Rachiosoma hondoense (Cooke, 1953)
Plate: RaHo-10
Series: Upper Cretaceous Group: Navarro Formation: Escondido
Locale: Elston Biry Road County: Medina Stage: Maastrichtian
Length: 39.0 mm Height: 17.7 mm
Specimen in photos: U.S.N.M. 108371 Holotype
Collector: USGS SURVEY
Repository: Paleobiology Dept., NMNH, Smithsonian Institution
Plate: RaHo-10
Series: Upper Cretaceous Group: Navarro Formation: Corsicana
Locale: San Antonio, Hwy 90 W County: Bexar Stage: Maastrichtian
Length: 20.0 mm Height: 8.0 mm
Specimen in photos: Collector: Dan Woehr
Repository: Private Collection
HOLOTYPE INFO:
Holotype: USNM 108371 (Cooke, 1953).
Locality: USGS 15555, bed of Hondo Creek, 1.25 miles south of Elston Biry School, Medina County, Texas.
Stratigraphy: Escondido Formation.
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: Rachiosomatidae Smith & Wright, 1993
Genus: RACHIOSOMA Pomel, 1883
Species: Rachiosoma hondoense Cooke, 1953
TYPE SPECIES: Cidaris delamarrei Desor, 1856
DIAGNOSIS
Test low hemispherical. Ambulacra are wide and polyporous (5 pore pairs per plate). Tubercles are imperforate and arranged in two rows in the ambulacra and two rows in the interambulacra (bordered by smaller tubercles). Apical system scar is large (Cooke, 1953).
STRATIGRAPHICAL OCCURRENCE & DISTRIBUTION
Group: Navarro
Formations: Escondido Formation.
Stratigraphic Units: Upper Cretaceous, Maastrichtian.
Counties: Medina.
Abundance: Rare; known only from the holotype.
DESCRIPTION
TOP: Length, width, height: 39.0 mm L × 17.7 mm H. Top domed: Low hemispherical.
AMB: Amb shape: Wide. Amb arrangement: Decorated with two rows of rather large primary tubercles. Areoles are tangent below the ambitus but separated above it. Partial rings of small tubercles encircle the primaries separating the rows, but are absent elsewhere below the ambitus (Cooke, 1953).
PORE PAIRS: Arrangement: Polyporous; zygopores arranged in slightly curved or broken arcs. Elements: Apparently five to each plate (quinqueminate).
TUBERCLES: Primary Perforated: No (Imperforate).
Primary Crenulate: Cooke describes them as "smooth," which implies non-crenulate, but the genus Rachiosoma is defined by crenulate tubercles.
Interambulacral Tubercles: Two rows of primary tubercles bordered on each side by a row of smaller tubercles and separated by large granules.
GRANULES: Distribution: Large granules separate the interambulacral tubercles.
FASCIOLE: Absent.
APICAL SYSTEM: Size: Large apical scar (distinguishing it from Porosoma). Shape: Probably pentagonal.
PERIPROCT (ANUS): (Unknown).
PERISTOME (MOUTH): Size: Less than half as wide as the horizontal diameter. Shape: Round. Lip: Strongly notched.
SPINES: Unknown.
SPECIMENS STUDIED & CONDITION: Holotype USNM 108371. The specimen has lost most of the upper side.
SIZE: Range 12.5 mm – 20.0 mm
ETYMOLOGY
Named for Hondo Creek (or the town of Hondo) in Medina County, near the type locality.
REMARKS & COMPARISONS
Remarks: Cooke (1953) referred this species to Rachiosoma rather than Porosoma due to the greater size of its apical scar.
Comparisons:
Remarks: (02-2026) Updated name to match the Neuter gender of Rachiosoma. Studied 2 specimens in Dan Woehr’s Collection and 2 specimens in Kevin Durney’s Collection.
Plate: RaHo-10
Series: Upper Cretaceous Group: Navarro Formation: Escondido
Locale: Elston Biry Road County: Medina Stage: Maastrichtian
Length: 39.0 mm Height: 17.7 mm
Specimen in photos: U.S.N.M. 108371 Holotype
Collector: USGS SURVEY
Repository: Paleobiology Dept., NMNH, Smithsonian Institution
Plate: RaHo-10
Series: Upper Cretaceous Group: Navarro Formation: Corsicana
Locale: San Antonio, Hwy 90 W County: Bexar Stage: Maastrichtian
Length: 20.0 mm Height: 8.0 mm
Specimen in photos: Collector: Dan Woehr
Repository: Private Collection
HOLOTYPE INFO:
Holotype: USNM 108371 (Cooke, 1953).
Locality: USGS 15555, bed of Hondo Creek, 1.25 miles south of Elston Biry School, Medina County, Texas.
Stratigraphy: Escondido Formation.
SYNONYMY
- Rachiosoma hondoensis Cooke, 1953
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: Rachiosomatidae Smith & Wright, 1993
Genus: RACHIOSOMA Pomel, 1883
Species: Rachiosoma hondoense Cooke, 1953
TYPE SPECIES: Cidaris delamarrei Desor, 1856
DIAGNOSIS
Test low hemispherical. Ambulacra are wide and polyporous (5 pore pairs per plate). Tubercles are imperforate and arranged in two rows in the ambulacra and two rows in the interambulacra (bordered by smaller tubercles). Apical system scar is large (Cooke, 1953).
STRATIGRAPHICAL OCCURRENCE & DISTRIBUTION
Group: Navarro
Formations: Escondido Formation.
Stratigraphic Units: Upper Cretaceous, Maastrichtian.
Counties: Medina.
Abundance: Rare; known only from the holotype.
DESCRIPTION
TOP: Length, width, height: 39.0 mm L × 17.7 mm H. Top domed: Low hemispherical.
AMB: Amb shape: Wide. Amb arrangement: Decorated with two rows of rather large primary tubercles. Areoles are tangent below the ambitus but separated above it. Partial rings of small tubercles encircle the primaries separating the rows, but are absent elsewhere below the ambitus (Cooke, 1953).
PORE PAIRS: Arrangement: Polyporous; zygopores arranged in slightly curved or broken arcs. Elements: Apparently five to each plate (quinqueminate).
TUBERCLES: Primary Perforated: No (Imperforate).
Primary Crenulate: Cooke describes them as "smooth," which implies non-crenulate, but the genus Rachiosoma is defined by crenulate tubercles.
Interambulacral Tubercles: Two rows of primary tubercles bordered on each side by a row of smaller tubercles and separated by large granules.
GRANULES: Distribution: Large granules separate the interambulacral tubercles.
FASCIOLE: Absent.
APICAL SYSTEM: Size: Large apical scar (distinguishing it from Porosoma). Shape: Probably pentagonal.
PERIPROCT (ANUS): (Unknown).
PERISTOME (MOUTH): Size: Less than half as wide as the horizontal diameter. Shape: Round. Lip: Strongly notched.
SPINES: Unknown.
SPECIMENS STUDIED & CONDITION: Holotype USNM 108371. The specimen has lost most of the upper side.
SIZE: Range 12.5 mm – 20.0 mm
ETYMOLOGY
Named for Hondo Creek (or the town of Hondo) in Medina County, near the type locality.
REMARKS & COMPARISONS
Remarks: Cooke (1953) referred this species to Rachiosoma rather than Porosoma due to the greater size of its apical scar.
Comparisons:
- Rachiosoma mortoni: R. hondoense appears to be a taller, less disc-shaped species than R. mortoni (de Loriol) and grew much larger than the few known specimens of that species (Cooke, 1953).
- Miocyphosoma bexarense: R. hondoense resembles M. bexarense but is distinguished by having 5 elements (quinqueminate) throughout, whereas M. bexarense is trigeminate adorally.
Remarks: (02-2026) Updated name to match the Neuter gender of Rachiosoma. Studied 2 specimens in Dan Woehr’s Collection and 2 specimens in Kevin Durney’s Collection.
