Phymosoma volanum (Cragin, 1893)
Plate: PhVo-10
Series: Upper Cretaceous Group: Washita Formation: Buda
Locale: County: Tarrant Stage: Cenomanian
Length: 22.9 mm Height: 9.0 mm
Specimen in photos: Hypotype
Repository: Non-vertebrate Paleo. Lab., Univ. of Texas Fragment
HOLOTYPE INFO:
Holotype: Originally described by Cragin (1893). Repository likely UT/BEG or USNM.
Locality: Big Elm Creek, below the Missouri, Kansas and Texas railway bridge, Denton County, Texas.
Stratigraphy: "Vola limestone" (Equivalent to Main Street/Grayson Formation), Washita Group.
SYNONYMY
SYSTEMATIC PALEONTOLOGY – TAXONOMY
Class: Echinoidea Leske, 1778
Subclass: Euechinoidea Bronn, 1860
Infraclass: Carinacea Kroh & Smith, 2010
Superorder: Calycina Gregory, 1900
Order: Phymosomatoida Mortensen, 1904
Family: Phymosomatidae Pomel, 1883
Genus: PHYMOSOMA Haime, 1853
Species: Phymosoma volanum (Cragin, 1893)
TYPE SPECIES: Diadema priscum Agassiz, 1840
DIAGNOSIS
Test small, subcircular to subpentagonal, and depressed. Ambulacra straight with sinuous pore zones. Interambulacra feature two rows of large primary tubercles and smaller lateral rows. Distinguished from P. texanum by being lower, more depressed, and having less fully defined outer tubercle rows (Clark & Twitchell, 1915).
STRATIGRAPHICAL OCCURRENCE & DISTRIBUTION
Group: Washita
Formations: Buda Limestone; Main Street Limestone; Grayson Marl ("Vola").
Stratigraphic Units: Upper Cretaceous, Cenomanian.
Counties: Tarrant, Denton.
Abundance: Uncommon.
DESCRIPTION
TOP: Length, width, height: 22.9 mm L × 9.0 mm H (Figured Specimen). Top shape: Subcircular to rounded-pentagonal (Cragin, 1893). Top domed: Slightly elevated upper surface.
SIDE: Ambitus: Moderately inflated. Side shape: Depressed profile; about three-fifths as high as broad (Cragin, 1893).
BOTTOM: Bottom shape: Depressed and concave.
AMB: Amb shape: Broad and slightly prominent. Amb arrangement: Two rows of primary tubercles (8 or 9 per row); tubercles decrease rapidly in size from ambitus to poles. Surrounded by narrow polygon-building strings of granules (Cragin, 1893).
PORE PAIRS: Arrangement: Uniserial from peristome nearly to apical system; crowded toward the apical system (Clark & Twitchell, 1915). Shape: Pores somewhat elongated in the direction of the belt (Cragin, 1893). Zones are sinuous.
TUBERCLES: Primary Perforated: No (Imperforate) (Cragin, 1893). Primary Crenulate: Yes. Interambulacral Tubercles: Two rows of large primary tubercles placed near the centers of the plates. A row of smaller cone-based primary tubercles is placed along either side of the area (close to the pore belt). Distribution: Numerous granules on the surface of the plates; not crowded but unevenly distributed (Cragin, 1893).
GRANULES: See Tubercles section.
FASCIOLE: Absent.
APICAL SYSTEM: Not fully described; Periproct is large.
PERIPROCT (ANUS): Size: Large. Shape: Subpentagonal (Clark & Twitchell, 1915).
PERISTOME (MOUTH): Size: Small; about one-third as broad as the test (Cragin, 1893). Shape: Circular. Lip: Distinct branchial incisions (Clark & Twitchell, 1915); two narrow and deep emarginations on each interambulacral lip (Cragin, 1893).
SPINES: Unknown.
SPECIMENS STUDIED & CONDITION: One specimen (Hypotype) housed at the Non-vertebrate Paleo. Lab., University of Texas. Specimen is a fragment.
ETYMOLOGY
Named for the "Vola" limestone (an obsolete name for the Upper Washita beds, specifically Main Street/Grayson), from which the type was collected.
REMARKS & COMPARISONS
Remarks: Cragin (1893) noted the strong resemblance to Pseudodiadema (Loriolia) texanum in general form but distinguished P. volanum by its imperforate tubercles (versus the perforate tubercles of Loriolia).
Comparisons:
Remarks (01-2026): Cragin's type was 17.5 mm wide.
Plate: PhVo-10
Series: Upper Cretaceous Group: Washita Formation: Buda
Locale: County: Tarrant Stage: Cenomanian
Length: 22.9 mm Height: 9.0 mm
Specimen in photos: Hypotype
Repository: Non-vertebrate Paleo. Lab., Univ. of Texas Fragment
HOLOTYPE INFO:
Holotype: Originally described by Cragin (1893). Repository likely UT/BEG or USNM.
Locality: Big Elm Creek, below the Missouri, Kansas and Texas railway bridge, Denton County, Texas.
Stratigraphy: "Vola limestone" (Equivalent to Main Street/Grayson Formation), Washita Group.
SYNONYMY
- Cyphosoma volanum Cragin, 1893
SYSTEMATIC PALEONTOLOGY – TAXONOMY
Class: Echinoidea Leske, 1778
Subclass: Euechinoidea Bronn, 1860
Infraclass: Carinacea Kroh & Smith, 2010
Superorder: Calycina Gregory, 1900
Order: Phymosomatoida Mortensen, 1904
Family: Phymosomatidae Pomel, 1883
Genus: PHYMOSOMA Haime, 1853
Species: Phymosoma volanum (Cragin, 1893)
TYPE SPECIES: Diadema priscum Agassiz, 1840
DIAGNOSIS
Test small, subcircular to subpentagonal, and depressed. Ambulacra straight with sinuous pore zones. Interambulacra feature two rows of large primary tubercles and smaller lateral rows. Distinguished from P. texanum by being lower, more depressed, and having less fully defined outer tubercle rows (Clark & Twitchell, 1915).
STRATIGRAPHICAL OCCURRENCE & DISTRIBUTION
Group: Washita
Formations: Buda Limestone; Main Street Limestone; Grayson Marl ("Vola").
Stratigraphic Units: Upper Cretaceous, Cenomanian.
Counties: Tarrant, Denton.
Abundance: Uncommon.
DESCRIPTION
TOP: Length, width, height: 22.9 mm L × 9.0 mm H (Figured Specimen). Top shape: Subcircular to rounded-pentagonal (Cragin, 1893). Top domed: Slightly elevated upper surface.
SIDE: Ambitus: Moderately inflated. Side shape: Depressed profile; about three-fifths as high as broad (Cragin, 1893).
BOTTOM: Bottom shape: Depressed and concave.
AMB: Amb shape: Broad and slightly prominent. Amb arrangement: Two rows of primary tubercles (8 or 9 per row); tubercles decrease rapidly in size from ambitus to poles. Surrounded by narrow polygon-building strings of granules (Cragin, 1893).
PORE PAIRS: Arrangement: Uniserial from peristome nearly to apical system; crowded toward the apical system (Clark & Twitchell, 1915). Shape: Pores somewhat elongated in the direction of the belt (Cragin, 1893). Zones are sinuous.
TUBERCLES: Primary Perforated: No (Imperforate) (Cragin, 1893). Primary Crenulate: Yes. Interambulacral Tubercles: Two rows of large primary tubercles placed near the centers of the plates. A row of smaller cone-based primary tubercles is placed along either side of the area (close to the pore belt). Distribution: Numerous granules on the surface of the plates; not crowded but unevenly distributed (Cragin, 1893).
GRANULES: See Tubercles section.
FASCIOLE: Absent.
APICAL SYSTEM: Not fully described; Periproct is large.
PERIPROCT (ANUS): Size: Large. Shape: Subpentagonal (Clark & Twitchell, 1915).
PERISTOME (MOUTH): Size: Small; about one-third as broad as the test (Cragin, 1893). Shape: Circular. Lip: Distinct branchial incisions (Clark & Twitchell, 1915); two narrow and deep emarginations on each interambulacral lip (Cragin, 1893).
SPINES: Unknown.
SPECIMENS STUDIED & CONDITION: One specimen (Hypotype) housed at the Non-vertebrate Paleo. Lab., University of Texas. Specimen is a fragment.
ETYMOLOGY
Named for the "Vola" limestone (an obsolete name for the Upper Washita beds, specifically Main Street/Grayson), from which the type was collected.
REMARKS & COMPARISONS
Remarks: Cragin (1893) noted the strong resemblance to Pseudodiadema (Loriolia) texanum in general form but distinguished P. volanum by its imperforate tubercles (versus the perforate tubercles of Loriolia).
Comparisons:
- Phymosoma texanum: P. volanum is very closely related to P. texanum (Fredericksburg Group). It is generally lower, more depressed, more subpentagonal in outline, and the four rows of interambulacral tubercles are less fully defined (Clark & Twitchell, 1915). Clark suggested P. texanum might be the ancestor of P. volanum.
Remarks (01-2026): Cragin's type was 17.5 mm wide.
