The Lower Cretaceous 

Ammonite Schloenbachia 

leonensis Conrad var. 

equidistans Cragin

U.S. GEOLOGICAL SURVEY BULLETIN 1641-A

The Lower Cretaceous 

Ammonite Schloenbachla 

leonensis Conrad var. 

equidistans Crag in

By William A. Cobban

Description and illustrations of an 

important zonal fossil from Texas

Contributions to Paleontology

U.S. GEOLOGICAL SURVEY BULLETIN 1641-A
DEPARTMENT OF THE INTERIOR 

WILLIAM P. CLARK, Secretary

U.S. GEOLOGICAL SURVEY 

Dallas L. Peck, Director

Library of Congress Cataloging in Publication Data

Cobban, William A.

The Lower Cretaceous ammonite Schloenbachia leonensis Conrad var. equidistans Cragin. 

(Geological Survey bulletin ; 1641-A) 

Includes bibliographical references. 

Supt of Docs no.: I 19.3:1641-A

1. Schloenbachia leonensis. 2. Paleontology Cretaceous. 3. Paleontology Texas. I. Title. II. Series. 

QE75.B9 no. 1641 [QE807.A5] 557.3 s [564'.53] 84-600321

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON: 1985

For sale by the U.S. Geological Survey 

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CONTENTS

Abstract 1 

Introduction 1 

Systematic paleontology 

References cited 3

PLATE

1. Mortoniceras equidistans.

FIGURES

1. Whorl section of Mortoniceras equidistans (Cragin) 2

2. External suture of Mortoniceras equidistans (Cragin) 3

Contents III

CONTRIBUTIONS TO PALEONTOLOGY

The Lower Cretaceous Ammonite Schloenbachia 

leonensis Conrad var. equidistans Cragin

By William A. Cobban

Abstract

Schloenbachia leonensis var. equidistans Cragin was 

based on one or more specimens from the Duck Creek Formation of Texas. Cragin briefly described the species, but did not 

illustrate it, and the whereabouts of his type or types is unknown. Only one specimen in the Cragin collections at Colorado 

College, Colorado Springs, Colo., fits Cragin's description, and 

this specimen is assumed to be the type. Cragin's variety has 

been elevated to species rank and usually assigned to Pervinquieria or Mortoniceras. The species is widely distributed in 

Texas and New Mexico, where it is a guide to one of the upper 

Albian ammonite zones.

INTRODUCTION

The holotype of Mortoniceras equidistans (Cragin), 

an important guide fossil to one of the late Albian ammonite zones of Texas, was never illustrated or adequately 

described. Cragin (1893, p. 241) merely named it as a 

variety of Schloenbachia leonensis (Conrad, 1857, p. 160, 

pi. 16, figs. 2a,b). Cragin briefly described his new form 

as having "the general proportions of the typical leonensis, but the ribs bear three coarse, equidistantly placed 

nodes: umbilical, intermediate, and ventro-lateral, of 

which the last-named is particularly large and prominent 

and, with the corresponding nodes of other ribs, gives 

to the venter a decidedly squarish, instead of the ordinary 

rounded appearance." Young (1967a, p. 67-69; 1967b, 

p. 27) raised Cragin's variety to species rank and assigned 

it to the genus Pervinquieria. As noted by Wright (1957, 

p. L406), Mortoniceras Meek (1876, p. 448) is a valid 

genus, and Pervinquieria Bohm (1910, p. 152) is a 

synonym.

In north and north-central Texas, Mortoniceras 

equidistans occurs in the lower part of the Duck Creek 

Formation. The species first appears with the ammonite 

Eopachydiscus marcianus (Shumard), but ranges higher 

than that form. Young (1967a, p. 67-69; 1967b, p. 27) 

noted this overlap, but recognized two zones, a lower of 

Eopachydiscus brazosensis [sic] and an upper of Pervinquieria equidistans. Eopachydiscus brazoensis (Shumard, 

1860) is a synonym of E. marcianus (Shumard, 1854).

Young (1967a, p. 67-69) recorded M. equidistans 

(as Pervinquieria equidistans) as a zonal index for many 

areas in Texas (south-central and north-central Texas, 

north Texas, High Plains, upper Concho and Llano river 

drainages, and Fort Stockton area). In addition to these, 

M. equidistans has been recorded from Trans-Pecos Texas 

in the Kent area (Brand and DeFord, 1958, p. 381) and 

farther southwest in the Eagle Mountains (Underwood, 

1963, p. 9, 10).

Mortoniceras equidistans has been found at three 

localities in New Mexico. The species occurs in the Smeltertown Formation of Strain (1968, p. 82; 1976, p. 78, 

79) at Sierra de Cristo Rey across the Rio Grande from 

El Paso, Tex. Specimens from this area were described 

by Bose (1910, p. 63, 78) as the new species Schloenbachia whitei and S. trinodosa. Farther northwest, in the 

Cooke Range near Deming, N. Mex., M. equidistans occurs in calcareous beds below the middle of the Sarten 

Sandstone. In the Tucumcari area in northeastern New 

Mexico, Stanton (1928, p. 407) recorded M. equidistans 

(as Pervinquieria equidistans) from beds now assigned to 

the Purgatoire Formation. Collections made later from the 

Mesa Rica Sandstone include excellent examples of M. 

equidistans and Eopachydiscus marcianus.

Outside the United States and Mexico, M. equidistans is known from England and Nigeria. Spath (1923, 

p. 76; 1932, p. 408) recorded the species from the middle 

part of the upper Albian of southern England first under 

the name of Inflaticeras kiliani (Lasswitz) and later as 

Mortoniceras (Pervinquieria) kiliani. The species described by Lasswitz (1904, p. 245, pi. 19, fig. 1; text 

fig. 6) as Schloenbachia kiliani is a synonym of M. 

equidistans, as noted by Young (1967b, p. 27). Reyment 

(1955, p. 33, pi. 5, fig. 3) figured a small specimen under 

the name of Mortoniceras (M.) kiliani from Nigeria.

Cragin's type or types of his Schloenbachia leonensis var. equidistans have not been located. Some of his 

invertebrate types, which were at the Bureau of Economic 

Geology, Austin, Tex., are now at the Texas Memorial 

Museum, Austin. A few of his invertebrate types are at 

Colorado College, Colorado Springs, Colo., where large 

collections of Cragin's fossils are housed. Others of his 

types are at the National Museum of Natural History, 

Washington, D.C.

Introduction 1
The Cragin collections at Colorado College include 

about a dozen ammonites from Texas from rocks now 

assigned to the Kiamichi, Duck Creek, and Fort Worth 

Formations. Only one specimen fits Cragin's description 

of his Schloenbachia leonensis var. equidistans. This 

specimen may be the holotype, but it has neither a label 

nor a number. The preservation indicates a Duck Creek 

source. Because of the importance of M. equidistans as 

a guide fossil, the single extant specimen in the Cragin 

collections is assumed to be the type and is herein redescribed and illustrated.

The present study was made possible by the loan 

of Cragin's specimen from Dr. Andy Cohen, Geology Department, Colorado College, Colorado Springs, Colo. Dr. 

Keith Young, Department of Geological Sciences, University of Texas at Austin, kindly gave advice on other possible locations of Cragin's original specimen.

SYSTEMATIC PALEONTOLOGY

Family BRANCOCERATTDAE Spath, 1934 

Subfamily MORTONICERATINAE Spath, 1925 

Genus MORTONICERAS Meek, 1876

Type species. Ammonites vespertinus Morton, 1834.

Mortoniceras equidistans (Cragin)

Plate 1, figures 1-3, text figures 1,2

1893. Schloenbachia leonensis Conrad var. equidistans Cragin,

p. 241. 

1904. Schloenbachia kiliani Lasswitz, p. 245, pi. 19, fig. 1;

text fig. 6. 

1907. Mortoniceras inflatum (Sowerby) var. kiliani Lasswitz?

Pervinquiere, p. 246.

1910. Schloenbachia whitei Rose, p. 63, pi. 1, figs. 6-9. 

1910. Schloenbachia trinodosa Bose, p. 78, pi. 9, fig. 4; pi.

10, figs. 1-4.

1922. Inflaticeras [Subschloenbachia] kiliani (Lasswitz). 

Spath, p. 102.

1923. Inflaticeras kiliani (Lasswitz). Spath, p. 76. 

1925. Schloenbachia (Inflaticeras) trinodosum Boese. Diener, 

p. 139.

1925. Schloenbachia (Inflaticeras) whitei Boese. Diener, p.

139.

1925. Gauthiericeras kiliani (Lasswitz). Diener, p. 153. 

1925. Inflaticeras kiliani (Lasswitz). Spath, p. 183. 

1925. Inflaticeras trinodosum (Bose). Spath, p. 183. 

1927. Pervinquieria kiliani (Lasswitz). Adkins, pi. 3, fig. 4.

1927. Schloenbachia trinodosa Bose. Bybee and Bullard, pi. 

4, fig. 3.

1928. Prohysteroceras whitei (Bose). Adkins, p. 229.

1928. Pervinquieria kiliani (Lasswitz). Adkins, p. 233, pi. 5,

fig. 4.

1928. Pervinquieria n. sp. 4. Adkins, p. 234, pi. 10, fig. 1. 

1928. Pervinquieria trinodosa (Bose). Adkins, p. 234.

1932. Mortoniceras (Pervinquieria) kiliani (Lasswitz). Spath, 

p. 408, pi. 38, figs. 1, 2; pi. 42, fig. 1; pi. 47, fig. 

1; text fig. 140.

1934. Prohysteroceras (Goodhallites) whitei (Bose). Spath, p. 

446.

1940. Pervinquieria kiliani (Lasswitz). Breistroffer, p. 75, 

138.

1940. Pervinquieria kiliani var. alstonensis Breistroffer, p. 

138, 139.

1940. Pervinquieria fallax var. kilianiformis Breistroffer, p. 

140.

1942. Pervinquieria kiliani (Lasswitz). Haas, p. 94.

1947. Neoharpoceras (Goodhallites) withei [sic] Bose. Breistroffer, p. 66.

1947. Pervinquieria kiliani (Lasswitz). Briestroffer, p. 66.

1955. Mortoniceras (Mortoniceras) kiliani (Lasswitz). Reyment, p. 33, pi. 5, fig. 3.

1957. Pervinquieria kiliani (Lasswitz). Young, p. 3

1957. Goodhallites whitei (Bose). Young, p. 19.

1967a. Pervinquieria equidistans (Cragin). Young, p. 67-69.

1967b. Pervinquieria equidistans (Cragin). Young, p. 27.

The probable type is an uncrushed, well-preserved 

internal mold of an adult that has nearly half a whorl 

of body chamber. Diameter of the shell is 169 mm, and 

that of the umbilicus is 71.7 mm (ratio of 0.42). Whorls 

are higher than wide, with a rectangular section (fig. 1). 

Greatest width is at the mid-flank tubercle. A prominent, 

narrowly rounded keel is present. The umbilicus is steep 

and has a narrowly rounded shoulder.

Ornament consists of trinodose prorsiradiate ribs 

that are narrower than the interspaces. On the penultimate 

whorl, ribs are slightly sinuous, and most originate in 

pairs from prominent umbilical bullae. A few umbilical 

bullae give rise to single ribs, and an occasional secondary 

rib arises low on the flank. There are 35 ribs on the penultimate whorl and 22 umbilical bullae. Twenty-five ribs 

and 22 umbilical bullae are present on the outer whorl. 

Ribs are straight but very prorsiradiate. On the younger

Figure 1. Whorl section, natural 

size at a diameter of 163 mm, of 

Cragin's specimen of Mortoniceras equidistans (Cragin).

2 Schloenbachia Leonensis var. equidistans Cragin
Figure 2. Part of the external suture, at a diameter of 

110 mm, of Cragin's specimen of Mortoniceras equidistans (Cragin). E/L indicates the position of the saddle 

separating the external lobe from the lateral lobe (L). 

Heavy, straight line marks middle of venter.

half of the body chamber, ribs become distantly spaced 

with interspaces two or three times as wide as the ribs. 

Weak, bullate mid-flank tubercles first appear on the last 

half of the penultimate whorl and gradually become 

stronger adorally. On the outer whorl, mid-flank tubercles 

become strong and nodate. Ventrolateral tubercles, which 

are visible only on the outer whorl, are large, strong, 

and nodate to slightly clavate. Faint spiral ornament is 

present on the ribs and tubercles on the outer whorl.

Part of the external suture is visible (fig. 2). Lobes 

and saddles have lengthy branches about as complex as 

those shown for other species of Mortoniceras by Spam 

(1932, fig. 130) and Haas (1942, pi. 22). The lateral lobe 

(L) is long, bifid, and probably as large as the partly 

visible E/L saddle.

REFERENCES CITED

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the Fort Stockton quadrangle: University of Texas Bulletin 

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Bohm, Johannes, 1910, Review of L. Pervinquiere; Etudes de 

paleontologie Tunesienne. 1, Cephalopodes des terrains secondaires: Neues Jahrbuch, 1910, v. 2, p. 149-155.

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p. 5-61, 10 pis.

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Senate Executive Document 108, p. 141-174, 21 pis.

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4th Annual Report, pt. 2, p. 139-246, pis. 24-46.

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443^96, pis. 49-56.

References Cited 3
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series: American Journal of Science, 5th ser., v. 16, p. 399- 

409.

Strain, W. S., 1968, Cerro de Muleros (Cerro de Cristo Rey), 

in Delaware Basin exploration, 1968 Guidebook: West 

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 1976, New formation names in the Cretaceous at Cerro 

de Cristo Rey, Dona Ana County, New Mexico, Appendix 

2, in Lovejoy, E. M. P., Geology of Cerro de Cristo Rey 

uplift, Chihuahua and New Mexico: New Mexico Bureau 

of Mines and Mineral Resources Memoir 31, p. 77-82.

Underwood, J. R., Jr., 1963, Geology of Eagle Mountains and 

vicinity, Hudspeth County, Texas: Texas University, Austin, Bureau of Economic Geology Quadrangle Map 26, scale 

1:48,000, illus., text of 32 p. [reprinted 1974].

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Arkell, W. J., and others, Mesozoic Ammonoidea, in 

Moore, R. C., ed., Treatise on invertebrate paleontology,

[pt.] L, Mollusca 4: New York and Lawrence, Kansas, 

Geological Society of America and University of Kansas 

Press, p. L402-L409.

Young, Keith, 1957, Upper Albian (Cretaceous) Ammonoidea 

from Texas: Journal of Paleontology, v. 31, no. 1, p. 1-33, 

10 pis.; reprinted as Texas University, Austin, Bureau of 

Economic Geology Report of Investigations 28.

 1967a, Ammonite zonations, Texas Comanchean (Lower 

Cretaceous), in Hendricks, Leo, ed., Comanchean (Lower 

Cretaceous) stratigraphy and paleontology of Texas: Permian 

Basin Section of Society of Economic Paleontologists and 

Mineralogists Publication 67-8, p. 65-70.

 1967b, Comanche Series (Cretaceous), south central 

Texas, in Hendricks, Leo, ed., Comanchean (Lower Cretaceous) stratigraphy and paleontology of Texas: Permian 

Basin Section of Society of Economic Paleontologists and 

Mineralogists Publication 67-8, p. 9-29.

4 Schloenbachia Leonensis var. equidistans Cragin

PLATE 1

[All figures natural size]

Mortoniceras equidistans (Cragin)

Figures 1-3. Rear, side, and front views of Cragin's specimen from the Duck Creek Formation of Texas.
U.S. GEOLOGICAL SURVEY BULLETIN 1641-A PLATE 1

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MORTONICERAS EQUIDISTANS

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