, 1915
DIAGNOSIS
Test small, depressed above and flat below. Ambulacra straight and wide at the ambitus; plates compound (typically one primary and three demiplates). Interambulacra feature long bare median areas on the upper surface; lower surface possesses six rows of small perforated and crenulated tubercles (Clark, 1915).
STRATIGRAPHICAL OCCURRENCE & DISTRIBUTION
Group: Washita, Fredericksburg
Formations: Del Rio Formation, Walnut Formation, (Undifferentiated Washita)
Stratigraphic Units: Lower Cretaceous (Albian) to Upper Cretaceous (Cenomanian)
Counties: Denton, McLennan, Travis
Abundance: Rare.
DESCRIPTION
TOP: Length, width, height: 19.9 mm L × 11.8 mm H (Holotype).
Top shape: Circular outline.
Top domed: Depressed upper surface (Clark, 1915).
SIDE: Side shape: Depressed near the peristome. Flat below.
BOTTOM: Bottom shape: Nearly flat (Clark, 1915).
AMB: Amb shape: Straight; narrow adapically, increasing in width to the ambitus, then decreasing toward the peristome (Clark, 1915).
Amb arrangement: Double row of about thirty tubercles in each area; tubercles become indistinct toward the apical disk.
Plating: Compound plates; generally composed of one primary and three demiplates (some with two demiplates).
PORE PAIRS: Arrangement: Irregular simple series in each poriferous zone (Clark, 1915).
TUBERCLES: Primary Perforated: Yes
Primary Crenulate: Yes.
Distribution: Interambulacra contain six rows of tubercles from the ambitus to the peristome. The central row of each half continues to the apical system but becomes less distinct.
Ambulacral Tubercles: ~30 per area.
INTERAMB: Width: About three times the width of the ambulacra.
Features: Characterized by long bare median areas on the upper surface approaching the ambitus (Clark, 1915).
GRANULES: Distribution: Miliary space covered with small granules (Clark, 1915).
FASCIOLE: Absent.
APICAL SYSTEM: Size: Medium (Diagnosis) to Small (Description) (Clark, 1915).
Plating: Genital plates are somewhat larger than the oculars, but rarely project farther than the latter.
PERIPROCT (ANUS): Shape: Nearly circular.
Location: Slightly subcentral.
PERISTOME (MOUTH): Size: Diameter is approximately 1/3 of the test diameter (Book Author).
Lip: Moderate buccal notches.
SPINES: Shape: Lanceolate, evenly tapered.
Ornamentation: Slightly striated on the shaft above the collar.
Base: Rounded and perforated; slightly smaller diameter than the shaft.
SPECIMENS STUDIED & CONDITION:
- Holotype: USNM 103703 (Denton Co.).
- Specimen 2: Collection of Matthew Harrel (McLennan Co., Del Rio Fm). Preserves spines.
- Specimen 3: Collection of Ed Elliott (Travis Co., Walnut Fm).
ETYMOLOGY
From the Latin argutus (clear, bright, or sharp), possibly referring to the distinct ornamentation.
REMARKS & COMPARISONS
Remarks: Clark (1915) originally placed this in Leptarbacia based on the compound plating and general form. Cooke (1946) moved it to Polydiadema due to the perforated tubercles described by Clark. However, modern classification (Kroh & Smith) retains Leptarbacia within the Stomopneustidae.
The observations by the Book Author regarding the spines (perforated base) and peristome size add significant morphological data not present in the original description of the holotype.
Remark (1-3-26): Clark describes the tubercles as perforated. Modern definitions of Leptarbacia (Stomopneustidae) typically require imperforate tubercles. If the tubercles are truly perforated, this species may belong in Polydiadema (Diadematidae), as Cooke suggested.
