Coenholectypus planatus (Roemer, 1849)

Remarks

• Collector:       Keith Minor

• Formation:     Fredericksburg Group, Walnut Formation

• Location:       Wise County, TX

• Dimensions for echinoid 1 (left):     diam = 34 mm, ht = 20 mm

• Dimensions for echinoid 2 (right):   diam = 19 mm, ht = 11 mm

• The two echinoids above are illustrative of the variation with this species.  The one in the far right image is a young echinoid that has very well preserved genital pores.  This pentaperforate arrangement places this species in the genus Coenholectypus instead of Holectypus.

• The echinoid in the far left image is older.  Notice that the apical system is a very small disk.  We've seen this on many mature specimens in our collection.  Both phenotypes shown above are figured in Clark & Twitchell (1915, see ref below).

• Clark & Twitchell also discuss the variable nature of this species and the consequent oversplitting of this echinoid.

• We want to bring your attention to the top ambulacrum on the left specimen.  Notice a conspicuous constriction on it.  It's symmetrical, and almost looks like a diagnostic feature.  However, we've never seen this on any other specimen in our collection nor on any figured in the literature.  A couple of echinoid collectors suggest that this may be a pathological feature, but it's anybody's guess as to what caused this apparent stunt in growth.  Apparently, the echinoid made it to adulthood just fine.

General Info

Synonyms (older names and misidentifications in the literature)

• 1849 Holectypus planatus Roemer

• 1852 Holectypus planatus Roemer; Roemer

• 1852 Holectypus planatus Roemer; Shumard

• 1853 Holectypus planatus Roemer; Giebel

• 1853 Holectypus planus Roemer; Giebel

• 1857 Holectypus planatus Roemer; Conrad

• 1858 Holectypus planatus Roemer; Desor

• 1859 Holectypus planatus Roemer; Gabb

• 1864 Holectypus planatus Roemer; Meek

• 1888 Holectypus sp. Roemer

• 1891 Holectypus planatus Roemer; Clark

• 1893 Holectypus planatus Roemer; Clark

• 1893 Holectypus charltoni Cragin

• 1893 Holectypus planatus Roemer; Cragin

• 1894 Holectypus charltoni Cragin; Cragin

• 1901 Holectypus planatus Roemer; Hill

• 1915 Holectypus planatus (Roemer); Clark & Twitchell

• 1927 Holectypus planatus (Roemer); Lambert

• 1928 Holectypus planatus (Roemer); Adkins

• 1947 Holectypus planatus (Roemer); Shimer & Shrock

• 1987 Coenholectypus planatus (Roemer); Akers & Akers

Stratigraphic Occurrence

• The report from the Washita group is most likely a different species.

• Fredericksburg Group, Goodland Formation of north Texas

• Fredericksburg Group, Walnut Formation of north and central Texas

• Fredericksburg Group, Finlay Formation (?) of Trans-Pecos Texas

• Trinity Group, Glen Rose Formation of north and central Texas

Age

• Coenholectypus planatus (Roemer) ranges from about the Hypacanthoplites comalensis (Scott) zone to the Oxytropidoceras (Oxytropidoceras) powelli (Young) zone, latest Early Albian Stage to earliest Late Albian Stage (~108–103 mya).

Geographic Occurrence (by no means an all-inclusive list)

• Bexar County, TX

• Denton County, TX

• Hudspeth County, TX

• Tarrant County, TX

• Travis County, TX

• Wise County, TX

Remarks

         For identification and descriptions, see:

• Clark, W. B.; Twitchell, M. W.  1915.  Mesozoic and Cenozoic Echinodermata of the United States.  United States Geological Survey Monograph 54, 65–66; plate XXV, figs 2a–f, 3a–c, and 4; plate XXVI, figs 1a–e.