Periarchus lyelli (Conrad, 1866)
Plate: PeLy-10
Series: Upper Eocene Group: Jackson Formation: Caddell in Texas
Locale: Broaddus in Texas County: San Augustine in Texas Stage: Priabonian
Length: 27.2mm Width: 26.1 mm Height: 5.3 mm
Specimen in photos: NPL 13491 Plates only.
Collector: (Not specified)
Repository: Non-vertebrate Paleo. Lab., Univ. of Texas
HOLOTYPE INFO:
Holotype: Originally described as Scutella lyelli by Conrad (1834). Type material collection status uncertain (likely ANSP).
Locality: Claiborne, Alabama.
Stratigraphy: "Nummulitic limestone" (Moodys Branch Formation / Gosport Sand equivalent).
SYNONYMY
SYSTEMATIC PALEONTOLOGY – TAXONOMY
Class: Echinoidea Leske, 1778
Subclass: Euechinoidea Bronn, 1860
Infraclass: Irregularia Latreille, 1825
Superorder: Neognathostomata Smith, 1981
Order: Clypeasteroida A. Agassiz, 1872
Family: Periarchidae Lambert, 1920
Genus: PERIARCHUS Conrad, 1866
Species: Periarchus lyelli (Conrad, 1834)
TYPE SPECIES: Spatangus altus Conrad, 1865 (=Periarchus altus)
DIAGNOSIS
Test subcircular to subovate with a greatly depressed, flat marginal area and a central, gently rounded, convex mound (tumid area). Ambulacral petals extend about halfway to the margin. Periproct is inframarginal. Food grooves fork symmetrically about halfway to the margin (Clark & Twitchell, 1915).
STRATIGRAPHICAL OCCURRENCE & DISTRIBUTION
Group: Jackson (Texas); Claiborne/Jackson (Alabama).
Formations: Caddell Formation (Texas); Moodys Branch Formation (Mississippi/Alabama); Castle Hayne Limestone (NC).
Stratigraphic Units: Paleogene, Upper Eocene (Priabonian).
Counties: San Augustine (Texas).
Abundance: One of the most abundant American Cenozoic echinoids; an index fossil for the Jacksonian stage.
DESCRIPTION
TOP: Length, width, height: 27.2 mm L × 26.1 mm W × 5.3 mm H (Note: Measurements likely from juvenile or NC specimen; adults reach 75mm+). Top shape: Variable; circular, subcircular, or subovate. Top domed: Greatly depressed and flat in the marginal area; rises centrally into a gently rounded mound with a flattened summit. The mound height varies with age.
SIDE: Side shape: Margin thin and slightly undulating, especially posteriorly. Profile: Domed center with flat brim.
: Bottom shape: Flat or nearly so; broad faintly depressed ring about midway between margin and peristome.
AMB: Amb shape: Narrow in petaloid region, wide at margin. Petals subelliptical to suboblong, slightly open at ends, extending about half the way to the margin. Amb arrangement: Subequal in length; odd petal (III) slightly longer.
PORE PAIRS: Arrangement: Inner rows straight and parallel; outer rows slit-like. Pairs conjugated. A few pairs occur beyond the ends of the petals.
TUBERCLES: Distribution: Whole surface covered with small uniform tubercles, slightly larger on the underside.
GRANULES: See Tubercles.
FASCIOLE: Absent.
APICAL SYSTEM: Location: Central or slightly eccentric anteriorly, situated on the flattened summit of the tumid mound. Plating: Madreporite is relatively large and flush. Genital pores: 5 genital pores near the edge of the madreporite.
PERIPROCT (ANUS): Shape: Circular or subcircular; very small (half diameter of peristome). Location: Oral surface (inframarginal); situated 1/3 to 1/2 the distance from the peristome to the posterior border.
PERISTOME (MOUTH): Shape: Subcircular. Size: Small. Location: Central or slightly anterior. Features: Ambulacral furrows are simple and straight for 1/2 to 2/3 distance to margin, then fork symmetrically at ~45°. Side branches are given off at right angles.
SPINES: Unknown.
SPECIMENS STUDIED & CONDITION:
ETYMOLOGY
Named in honor of Sir Charles Lyell (1797–1875), the pioneering British geologist who established the Tertiary epoch divisions (Eocene, Miocene, etc.).
REMARKS & COMPARISONS
Remarks: The text provided matches the description in Clark & Twitchell (1915) verbatim, though attributed to Cooke (1942) in the input. Periarchus lyelli is the "Guide Fossil" for the Late Eocene (Jacksonian) of the Gulf Coast.
Comparisons:
Plate: PeLy-10
Series: Upper Eocene Group: Jackson Formation: Caddell in Texas
Locale: Broaddus in Texas County: San Augustine in Texas Stage: Priabonian
Length: 27.2mm Width: 26.1 mm Height: 5.3 mm
Specimen in photos: NPL 13491 Plates only.
Collector: (Not specified)
Repository: Non-vertebrate Paleo. Lab., Univ. of Texas
HOLOTYPE INFO:
Holotype: Originally described as Scutella lyelli by Conrad (1834). Type material collection status uncertain (likely ANSP).
Locality: Claiborne, Alabama.
Stratigraphy: "Nummulitic limestone" (Moodys Branch Formation / Gosport Sand equivalent).
SYNONYMY
- Scutella lyelli Conrad, 1834
- Periarchus lyelli (Conrad) Conrad, 1866
SYSTEMATIC PALEONTOLOGY – TAXONOMY
Class: Echinoidea Leske, 1778
Subclass: Euechinoidea Bronn, 1860
Infraclass: Irregularia Latreille, 1825
Superorder: Neognathostomata Smith, 1981
Order: Clypeasteroida A. Agassiz, 1872
Family: Periarchidae Lambert, 1920
Genus: PERIARCHUS Conrad, 1866
Species: Periarchus lyelli (Conrad, 1834)
TYPE SPECIES: Spatangus altus Conrad, 1865 (=Periarchus altus)
DIAGNOSIS
Test subcircular to subovate with a greatly depressed, flat marginal area and a central, gently rounded, convex mound (tumid area). Ambulacral petals extend about halfway to the margin. Periproct is inframarginal. Food grooves fork symmetrically about halfway to the margin (Clark & Twitchell, 1915).
STRATIGRAPHICAL OCCURRENCE & DISTRIBUTION
Group: Jackson (Texas); Claiborne/Jackson (Alabama).
Formations: Caddell Formation (Texas); Moodys Branch Formation (Mississippi/Alabama); Castle Hayne Limestone (NC).
Stratigraphic Units: Paleogene, Upper Eocene (Priabonian).
Counties: San Augustine (Texas).
Abundance: One of the most abundant American Cenozoic echinoids; an index fossil for the Jacksonian stage.
DESCRIPTION
TOP: Length, width, height: 27.2 mm L × 26.1 mm W × 5.3 mm H (Note: Measurements likely from juvenile or NC specimen; adults reach 75mm+). Top shape: Variable; circular, subcircular, or subovate. Top domed: Greatly depressed and flat in the marginal area; rises centrally into a gently rounded mound with a flattened summit. The mound height varies with age.
SIDE: Side shape: Margin thin and slightly undulating, especially posteriorly. Profile: Domed center with flat brim.
: Bottom shape: Flat or nearly so; broad faintly depressed ring about midway between margin and peristome.
AMB: Amb shape: Narrow in petaloid region, wide at margin. Petals subelliptical to suboblong, slightly open at ends, extending about half the way to the margin. Amb arrangement: Subequal in length; odd petal (III) slightly longer.
PORE PAIRS: Arrangement: Inner rows straight and parallel; outer rows slit-like. Pairs conjugated. A few pairs occur beyond the ends of the petals.
TUBERCLES: Distribution: Whole surface covered with small uniform tubercles, slightly larger on the underside.
GRANULES: See Tubercles.
FASCIOLE: Absent.
APICAL SYSTEM: Location: Central or slightly eccentric anteriorly, situated on the flattened summit of the tumid mound. Plating: Madreporite is relatively large and flush. Genital pores: 5 genital pores near the edge of the madreporite.
PERIPROCT (ANUS): Shape: Circular or subcircular; very small (half diameter of peristome). Location: Oral surface (inframarginal); situated 1/3 to 1/2 the distance from the peristome to the posterior border.
PERISTOME (MOUTH): Shape: Subcircular. Size: Small. Location: Central or slightly anterior. Features: Ambulacral furrows are simple and straight for 1/2 to 2/3 distance to margin, then fork symmetrically at ~45°. Side branches are given off at right angles.
SPINES: Unknown.
SPECIMENS STUDIED & CONDITION:
- NPL 13491: Fragment (plates only) from Broaddus, Texas.
- Reference Specimen: Complete specimen from North Carolina (likely Castle Hayne Fm) shown in photos for morphology.
ETYMOLOGY
Named in honor of Sir Charles Lyell (1797–1875), the pioneering British geologist who established the Tertiary epoch divisions (Eocene, Miocene, etc.).
REMARKS & COMPARISONS
Remarks: The text provided matches the description in Clark & Twitchell (1915) verbatim, though attributed to Cooke (1942) in the input. Periarchus lyelli is the "Guide Fossil" for the Late Eocene (Jacksonian) of the Gulf Coast.
Comparisons:
- Periarchus pileussinensis: P. lyelli has a lower, more rounded central mound compared to the higher, conical mound of P. pileussinensis.
- Periarchus protuberans: P. lyelli lacks the swollen anterior interporiferous area characteristic of P. protuberans.
- Scutella tuomeyi: P. lyelli differs by not being posteriorly truncated and having the periproct further from the posterior border.
