Mecaster austinensis Thompson, 2020
Plate: MeAu-10
Series: Upper Cretaceous Group: Austin Formation: Austin Chalk
Locale: Austin County: Travis Stage: Santonian
Length: 29.5 mm Width: 28.4 mm Height: 21.3 mm
Collector: William Thompson
Specimen in Photo: NPL 94343
Repository: Non-vertebrate Paleo. Lab., Univ. of Texas
HOLOTYPE INFO:
Holotype: NPL 94343
Paratypes: NPL 94344, NPL 94345
Locality: Austin, Travis County, Texas.
Stratigraphy: Austin Chalk, Austin Group.
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: Hemiasteridae H. L. Clark, 1917
Genus: MECASTER Pomel, 1883
Species: Mecaster austinensis Thompson, 2020
TYPE SPECIES: Hemiaster fourneli Agassiz & Desor, 1847
DIAGNOSIS
A small Mecaster from the Austin Chalk characterized by a less pronounced oral feeding system (phyllodes/food grooves) compared to M. texanus. The widest part of the test is positioned distinctly anterior of the midpoint.
STRATIGRAPHICAL OCCURRENCE & DISTRIBUTION
Group: Austin
Formations: Austin Chalk.
Stratigraphic Units: Upper Cretaceous, Santonian.
Counties: Travis, Hays, Maverick.
Abundance: 15 specimens studied.
Associated Fauna: Found associated with Phymosoma bybeei and Baculites sp.
DESCRIPTION
TOP: Length, width, height: 29.5 mm L × 28.4 mm W × 21.3 mm H.
Top shape: Heart-shaped with broad sides and a broad base. Deep anterior sulcus present. Widest part is located at the anterior third (1/3 from anterior edge).
Top domed: Apical system central.
SIDE: Ambitus: Positioned below the midpoint; lowest on the posterior end.
Side shape: Inflated with sides somewhat angular. Posterior end truncated.
Profile: Apex positioned posterior of center (1/3 from posterior end).
BOTTOM: Bottom shape: Flat.
Plastron: Prominentrominent.
AMB: Amb shape: Paired petals are deeply sunken and open-ended.
Amb arrangement: Anterior paired petals are straight and longer than the posterior pair. Posterior paired petals are deeply sunken, downwardly flexed, and shorter than the anterior pair.
Unpaired Amb: Anterior ambulacrum (III) is broad, lanceolate, and extends to the peristome.
PORE PAIRS: Shape: Pores in paired petals are elongated and conjugate.
Unpaired Amb Pores: Round, close together, and separated by a small bead.
TUBERCLES: Distribution: Tubercles on the anterior surface are surrounded by fine granulation.
Size Comparison: Tubercles on the adoral surface are described as larger than those on the aboral surface.
GRANULES: Distribution: Fine granulation surrounds anterior tubercles.
FASCIOLE: Peripetalous: Present.
APICAL SYSTEM: Location: Central.
Type: Ethmolytic.
Genital pores: Four gonopores.
Plating: Large madreporite.
PERIPROCT (ANUS): Shape: Taller than wide with rounded ends.
Location: Situated high on the truncated posterior end.
Visibility: Visible from above.
PERISTOME (MOUTH): Shape: Reniform ("D" shaped).
Location: Located 20% of test length from the anterior edge.
Lip: Downward facing with a protruding lip.
Features: Shallow food tracks (feeding system) lead to the peristome. Easily seen from the anterior end.
SPINES: Unknown.
SPECIMENS STUDIED & CONDITION: Fifteen specimens found in Austin, Texas. Holotype NPL 94343; Paratypes NPL 94344 and NPL 94345. Specimen from Maverick County (Kevin Durney collection) also noted.
ETYMOLOGY
Named after the Austin Chalk Formation, the stratigraphic unit where the species is found.
REMARKS & COMPARISONS
Remarks: Cooke (1953) lumped specimens from the Anacacho Limestone, Taylor Marl, and Austin Chalk under Hemiaster (Mecaster) texanus, noting that Austin Chalk specimens were often poorly preserved. Examination of well-preserved material confirms M. austinensis is distinct from M. texanus.
Comparisons:
Plate: MeAu-10
Series: Upper Cretaceous Group: Austin Formation: Austin Chalk
Locale: Austin County: Travis Stage: Santonian
Length: 29.5 mm Width: 28.4 mm Height: 21.3 mm
Collector: William Thompson
Specimen in Photo: NPL 94343
Repository: Non-vertebrate Paleo. Lab., Univ. of Texas
HOLOTYPE INFO:
Holotype: NPL 94343
Paratypes: NPL 94344, NPL 94345
Locality: Austin, Travis County, Texas.
Stratigraphy: Austin Chalk, Austin Group.
SYNONYMY
- Mecaster texanus (Akers & Akers, 1987; p. 107) (partim)
- Hemiaster (Mecaster) texanus (Cooke, 1953) (partim)
- Mecaster texanus (Morgan, 2016; p. 138-139) (partim)
- Mecaster texanus (Thompson, 2016; p. 134-135) (partim)
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: Hemiasteridae H. L. Clark, 1917
Genus: MECASTER Pomel, 1883
Species: Mecaster austinensis Thompson, 2020
TYPE SPECIES: Hemiaster fourneli Agassiz & Desor, 1847
DIAGNOSIS
A small Mecaster from the Austin Chalk characterized by a less pronounced oral feeding system (phyllodes/food grooves) compared to M. texanus. The widest part of the test is positioned distinctly anterior of the midpoint.
STRATIGRAPHICAL OCCURRENCE & DISTRIBUTION
Group: Austin
Formations: Austin Chalk.
Stratigraphic Units: Upper Cretaceous, Santonian.
Counties: Travis, Hays, Maverick.
Abundance: 15 specimens studied.
Associated Fauna: Found associated with Phymosoma bybeei and Baculites sp.
DESCRIPTION
TOP: Length, width, height: 29.5 mm L × 28.4 mm W × 21.3 mm H.
Top shape: Heart-shaped with broad sides and a broad base. Deep anterior sulcus present. Widest part is located at the anterior third (1/3 from anterior edge).
Top domed: Apical system central.
SIDE: Ambitus: Positioned below the midpoint; lowest on the posterior end.
Side shape: Inflated with sides somewhat angular. Posterior end truncated.
Profile: Apex positioned posterior of center (1/3 from posterior end).
BOTTOM: Bottom shape: Flat.
Plastron: Prominentrominent.
AMB: Amb shape: Paired petals are deeply sunken and open-ended.
Amb arrangement: Anterior paired petals are straight and longer than the posterior pair. Posterior paired petals are deeply sunken, downwardly flexed, and shorter than the anterior pair.
Unpaired Amb: Anterior ambulacrum (III) is broad, lanceolate, and extends to the peristome.
PORE PAIRS: Shape: Pores in paired petals are elongated and conjugate.
Unpaired Amb Pores: Round, close together, and separated by a small bead.
TUBERCLES: Distribution: Tubercles on the anterior surface are surrounded by fine granulation.
Size Comparison: Tubercles on the adoral surface are described as larger than those on the aboral surface.
GRANULES: Distribution: Fine granulation surrounds anterior tubercles.
FASCIOLE: Peripetalous: Present.
APICAL SYSTEM: Location: Central.
Type: Ethmolytic.
Genital pores: Four gonopores.
Plating: Large madreporite.
PERIPROCT (ANUS): Shape: Taller than wide with rounded ends.
Location: Situated high on the truncated posterior end.
Visibility: Visible from above.
PERISTOME (MOUTH): Shape: Reniform ("D" shaped).
Location: Located 20% of test length from the anterior edge.
Lip: Downward facing with a protruding lip.
Features: Shallow food tracks (feeding system) lead to the peristome. Easily seen from the anterior end.
SPINES: Unknown.
SPECIMENS STUDIED & CONDITION: Fifteen specimens found in Austin, Texas. Holotype NPL 94343; Paratypes NPL 94344 and NPL 94345. Specimen from Maverick County (Kevin Durney collection) also noted.
ETYMOLOGY
Named after the Austin Chalk Formation, the stratigraphic unit where the species is found.
REMARKS & COMPARISONS
Remarks: Cooke (1953) lumped specimens from the Anacacho Limestone, Taylor Marl, and Austin Chalk under Hemiaster (Mecaster) texanus, noting that Austin Chalk specimens were often poorly preserved. Examination of well-preserved material confirms M. austinensis is distinct from M. texanus.
Comparisons:
- Mecaster texanus: M. austinensis differs by having its widest point further removed from the midpoint (anterior), whereas M. texanus is widest slightly above the midpoint. The petals of M. austinensis are not as broad or deeply sunken as those of M. texanus. The posterior truncation below the periproct is less depressed in M. austinensis. Additionally, the peristome in M. texanus is very deeply sunken with a sunken lip, whereas in M. austinensis, the peristome is only slightly sunken with the lip nearly flush with the plastron.
