​Goniopygus zitteli Clark, 1891
Plate: GoZi-10
Series: Lower Cretaceous                       Group: Fredericksburg                            Formation: Edwards
Locale: Austin                                         County: Travis                                      Stage: Albian
Length: 27.5 mm                                     Height: 16.3 mm
Specimen in photo:                                 Collection of Ed Elliott
HOLOTYPE INFO:
Holotype: USNM 12320 — National Museum of Natural History, Smithsonian Institution (Clark & Twitchell, 1915)
Locality: Round Rock, Williamson County, Texas (Clark, 1893)
Stratigraphy: Edwards Limestone, Fredericksburg Group (Clark & Twitchell, 1915)
SYNONYMY

• Goniopygus guadalupae (Roemer, 1852)

SYSTEMATIC PALEONTOLOGY – TAXONOMY
Class: Echinoidea Leske, 1778
Subclass: Euechinoidea Bronn, 1860
Infraclass: Carinacea Kroh & Smith, 2010
Superorder: Echinacea Claus, 1876
Order: Arbacioida Gregory, 1900
Family: Acropeltidae Lambert, 1911
Genus: GONIOPYGUS L. Agassiz, 1838
Species: Goniopygus zitteli Clark, 1891
TYPE SPECIES: Goniopygus major L. Agassiz, 1838
DIAGNOSIS
Test subconical, circular in outline; upper surface elevated; base flattened. Ambulacra broad, with a double row of alternating tubercles (18 to 20 in each series). Interambulacra broader than ambulacra, with a double row of primary tubercles (10 to 12 in each row). Apical system prominent, compact. Peristome large, with shallow incisions (Clark & Twitchell, 1915).
STRATIGRAPHICAL OCCURRENCE & DISTRIBUTION
Group: Fredericksburg; Washita
Formations: Edwards Limestone (Clark & Twitchell, 1915); Georgetown Formation
Stratigraphic Units: Lower Cretaceous, Albian
Counties: Williamson (Type); Travis, Pecos, Borden
DESCRIPTION
TOP: Shape: Circular (Clark & Twitchell, 1915)
Top domed: Upper surface very much elevated; subconical (Clark & Twitchell, 1915)
SIDE: Ambitus: 30% from flat base
Side shape: Sub-globulose
BOTTOM: Bottom flat: Flat and slightly depressed around the peristome (Clark & Twitchell, 1915)
AMB: Amb shape: Broad; increasing regularly in width from apical disk to ambitus, decreasing toward peristome (Clark & Twitchell, 1915)
Amb primary tubercles: Double row of 18 to 20 tubercles in each area; rapidly increase in size from apical system downward (Clark & Twitchell, 1915)
Amb primary zones: Poriferous zones arranged in a double row (bigeminate?); multiplied/increased at peristome (Clark & Twitchell, 1915). Uniserial.
INTERAMB: Interamb width: About 1.5 times the width of ambulacra (Clark & Twitchell, 1915)
Interamb primary tubercles: Double row of large tubercles, 10 to 12 in each series. Considerably larger than ambulacral tubercles (Clark & Twitchell, 1915).
PORE PAIRS: Shape: Round
Arrangement: Uniserial; Double row on each zone (Clark & Twitchell, 1915)
TUBERCLES: Primary Perforated: Imperforate (Clark & Twitchell, 1915)
Primary Crenulate: No, smooth bosses (Clark & Twitchell, 1915)
Primary Boss: Smooth (Clark & Twitchell, 1915); Scrobiculate
Primary Areola shape: Wide circular areolas (Clark & Twitchell, 1915)
Granules: Miliary space wide; nearly smooth in upper portions, sparsely covered with granules toward peristome (Clark & Twitchell, 1915).
APICAL SYSTEM: Size: Large, compact, prominent (Clark & Twitchell, 1915)
Shape: Star-shaped appearance due to elongated genitals (Clark & Twitchell, 1915)
Plates: Genital plates angular at outer extremity; inner edge has semicircular depression containing a small mammillated tubercle (Clark & Twitchell, 1915).
PERIPROCT (ANUS): Shape & Size: Oval (Adult); Lemon shaped (Juvenile)
Structure: Sits in triangular depression with a platform in 3 corners with a granule
PERISTOME (MOUTH): Size: Large; covering quite 1/2 diameter of base (Clark & Twitchell, 1915)
Lip: Divided into ten lobes by ten shallow incisions (Clark & Twitchell, 1915)
SPINES: Shape: Paddle shaped, with two sides flatter. On paddle side is a depressed large cup-like indentation. Widest spot at 30% from end. Tip end bends downward.
Ornamentation: Linear serrations run from collar for entire shaft to rounded tip.
SPECIMENS STUDIED & CONDITION   Many juveniles and adult specimens studied.
ETYMOLOGY:
Named in honor of Karl Alfred von Zittel (1839–1904), a prominent German paleontologist (Clark, 1891).
REMARKS & COMPARISONS
Remarks: Ontogenetic differences are significant; juveniles have a taller ambitus and relatively larger apical system . Cooke (1946) suggested Goniopygus saleniaformis and Goniopygus texanus are likely immature variants of G. zitteli.
Comparisons:

• Echinus patagonensis (D'Orbigny): Similar, but margin is sharper and apical system differs.
• Goniopygus major: Similar, but G. zitteli has increased pores around the peristome and a more compact apical system (Clark & Twitchell, 1915).

Remarks: Vadet, A. & Nicolleau, P., 2020 in Revision des Goniopygidae is proposing a method to classify and group Goniopygus species based on a specific biometric character: the size of the secondary tubercles in the ambulacral zone at the ambitus (equator). They lump all the Texas Albian/Cenomanian as (Cyphopygus zitteli) and that all others: (G. whitneyi, G. quartus, G. zitteli, G. texanus, G. saleniaformis, G. goodlandensis)  are just growth stages They all possess small secondary ambulacral tubercles, suggesting that spine morphology and secondary tubercle size are the true diagnostic features for separating these echinoids, rather than the minor differences in shape or pore arrangement often used by earlier authors (like Ikins or Whitney) to name new species.
Remarks (12-2025): I cannot follow renaming to Cyphopygus. Pomel new genus Cyphogygus possessed five perianal tubercles and Goniopygus as having only three or four. Lambert and Mortensen rejected Pomel's genus. Taxonomists concluded that counting these tubercles was not a reliable way to define a whole new genus.  I continue to treat Cyphopygus as a subjective junior synonym of Goniopygus.
Remarks (12-2025): Clark & Twitchell state "double row on each poriferous zone" (likely implying bigeminate, common in Arbacioida).