Rhynchonellids from the Bagh Beds

RHYNCHONELLIDS FROM THE BAGH BEDS. BY G. W. CI-IIPLONKER, M.Sc. (From the Department oJ Geology, Benares Hindu University, Benc~res.) Received March 10, 1938. (Communicated by Prof. L. Rama Rau, ~t.A., F.C.S.) Introd~fction. THE Rhynehonellids from the Bagh Beds now under investigation were first identified by Duncan 5,~2 with Rhynchonella depressa Sowerby (non d'Orbigny), from the Upper Green Sand of England and Europe. Later on Bose 7 reported, among the rhynchonellids collected by him from these strata, the presence of Rhynchonella plicatiloides Stoliczka, of the Trichinopoly and Ariyalur Groups (Turonian and Senonian) of Southern India. But till now these rhynchonellids were not examined critically with a view to studying their phylogeny, as was done by Buckmang, 1~ and others14,15.24.25.~6.s~ ,to.. in the-case of other rhynchonellid faunas, the results of which are summar- ised by Buckman TM and Thomson. 5~ The methods of securing the internal casts, by removing the tests of fossils, for exposing their internal characters such as the muscle marks, aiad of studying the brachiaI skeletons in serial sections, both transverse and longitudinal, by grinding down the burnt specimens are fully described by Buckman,l~ is Thomson 5~ and Muir- Wood." In the author's opinion, however, serial sections although helpful in a general way are not useful in separating closely allied forms. The absence of restorations of cardinalia or brachial features in spite of multi- tudinous serial sections figured, for example, by Muir-Wood and others, reduces their intrinsic value. The few serial sections here figured are given to show the general features of the internal characters, and not to indicate their distinctions from closely allied forms. The rhynchonellids described here were collected by the late Prof. K. K. Mathur and by the present writer from the neighbourhood of Chirakhan Oat. 22 ~ 22' 30", long. 75 ~ 7' 30"), Indore State, Central India, where these beds were first noticed by Col. Keatinge '7 who referred them to the Creta- ceous. Though rhynchonellids are reported to occur in both the Deola- Chirakhan Marl and the Coralline Limestones,~.~.2s, 40 their relative abund- ance in these different beds is uncertain; because the Limestones are so hard that the fossils contained in them are available only after weathering 300 Rhynchouellids from the Bag; Beds 301 of the rock; and thus they occur mixed up with the fossils from the Marl. The method of burning pieces of the Coralline Limestones, as was done by the Shan Navvy in case of Burma fossils, la might enable one to secure fossils from the Coralline Limestones themselves; but the process is not very desirable, because many of the external characters of the fossils are lost, as happened in the case of the Burma Brachiopoda? 3 Owing to this unfavourable nature of the Limestones, a collection of fossils could not be made bed by bed; and though the phylogeny of the present rhynchonellids could be studie:t, the possible evolutionary stages of their various characters cou'd not be traced successively from one bed to another. Measurements in Rhynchonellids. Since dimensions are often useful along with other characters, in ~eparating brachiopod, especially rhynehonellid and terebratulid species, it is essential that such factors should be.selected for nleasurements as are always obtainable, and that no other factors influence them. As beaks are not always preserved in fossil brachiopods, a measurement intended for the base-line would not be satisfactory if it included the beak; again, if the distance from the dorsal umbo to the middle point of the dorsal fold be chosen as the base-line measurement, species like Burmirhynchia depressa Buckman, ~s with feebly developed fold, cannot be adequately compared with species like Rhynchone3la acuta,la with much everted fold; because the increased dorsal throw of the fold does not necessarily mean an increase in length. So Buckman ~3 chose for the base-line measurement, the distance from the dorsal umbo to the anterior end of lateral comnfissure, i.e., AA' in Yigs. 1 and 2 ; all the other measurements which follow are expressed as percentages of the base-line, and are always in the same order as enumerated here. A second measurement, to express the rostration is taken from the beak to the anterior end of the lateral commissure, i.e., A'B in Figs. 1 and 2. AC in Figs. 1 and 2 measured from the dorsal umbo to the furthest point on the fold repre- sents the degree of protrusion of the fold. The fourth and fifth measurements are DD' in Fig. 3 for breadth and EE' in Fig. 3 for thickness. Yet another measurement A'C in Figs. 1 and 4 intended to express the extent of the dorsal projection of the fold may be taken in case of much everted folds as in Rhynchonella acuta; it is, however, not always necessary nor quite easy to take. As is evident from Fig. 2 the base-line AA' and the rostration A'B are measured in a direction of which the obliquity becomes more or less accen- tuated according to the breadth of the test and of the fold. So in the case of species with broad tests and relatively narrow folds and those with 302 G. W. Chiplonker narrow tests and relatively broad fold, the obliquity of the base-line AA' being variable in opposite directions, other dimensions which are reduced to i ~ M I c K ', c"~~ ~' , FIG 2 FIG.I E c FIC ~ rig 4 its percentages, will be affected in opposite senses and will give fairly approximate values for species which may be quite different from each other. Thus, with such a variable base-line one cannot have a true comparison between different species. To give a true comparative significance to these measurements, the base-line must be chosen in a direction independent of all variations in the breadth of the test and of the fold. This is done by taking the base-line measurement along a perpendicular between the parallels passing through the dorsal umbo and through the anterior end of lateral commissure, i.e., A'F in Fig. 2. Similarly, the measurement for the degree of rostration should be taken along a perpendicular between the parallels pa~sing through the beak and through the anterior end of the lateral commissure, i.e., A'G in Fig. 2. While it thus gives a theoretically true base-line, the practical advantage gained is that the protrusion (or retraction) of the fold is expressed true to its value (i.e., above, below or equal to 100 as the case may be). The importance of taking measurements in the new way as described above was very keenly felt while working with the cretaceous rhychonellids with which the writer has been busy for some time past. According to the older system of measurement, which the writer followed in the beginning, the percentage value obtained for the protrusion of the fold was les~ than or equal to 100, unless the fold protruded beyond a certain limit, i.e., unless AC (in Fig. 2) subtended at A' an angle greater than that subtended by AA' at the point C. Thus in some individuals with a weakly developed fold, the percentage value of the protrusion would be equal to or less than 100, though the specimen, when viewed in profile, would actually show the R/~ync~o~ze//ids from t~e Ba~/z Beds 303 fold to be of a protrusive nature. It must, here, be clearly understood that a fold does not necessarily protrude beyond the anterior end of the lateral commissure, even if the percentage value of the protrusion is more than 100; because in such cases the dorsal throw of the fold has, to a certain extent, masked its protrusive nature. The case is too evident to be explained at any greater length and need only be illustrated by a comparative table giving the values as obtained by the two different methods of measurement. A few species from Namyau Beds of Burma, which are already described by Buckman, 13 are chosen for the purpose. The base-line chosen for the rhynchonellids is not suitable in the case of the terebratulids, because in the latter group the anterior end of the lateral commissure is not well defined ; so the base-line and rostration are measured from the middle point of the anterior margin to the dorsal umbo and to the beak respectively. TM It is obvious that the modification here proposed for measurement of the base-line and the rostration is not applicable to terebratulids. While working with British Chalk terebratulids, Dr. M. R. Sahni .2 found it useful to have a measurement (which might be used with advantage in rhynchonellids also) for the distance of highest point of the dorsal valve from the beak, i.e., KL in t~ig. 1 ; but since it includes the effect of a second factor, namely the rostration, which is always represented separately, specimens with low beak and the highest point of the dorsal valve away from its umbo, will give values fairly approximate to those of specimens with high beak and the highest point of the dorsal valve nearer the umbo. Moreover, it is not always that beak is preserved in fossils, especially in rhynchoneUids. So, if the distance of the highest point of the dorsal valve is measured from the dorsal umbo, i.e., from K to K', the effect of the second variable factor of the rostration will be eliminated, and as the dorsal umbo is preserved in most cases this measurement can always be obtained.

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