Cambrian life explosion in fray: Evidence from more than 1 b.y. old animal body fossils and skeletonization event
Rajesh K. VishwakarmaPutative outburst of animal phyla in the Cambrian age, mainly, evidenced by the absence of fossils of triploblastic metazoans from rocks predating the Cambrian1, now appears to be inadaptable following a recent discovery of more than 1-Ga-old triploblastic metazoans from India2. But another simultaneous and independent discovery of ancient brachiopods and the shelly fossils indicating an earliest Cambrian age3 is likely to reiterate the Cambrian fossil explosion about 540 million years ago. The brachiopods and shelly fossil fauna (for convenience read B-Sf as brachiopods and shelly fossils respectively), in particular are so far considered as a useful tool for biostratigraphic correlation of the PrecambrianCambrian boundary in chertphosphorite Member of the Tal Formation, lesser Himalaya, with that of a far distant (> 500 km) occurring Rohtas limestone and shale of the lower Vindhyan Semri Group (Vindhyan Supergroup) in the peninsular India3. Since B-Sf is important evidence for the occurrence of small animal body fossils with hard parts, in the early Cambrian, a re-examination of the age of the sequence in which B-Sf are found to occur is warranted. Surprisingly, isotope geochronological aspect has not yet been properly scrutinized for upholding biochronostratigraphic correlation. Secondly, it is also aimed to assess whether a generalized concept of explosion of diverse skeletonized taxa at the beginning of the Cambrian age has got any role to play with the recently recognized fossil record of B-Sf. The studies presented here may ultimately help in unfolding the mystery of the evolution of Precambrian life worldwide.
Focus on geology and
As B-Sf fossils have generated commotion amongst earth scientists with
regard to the age revision of the Vindhyan Supergroup, i.e. from the existing
ProterozoicCambrian age to terminal ProterozoicCambrian, it is first of all
necessary to look over the geological and geochronological aspects of the Vindhyan
Supergroup (Figure 1) as a whole in order to resolve the sharp discrepancy as mentioned.
Geologically, the lower Vindhyan of the Vindhyan
Figure 1. Stratigraphic column14,19 of more than 4500 m thick Vindhyan Supergroup rocks. S, Semri Group; K, Kaimur Group; R, Rewa Group and B, Bhander Group. Erinpura granite (E) and Malani volcanics (M) are coeval in origin.
Supergroup is represented by the Semri Group rocks4 which starts with the basal conglomerate and sequential occurrence of limestone, porcellanite, the Kheinjua formations (shows trace fossils of triploblastic metazoa2), olive shale, fawn limestone, glauconitic sandstone and the Rohtas formations (known for containing brachiopods5, recently it has shown rich occurrence of B-Sf3) alternate layers of shale and limestone of predominantly continental shallow marine environment. The shaly beds of the Rohtas Formation are followed upward by the upper Vindhyans Kaimur Group (consisting of lower grits, conglomerate, sandstone and breccia and subsequent sandstone, shale, quartzite and intermittent conglomerate), Rewa Group made up of alternating sequences of shale and sandstone, and then the topmost Bhander Group where the shalesandstone sequence is punctuated by limestone beds. All these rocks are mostly undeformed and unmetamorphosed, as a result, they are suitable for radiometric dating. A much modified version of Vinogradov and Tugarinovs6 age data, as given by Kreuzer et al.7, points to 1080 ± 40 Ma age of the upper Semri Group glauconitic sandstone pertaining to Kheinjua Formation. The age of the overlying Rohtas Formation, on the other hand, is comprehensible by taking into account the intriguing situation arising from an intrusion of 1067 ± 31 Ma old diamondiferous Majhgawan kimberlite pipe (which purports a precise age of emplacement obtained from RbSr analyses of acid leached phlogopite mica from kimberlite rock of Majhgawan area8) into the older Semri Group rocks and then partly into some of the younger (~ 890 Ma) (ref. 7) Kaimur Group rocks. As a matter of fact, this geological set-up may provide insight into: (i) the age of the Semri Group rocks as older than about 1067 Ma, and (ii) substantial erosion and denudation of the rocks of the upper Rohtas Formation. Both are dealt below.
The erosion was so severe that the diamondiferous kimberlite, within the Rohtas Formation, remain standed as these withstood erosion. The processes must have operated quite gradually over a considerable span of time, and so we find major unconformity atop the Rohtas Formation. Later, these were one of the sites of subsequent sedimentation corresponding to the Kaimur Group. Wherever it occurred around parts of the pre-existing kimberlite pipes, a false impression of igneous intrusion into the younger Kaimur beds is likely to be imposed upon. These conditions were possible, since the Semri Group rocks, lying over the old (Paleoproterozoic) Bijawar Group and under the young Kaimur Group, had been present in the nearby areas. Furthermore, the complete absence of deep-seated fundamental fractures (which are necessarily required for the emplacement of kimberlite) and any changes in the structural pattern of the rocks, mainly, from the force exertion due to magmatic intrusion9 rightly provides impetus to the view offered on sedimentation over and around Majhgawan diatreme. Hence, these assessments maintain significance of the radiometric dating. And it also helps to adduce the minimum possible age of the Rohtas Formation as 1067 Ma, or if not exact, then younger than 1080 Ma, because the underlying Kheinjua Formation rock is of 1080 Ma age. Obviously then, other lower Semri group rocks ought to be older than 1080 Ma age.
The suggested Mesoproterozoic age for the Rohtas Formation is further explicable from KAr dating of the lower Kaimur group glauconites (940 ± 30 to 910 ± 30 Ma with a mean of 940 ± 90 Ma) (ref. 6) which occur above Rohtas Formation of ³ 1067 Ma age. Because both the stratigraphy and geochronology of the upper Semri and the lower Kaimur Groups are in tune with each other, the genuinity of radiometric dating is upheld. As regards to precision, a modified age data of 890 ± 40 Ma, given by Kreuzer et al.7 on the basis of recalculation of Vinogradov and Tugarinovs radiometric data6, can be taken into consideration. This radiometric age also seems to justify the relatively younger isotopic age of the middle Kaimur Group rock (~ 725 Ma) (ref. 10) than the age presented here for the lower Kaimur Group. Particularly the age of 725 Ma points to Neoproterozoic age for the middle Kaimur Group. It has been obtained from the lead isotope dating of galenas found at the transition zone of Ghaghar sandstone and Bijaigarh shale, and it indicates the age of both sedimentation and galena mineralization10. Because galena lead isotope compositions hardly change during mobilization of galena ores due to deformation and/or metamorphism11, the reported lead age for the middle Kaimur is primary.
Though the evidences demonstrate that the Kheinjua and Rohtas Formations are Mesoproterozoic in age, it is often opined that the age of the Vindhyan sediments could be younger than 745 Ma. This is because of the occurrence of Malani volcanic suit (745 ± 10 Ma) (ref. 12) at the contact of overlying Trans-Aravalli Vindhyan rocks, supposedly of the lower Semri Group. This problem can be dealt with convincingly in an alternative way, i.e. either the Malani volcanics are an intrusive phase into the lower Semri Group having basal conglomeratic horizon, or they simply belong to the lower part of the overlying middle Kaimur Group which also constitutes conglomerate (Figure 1). To settle this controversy, the latter view requires serious thought on account of the fact that there is a scope for changing the stratigraphic position of the Malani volcanics vis-à-vis the Trans-Aravalli Vindhyan rocks well within limits of Neoproterozoic to Early Cambrian age. For example, Malani volcanics are deemed equivalent to the upper Vindhyan rocks13, whereas Ravindra Kumar14 has included it right at the base of the upper Vindhyan Group (i.e. around 1000 to 1070 Ma, inferable from this study). But in the same work of Ravindra Kumar, the Malani volcanic rock is placed just below the Bhander Group sandstone, mainly, Jodhpur sandstone. This possibility seems improbable because by the time the Jodhpur sandstone got deposited in the Bhander Group, the thickest pile of Rewa Group rocks as well as the whole upper and middle Kaimur Group rocks (~ 725 Ma) including relatively old Malani volcanics (~ 745 Ma) were already existing. All this, therefore, help to substantiate terminal ProterozoicCambrian age to the uppermost Vindhyan rocks, viz. Bhander group15. Yet, these aspects may still be verified from more geochronological study of the Trans-Aravalli Vindhyan rocks resting over Malani volcanics. But for now it is certain that either of the two aforementioned aspects on the relationship between Vindhyan and Malani volcanics would be the only possibility, mainly because in the overall scenario of the Vindhyan Basin development we find somewhat gradually decreasing chronological order of depositon (Table 1). Interestingly, it reflects an incident in which one expects sedimentary rocks of Mesoproterozoic to Neoproterozoic age well after the formation of old basin during ArcheanPaleoproterozoic time, rather than a sudden onset of geological processes for any basin sediment deposition at terminal NeoproterozoicEarly Cambrian age after the extremely long-continued, yet unrealistic gap of about 1850 Ma to 1150 Ma, more particularly in predominantly marine palaeoenvironment.
Primitive evolution of life
In view of the above, it is, therefore, logically admissible that the shelly body fossils, B-Sfs, from the Rohtas Formation could reveal the specialized B-Sfs record as the oldest known fossil fauna that can be definitely assigned to 1070 Ma age. This in turn manifests that: (1) These fossil records represent the most primitive ancestral fossils of animal lineage belonging to that of the earliest Cambrian, and supports a fact16 that the Cambrian animals had some familiar-looking predecessor. Hence, in a situation like this it would be naive to consider B-Sfs as a potential biostratigraphic tool for assigning PrecambrianCambrian transition, and (2) the said interpretation now helps to extend the skeletonization event more than 500 million years to 1070 million years back. Traditionally the event was believed to have occurred during early Cambrian time at 540 Ma (ref. 2).
Why does the opinion about brachiopod and shelly fossils Proterozoic ancestry sound so realistic? In fact, the key is the very slow evolution of life17. Some given evidences elsewhere do appear to corroborate the viewpoint held here, which suggests that the Vendian and Cambrian fossils18 and even the present-day forms of lives are the offshoot from relatively very primitive lives present well within the Precambrian time16, e.g. close to a billion years ago, that is at least true in the context of invertebrate divergence from chordates1. The present study, however, for the first time recognizes the earliest ever known age of the shelly animal phylogenesis about 1070 million years ago.
Hence, all these may bespeak: (1) Like sedimentary rock
formation, life too involves quite gradual process of development, as a result, an
of life at about 540 Ma seems incongruous, and (2) the fossils being con-
ventionally used as a potential biochronostratigraphic tool in the Precambrian/Cambrian sequences need to be reassessed and supported by isotope geochronological data.
ACKNOWLEDGEMENTS. I have been encouraged by Lt. Col. (Dr.) V. B. Vishwakarma, Mrs Shail and Mohini. The study is greatly benefited from comments and critical review by Prof. M. S. Srinivasan, and valuable support from Prof B. K. Chatterjee, Head of my Department, Dr H. P. Sengupta and the Council of Scientific and Industrial Research.
Rajesh K. Vishwakarma is in the Department of Geology, Banaras Hindu University, Varanasi 221 005, India.
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