No. 1902: January S, 1919 On a New Exogyra from the Del Rio Clay and Some Observations on the Evolution of Exogyra in the Texas Cretaceous BY Emil Bose Bureau of Economic Geology and Technology Division of Economic Geology J. A. Udden, Director of the Bureau and Head of the Division PUBLISHED BY THE UNIVERSITY SIX TIMES A MONTH, AND ENTERED Al SECOND-CLASS MATTER AT THE POSTOFFICE AT AUSTIN, TKXAS, UNDER THE ACT OF AUGUST 24, 1'12 The benefits of education and of useful knowledge, generally diffused through a community, are essenHal to the preservation of a free govern· ment. Sam Houston Cultivated mind is the guardian genius of democracy.... It is the only dictator that freemen acknowl· edge and the only security that free· men desire. Mirabeau B. Lamar ON A NEW EXOGYRA FROM THE DEL RIO CLAY AND SOME OBSERVATIONS ON THE EVOLUTION OF EXOGYRA IN THE TEXAS CRETACEOUS1 BY EMIL BOSE Nearly twenty years ago attempts were made to subdivide the Texas Cretaceous, basing stratigraphical zones on the occurrence of oysters, especially Gryphaeas and Exogyras. That such a subdivision must be doomed to failure will be evident to any paleontologist on account of the extreme variability of oysters, Exogyra included. But these at­tempts are interesting, in so far as they showed that Ex­ogyras occur in most of the Cretaceous beds of Texas and that all these may be related to each other. Exogyras occur practically from the lowest stratum of the Trinity form­ation, which corresponds to the Aptian, to the highest Cre­taceous, the Navarro beds; which in their upper part, the higher Escondido beds, certainly represent the Maestrich­tian. In order to show the evolution of Exogyra in the Texas Cretaceous a thorough collection of specimens of this genus in the different beds and a redescription of the known species would be absolutely necessary; first, because many forms are still undescribed; and second, because in most of the described species the development from the smallest to the mature individuals has not been sufficiently taken into account. The very small specimens are often the most important ones, because their ornamentation is frequently quite different from that of the full grown individuals and demonstrates from which forms in lower beds the species has branched off. Often the ornamentation on the very first part of the beak of mature specimens shows also this original ornamentation, but generally the beak has been corroded by the movement of the shell in the water or b¥" the grinding effect of sand in currents. In these cases the beak will appear smooth, although the younger specimens show a decided ornamentation. In Exogyra the evolution 1The manuscript for this paper was submitted in June, 1919; the bulletin was issued in September, 1919. (Ed.) of the beak is of an importance similar to that of the suture and the smaller whorls in the ammonites. More than twelve years ago, when I described the fauna of the Cerro de Muleros near Ciudad Juarez, Chihuahua, Mexico, I felt convinced that most of the Texas Exogyras had developed from a common tribe, but at that time I did not have enough material and there seemed to exist serious gaps which in part have been filled since that date. One of them especially was between the Vraconnian and the highest Cenomanian ; or, m:;ing local terms, between the uppermost Edwards limestone and the Buda limestone. This latter one has been recently filled through a discovery made by Mr. W. R. Cartledge, who found a new Exogyra in the upper part of the Del Rio beds in the region near the quick­silver deposits of the Terlingua district; a species which will be described farther on. I do not propose to prove that the evolution of the Texan Exogyras was entirely continuous, because my material is still very incomplete; but I wish to open a road for some­body else who may want to demonstrate in what manner Exogyra developed in Texas during the Cretaceous. It will probably be found that such an evolution existed and was at least partly parallel to the development of this genus in southern Europe and northern Africa; possibly in the later stages also in Asia. Thus the content of the following paper will be merely a sketch, but it may point out a way for a monographical description of the Cretaceous species of Exogyra in Texas. The oldest species of Exogyra in the Texas Cretaceous seemingly occurs in the lower part of the Trinity formation, the Basement sands or Travis Peak formation of Travis County. This species has been described, although net fig­ ured, by Cragin1 as Exogyra Hilli. He asserts that it has also been found in Arkansas, where it was described by R. T. Hill.2 It is a small form somewhat similar in out­ 1F. W. Cragin, A contributicn to the invert. Paleontol. of the Texas Cretaceous. 4th An. Rep. Geol. Surv. Texas, 1893, p. 186. 2R. T. Hill, The Mesozoic geology of southwestern Arkansas. An. Rep. Geo!. Surv. Arkansas, 1888, vol. 11, p. 131 (under the name of Ostrea Franklini), pl. 5, fig. 1-10 (not pl. 5, fig. 11-18; pl. 6, fig. 19-27; pl. 7, fig. 28-30). line to immature specimens of Ex. texana, with 5 to 8 radial folds on the anterior slope, a distinct umbonal ridge, and a steep anterior slope. The upper valve is practically smooth. This form seems to be intimately related to Exogyra Bous­singaulti d' Orb.1 which occurs approximately in the cor­responding horizon of Europe. A similar form has been found in the Aptian of San Juan Raya, Puebla, Mexico. This species seems to have branched off into several forms in the next higher division of the Trinity formation, some of which have been described and very imperfectly figured by Cragin as Ex. paupercula2 and Ex. weatherfordensis.' These species are quite frequent in the Alternating beds (Glenrose), the middle and upper part of the Trinity form­ation. Both seem to be related to Ex. Hilli and Ex. Bous­singaulti, although the distinguishing feature in Ex. pau­percula is that the beak becomes free from the rest of the shell as in Ex. arietina. E x. paupercula has only 2 to 4 irregularly-topped, angular, radial folds on the lower valve. while the upper valve seems to be smooth. I have not been able to see the original specimens. Much more similar to the Ex. Boussingaulti group is the Ex. weatherfordensis (pl. 1, fig. 1, 2) with a strong um­bonal ridge, 6 to 12 oblique plications on the anterior slope, and a greater number of narrow, subradiate, or curved pli­cations on the posterior slope. The upper valve is smooth, but the margin is often scalloped corresponding to the end­ing of the ribs on the lower valve. This is, at least, what I take to be Ex. weatherfordensis, a species which has not been very well figured. In the collection of the University of Texas exists a number of specimens collected by Taff at one quarter of a mile west of Weatherford, labeled E. Boussingaulti d'Orb. This species certainly is very similar to the French form and a near rela­ tive of it, and at the same time also of Ex. Hilli. It does 1A.d' Orbigny, Voy. dans l'Amerique merid. t. 111, 4me part, Pal., p. 91, pl. 18, fig. 20; pl. 20, fig. 8, 9. Idem, Paleont. franc;., Terr. Cret. t. 111, p. 702, pl. 468. 2Cragin, loc. cit., p. 186, pl. 30, fig. 7, 8. ardem, ibidem, p. 188, pl. 45, fig. 7-10. not seem possible to prove that it is the type of Ex. weather­fordensis but it appears at least to be viary similar to the specimens described and figured by Cragin; and it certainly proves that a very near relative of Ex. Boussingaulti lived in the Texas sea at about the epoch of the European species. The specimens from a place four miles west of Montell, also cited by Cragin, may belong to the same species, but they are mainly upper valves, and not very characteristic. These last two species seem to grow larger than Ex. Hilli and to a certain degree lead over to Ex. texana and Ex. arietina. The first of these two species is found well de­veloped in the Walnut clays, the base of the Fredericksburg formation. This species has been described and figured by several authors, first by Roemer/ then by Conrad, Gabb, Coquand, White, Hill and Vaughan, and by myself.2 The species is nearly related to Ex. flabellata Goldfuss of the Cenomanian of Europe, but it is in general broader, the ribs are more numerous, and the ridge is nearer to the anterior side; the European species is generally a little younger than Ex. texana, although this latter passes through quite a num­ber of horizons. In the Walnut clay it is associated with Engonoceras, Schloenbachia group of Sch. Belknapi, and Schloenbachic;, group of Sch. acutocarinata, and these beds probably represent the Vraconnian or the Albian. It is found also in the Comanche Peak limestone and the Ed­wards limestone; both still belonging to the Vraconnian or Albian. At the Cerro Muleros it occurs throughout the whole Vraconnian. It should not be overlooked that the Comanche Peak lime­stone seemingly contains another species of Exogyra, di­rectly derived from the main tribe and exceedingly similar to Ex. Boussingaulti; it is distinguished from Ex. texana by its smooth upper valve, is not very frequent, and I have not seen more than a very few specimens. Ex. texana is certainly derived from the tribe of Ex. Boussingaulti represented by Ex. weatherfordensis, but the younger species grows much broader, the umbonal ridge 1Roemer, Kreidebild. v. Texas, 1852, p. 69, pl. 10, fig. la-le. 2Bose, Cerro de Muleros, Bol. Inst. geol. de Mexico, 25. P. 112, pl. 20, fig. 14-16; pl. 21, fig. 1-11; pl. 22, fig. 1·9. moves nearer to the anterior margin, which becomes ex­ceedingly steep, while the posterior slope becomes flattened or even concave. The upper valve develops a great number of somewhat irregular radial ribs, the umbonal ridge be­comes very pronounced and the anterior slope quite steep. A conscientious study of the forms from different horizons may show that even within the species a certain develop­ment takes place, in so far as the umbonal ridge is nearer to the anterior margin in the older specimens than in the younger ones, and that the ribs grow coarser in the indi­viduals found in the higher horizons. Ex. texana nearly always occurs in great numbers and with it begins the rich development and subdivision of the principal tribe. Before we go any farther we shall have to mention an­other form developed during th~ Albian or the Vraconnian and apparently the ancestor of several younger species. This is the form described by Cragin1 as Ex. plexa. Under this name Cragin seems to have united at least two differ­ent species; I refer his name to one of these which looks entirely like an enlarged juvenile specimen of Ex. arietina (compare the description of this species given below). The exact types could not be found, but there is sufficient ma­terial from the original localities. (Compare pl. 1, figs. 3, 4.) I regard as the type the specimen figured by Cragin in his figures 3, 5, 6 (not 4). It is entirely covered with fine radial ribs which cross the prominent umbonal ridge obliquely as in the juvenile specimens of Ex. arietina. The specimens occur in the bed of Ex. texana below the Kiami­tia clays on the Texas and F acific railroad, three miles west of Benbrook. The whole shell is spirally curved similar to the young Ex. arietina although the size is much larger. The beak is still pressed down against the body of the shell, but a further spiral development would detach the beak from the rest of the valve. This species has probably developed from the group of Ex. Boussingaulti (Ex. weatherfordensis) losing the thick 1Cragin, loc. cit., p. 187, pl. 30, fig. 3.-0. folds and developing fine ribs, but retaining the high um­bonal ridge. In the same strata we find another species, of which a specimen not very characteristic has been illustrated by Cragin in his fig. 4. The upper part (about 15 mm. in height) is practically of the same form and ornamentation as Ex. plexa typ. but from there on it begins to develop on the anterior side four to five strong radial folds begin­ning near the umbonal ridge and going down obliquely with the ridge to the antero-inferior margin. They are much stronger and sharper than shown in Cragin's figure and are covered by fine radial ribs. They are fainter on the mould and there appear smooth. As our material is not sufficient to allow a complete description we shall designate the species as Ex. nov. sp. aff. plexa. (Pl. 1, fig. 5). Ex. plexa seems to lose the fine ribs on the lower margin in larger specimens, as is shown by an individual from the original locality. This feature is still more evident in larger specimens found at the base of the Fort Worth limestone in Duck Creek, Grayson County. This is evidently a younger form of the same group, but possibly a different spec~e::i or variety. It grows much larger than the type, show;; ap­proximately the same ornamentation on the umbonal region, but becomes entirely smooth on the inferior margin. Another form from the Kiamitia clays found on little Mineral Creek, north of Pottsboro, Grayson County, shows an ornamentation very similar to that of Ex. plexa on the umbonal region, but develops a rather deep furrow along the umbonal ridge on the posterior side and several obtuse folds covered with fine radial ribs on the anterior side near the antero-inferior margin. It is most probably a descend­ant of Ex. nov. sp. aff. plexa, mentioned above (Pl. 1, fig. 6.) About the development of Exogyra in the Georgetown formation, i. e., the lower and middle part of the Cenoman­ ian, little is known. There is a large species frequently found in this forma­ tion and it has generally been called Ex. americana or Ex. Walkeri. It was originally described by Marcou as Gry­ phaea sinuata var. americana, and Cragin cites it as Ex. americana Marcou. Unfortunately, only very large speci­men$ are described and figured, but the individual illus­trated by Marcou shows that the younger specimens must have strong, fold-like, radial ribs in the umbonal region. Therefore I presume that this broad and flat species with its obtuse ridge near the anterior margin developed from a broad, coarsely-ribbed group, probably one derived from Ex. texana. E xogyra becomes exceedingly frequent on top of the Georgetown formation in the Del Rio clay which represents part of the upper Cenomanian. The principal species is Ex. arietina Roem. which we have redescribed farther on. It is evidently a descendent of Ex. plexa; it retains the ornamentation of this species and its form on the umbonal region and in the very young indi­viduals, but later on becomes entirely smooth and its beak becomes detached because the whole valve coils spirally. A second species is Ex. Cartledgei nov. sp., described be­ low. It certainly was derived from Ex. nov. sp. aff. plexa, developing the strong radial ribs still more, while the beak became somewhat detached from the rest of the shell by the coiling of the valve. The strong umbonal ridge is still in existence, but the fine ribs on the beak were lost; or at least are not preserved on the specimens I have in hand. This species grows much larger than either Ex. nov. sp. aff. plexa or Ex. arietina. Cragin1 described another species from the Del Rio clay which he called Ex. Drakei. Unfortunately his figures are very deficient, but his originals show that the species is entirely different from Ex. Cartledgei. It is covered with irregular but strong ribs on the lower valve, not really coiled, but rather bent over to one side, as shown in Cragin's figure 11. The upper valve is very similar to that of Ex. arietina and Ex. Cartledgei. In the next higher horizon, the Buda limestone and the corresponding strata of the Cerro de Muleros, which, ac­cording to the echinoid fauna described by Whitney, and 1Cragin, loc. cit., p. 184, pl. 29, fig. 8-11. the ammonites, represents the very uppermost part of the Cenomanian, several Exogyras occur which are larger than most of those in beds below, with only the possible excep­tion of Ex. americana. The species found in the Buda limestone is Ex. Clarki Shatt.,1 but the specimens figured by Shattuck represent only medium-sized individuals and the species should be re­described and figured. It is a very broad and not very thick species very similar to Ex. americana; the medium­sized specimens show rather strong radial ribs, while the mature specimens are almost smooth. Very juvenile speci­mens do not seem to have been found. The species evi­dently is a direct descendent of the older E x. americana. In the corresponding strata of the Cerro de Muleros an E xogyra is found in great numbers, which I took formerly to be identical with Ex. Clarki and which on account of its great similarity with Ex. ponderosa I described as Ex. pon­derosa var. Clarki.2 The medium-sized specimens of this species really are extremely similar to Ex. Clarki while the full grown individuals are entirely different. I therefore rename the species from the Cerro de Muleros and call it Exogyra Whitneyi in honor of Prof. F. L. Whitney, whose beautiful studies about the echinoid and mollusk fauna of the Buda limestone have given us the first idea of its rich fauna and its real age. I have figured especially the juvenile and medium-sized specimens of Ex. Whitneyi in the work cited above. If we compare figs. 5, 6, 7, 11 on plate 26 with specimens of Ex. plexa, we can find nearly no difference. Comparing figs. 4, 8, 10, 12 on the same plate with Ex. Cartledgei and Ex. Drakei we see at once that the similarity is extremely great, although the beak is not quite as much coiled spirally as in Ex. Cartledgei. The upper valve figured in fig. 9 of the same plate is very similar to that of E x. Cartledgei. The mature forms are entirely different from the younger 1Shattuck, Buda Limestone, Bull. U. S. Geol. Surv. no. 205. 1903. p. 22, pl. 10, 11. 2E. Bose, Cerro de Muleros, p. 115, pl. 23, fig. 17; pl. 24, fig. 6; pl. 25, fig. 8; pl. 26, fig. 4-11. On a New Exogyra from Del Rio Clay ones; the ribs mostly are destroyed by erosion and the shell becomes entirely smooth, the beak detached from the body. It brings to mind at once the elongate varieties of Ex. pon­derosa but notwithstanding the great similarity between the two species I have become convinced that they should be separated specifically on account of the somewhat different form of the younger specimens, the generally broader shape of the mature Ex. ponderosa, and the extremely different age of the beds in which both species occur. Those small individuals from Cerro de Muleros which I have taken to be juvenile forms of Ex. Whitneyi show an astounding similarity to a small Exogyra extremely fre­quent in the Mainstreet limestone of Denison. This form has been frequently cited under the name of Exogyra arie­tina, but is easily distinguished from it by its rather strong ribs and the form of the beak pressed against the body of the shell. The real Ex. arietina occurs in a somewhat higher horizon of the same locality; that is to say, in the Grayson Marls. The similarity between the small Exogyra of the Mainstreet limestone and the small individuals from the red sandstone of Cerro Muleros is so great that I have become somewhat doubtful if these latter forms are really the ju­venile forms of Ex. Whitneyi and not perhaps a different species identical with or very similar to the small Exogyra in the Mainstreet limestone, although they certainly occur together with Ex. Whitneyi in the red sandstone of Cerro Muleros. It is of course possible that the small Exogyra of the Mainstreet limestone is at the same time a prede­cessor of Ex. Whitneyi and of Ex. arietina. A species very nearly related to Ex. Whitneyi is Ex. f erox, if it is not only a variety. Ex. fer ox was described by Cragin1 ; unfortunately his figures are so deficient that one cannot get an adequate idea of the shape, and the upper valve of the original has been lost. The lower valve is very similar to that of the mature Ex. Whitneyi, but is still more elongate; no closer comparison can be made before the ju­ 1Cragin, loc. cit., p. 185, pl. 32, fig. 1; pl. 33, fig. 5; pl. 34, fig. 1; pl. 36, fig. 6. University of Texas Bulletin venile stages of Ex. ferox have been found and described. The species was found in the so-called lower Cross Timber (Woodbine) sands, which correspond in age probably to the Buda limestone; at least, they also belong in their lower part to the upper Cenomanian. It is difficult to say to what group belongs the form cited by Cragin1 as Ex. columbella Meek. It certainly has noth­ing to do with the type of that species. The specimen which came from the lower Cross Timber sands of Timber Creek, two miles below the Dallas-Lewisville Road, Denton County, is so much corroded that the surface appears nearly smooth, but on close inspection one can easily see that it had been covered by thick, rounded, rather strong ribs. It might be a very young individual belonging to the group of Ex. Clarki. From the next higher beds, the Eagle Ford shales, which correspond to the Turonian, very few fossils have been de­scribed in Texas, and among these only one Exogyra is cited under the name of Ex. columbella Meek. This is a rather small form with thin shell and covered by small, quite regular, radial ribs, which bifurcate at the umbonal ridge. It seems to be very possible that this species is a branch of the Ex. Clarki group, if it is not directly derived from Ex. arietina through the so-called Ex. columbella Cragin. This may only be decided when larger collections in the Eagle Ford shales or the upper part of the Wood­bine sands shall have been made. In Colorado a large species, Ex. suborbiculata Lam., occurs, which as Stanton justly remarks, is intimately related to the European group of Ex. columba, but this larger form has not yet been found in Texas. Also in the next higher formation, the Austin Chalk, very few Exogyras are known, although they can be found in great numbers in it. The Austin Chalk corresponds in its lower part to the Emscherian, in its middle to the lower Santonian, and in its upper part to the middle Santonian. I have not seen any Exogyra in the lower part, but it is 1Cragin, loc. cit., p. 184. possible that Ex. laeviuscula Roemer or a related form oc­curs there, as it certainly does in the middle portion. This very characteristic form is entirely smooth, but if the younger specimen figured by Roemer in Pl. 9, fig. 3c, really belongs to this species, we might suppose that it is derived from Ex. nov. sp. aff. plexa through Ex. Cartledgei and Ex. columbella Cragin (not Meek) having lost entirely the finer ribs. An undescribed and unstudied small Exogyra is very fre­quent in the middle part of the Austin Chalk (zone of Mortoniceras texanum) on the Rio Grande. In the upper part of the Austin Chalk near Austin we find a great number of a large Exogyra which commonly has been called Ex. ponderosa, although it is specifically quite different. It has a broad and not very convex lower valve, with a rounded ridge and an extremely steep slope on the anterior margin, and is very similar to the large specimens of Ex. Clarki and to a certain degree to Ex. amer­icana. These species with very low right, valves probably form an independent branch and are derived from Ex. tex­ana. I have not seen any of this tribe in higher strata of Texas, but a near relative, Ex. tamulica Stol., appears in the higher Senonian of India. The Exogyras of the higher strata of Texas, the Taylor marls (upper Santonian), Navarro beds (Campanian) and the higher Escondido beds (Maestrichtian) are very little known, practically only two species having been dt::scribed: Ex. ponderosa Roem. from the Taylor marls and Ex. costata Say from the Navarro bed:3, which seems to continue into the base of the higher Escondido beds. Exogyra ponderosa has been first and very well described and figured by Roemer.1 In recent years it has been re­ described and figured by Stephenson.2 His figures are very characteristic but unfortunately he has not tried to describe and figure the development of the species from the 1Roemer, Kreidebild. v. Texas, p. 71, pl. 9, fig. 2. 2L. W. Stephenson, Cret. dep. on the Eastern Gulf region. U. S. Geol. Surv. Prof. Paper 81, 1914, p. 46, pl. 13, fig. 5-7; pl. 14; pl. 15, fig. 1-3. youngest to the mature individuals. Roemer mentions that specimens about 2.5 cm. long show distinct, unequal, radial folds which begin to grow indistinct near the margin of the valve. Stephenson's Pl. 13, fig. 5 shows clearly this kind of ribs on the beak of a medium-sized specimen. If we imagine this beak detached from the valve and in the form of a young specimen, it would have exactly the shape and ornamentation of a young specimen of our young Ex. Whit­neyi from the Cerro de Muleros, and be extremely similar to the young of Ex. arietina and to the mature specimens of Ex. plexa. Stephenson (loc. cit. p. 49, pl. 15, fig. 4; pl. 16, fig. 1, 2) has described a number of shells as Ex. pon­derosa var. erraticostata Steph. It will be rather difficult to sustain this variety because the figure of Roemer's type shows very similar ribs and thus should be part of this variety. Ex. costata has been quite frequently described and fig­ured ; recently Stephenson1 has very fully described the ma­ture specimens and also has illustrated these profusely. Unfortunately also in this case we lack the exact knowledge of the ornamentation of very young specimens. The species is possibly derived from Ex. ponderosa or at least from a form of which Ex. ponderosa is a branch, but this cannot be proven at the present time. Specimens should be espe~ cially studied in the zone where Ex. ponderosa and Ex. cos­tata appear to occur together and where the real antecessor of the latter species may be found. The preceding sketch of the evolution of Exogyra in the Texas Cretaceous necessarily has to be deficient, incomplete and in a great part hypothetical, but at least it shows that all the species, even the largest which are practically smooth, came originally from a rather small costate form; and that the great diversity of forms in the upper strata is simply due to the subdivision of a tribe represented at the base by forms that are intimately related to each other and at the same time to Ex. Boussingaulti of the European Aptian. It shows also that some of the large species in the istephenson. Joe. cit. n. 50 . pl. 16, fig. 3, 4; pl. 17, fig. 1, 2; pl. 18; pl. 19, fig. 1-4 ; pl. 20 fig 1. higher horizons may not be intimately related, notwith­standing the very similar shape, but rather be convergent forms. A monograph of the Texas Cretaceous Exogyras would probably show a still much greater variety of forms which were derived from the same original tribe, and at the same time it would not at all be difficult to demonstrate that in Europe, Northern Africa and Asia quite a similar evolution has taken place. -·­ ~ 0) Nowrtv bethNowrtv bdu/h £09/0 rord s/Jo/11s Eagle Ford.s/K;/os Tvronhn ~ I I : ""· ---; ~ I Ex Odrl