Hemiaster bexari - not Hemiaster wetherbyi

[Bill Thompson]

I wanted to clear up the name of a Texas echinoid.

 

Hemiaster bexari   (Clark, 1915)

and

Hemiaster wetherbyi   de Loriol, 1887

 

They are not the same.

I studied type specimens of both at the Smithsonian in 2015 and 2016.

 

The following red & blue text is from my newly published book:  Fossil Echinoids of Texas   

 

Remark by Book Author:  This species  (Hemiaster wetherbyi)  can be separated from Hemiaster bexari because of the beads between the pore pairs in the unpaired amb.

 

Hemiaster bexari   (Clark, 1915)

Plate: 85

Series: Upper Cretaceous                   Group: Navarro                                   Formation: Corsicana

Locale: San Antonio                           County: Bexar

Length: 22.9 mm                                Width: 21.6 mm                                  Height: 15.6 mm

Specimen in photo:                             U.S.N.M. 8330                                    Holotype

Paleobiology Depart.                          NMNH, Smithsonian Institution

 

Description from:  Clark & Twitchell 1915  The Mesozoic and Cenozoic Echinodermata of the United States

Determinative characters:  Test small, upper surface nearly flat, elevated, lower surface rounded; sides inflated; apex nearly central, posterolateral ambulacra very short, interambulacra prominent. Peristome small, only slightly depressed, near anterior margin. Periproct small, situated high on truncated surface of posterior margin.

Description:  This small species has a broad, nearly flat upper surface, rising toward the center, which is slightly elevated. The lower surface is rounded, sloping from near the center in all directions. The sides are inflated.

The ambulacra are unequal, the posterolateral pair being very short. The poriferous zones are broad and shallow and but little depressed. The anterior ambulacrum is broad and shallow. The surface is covered with small tubercles. The peripetalous fasciole is distinct.

The peristome is small, only slightly depressed, and near the anterior margin. The periproct is small and situated high on the truncated surface of the posterior margin.

Geologic horizon:  Washita group of the Comanche series, Cretaceous.

 

Hemiaster wetherbyi   de Loriol, 1887

Plate:  88

Series: Upper Cretaceous                   Group: Navarro                                   Formation: Corsicana in Texas

Locale: San Antonio, Texas               County: Bexar, Texas

Length:  24.4 mm                               Width: 23.1 mm                                  Height: 17.4 mm

Specimen in photo:                             U.S.N.M. 108398                               Type

Paleobiology Depart.                          NMNH, Smithsonian Institution

 

Description from:  Cooke 1953  American Upper Cretaceous Echinoidea

Test small, plump, subovate; upper surface somewhat higher behind; lower surface inflated; margin rounded, truncated behind. Apical system behind the center; pores inconspicuous; madreporite large and tumid.

Paired petals wide, straight, closed,, moderately sunken; anterior pair twice as long as posterior pair; poriferous zones wider than interporiferous; pores elongate-oval to drop-shaped; pores of each zygopore connected by a raised thread. Anterior petal straight, open, moderately sunken, the depression barely indenting the margin; pores small, zygopores short, pores separated by a conspicuous bead. Peripetalous fasciole broad, conspicuous, barely indented between the petals.

Peristome anterior, one-third the total length from the margin, strongly labiate behind.

Periproct small, slightly elongated longitudinally; well up on the vertical posterior truncation. Tubercles large, close together, scrobiculate; larger and more deeply scrobiculate on lower surface. Sternum bordered by broad, straight rugose bands.

Geologic horizon:  Navarro group and equivalents (Maestrictian).

Comparisons:  This species appears to have been mistaken for Hemiaster stella (Morton), an Eocene species from New Jersey, by Morton (1834), D'Orbigny (1854-1860), and Clark (1915), but it is not closely related to that species, differing in shape, in the arrangement of its apical plates, and in other features. It is higher in front and lower behind than Hemiaster bufo as figured by D'Orbigny (1854-1860). It is easily distinguishable from Hemiaster arcolensis Cooke, which occurs in a lower zone, by its sunken tubercles.

 

Pictures of both types are in the book.

I hope this helps.

 

Sincerely;

Bill Thompson

 

[Ramon]

 

 

 

The most numerous fossil that I find on the Corsicana formation is Turritella Vertebroides.

 

 

Great find!!!!