3 General Features of Insect History
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3 General Features of Insect History 3.1 respectively, events that are hardly possible to be dated precisely). Indeed, the constant number of taxa recorded in two succeeding time Dynamics of Insect Taxonomic intervals may be due to the absence of any change, or may be due to high Diversity but equal numbers of taxic origins and extinctions (equal taxic birth and death rates). V.YU. DMITRIEV AND A.G. PONOMARENKO Because of these and related problems discussed at length by ALEKSEEV et al. (2001), a few relatively safe and informative methods have been selected. One of them is the momentary (= instantaneous) diversity on the boundary of two adjacent stratigraphic units which is The study of past dynamics of the taxonomic diversity is a complicated calculated as the number of taxa crossing the boundary (recorded both job, threatened by numerous traps and caveats connected mostly with above and below it). This permits us to avoid or minimise the errors improper or insufficiently representative material and inappropriate cal- caused by the unequal duration of the stratigraphic units, as well as by culating methods (ALEKSEEV et al. 2001). It is self-evident that the taxa irregular distribution of the exceptionally rich fossil sites that fill the used as operational units in the diversity calculation should be long liv- fossil record with numerous short lived taxa, a kind of noise in ing enough to have their duration at least roughly recordable in the fossil diversity dynamics research. record available, and yet not too long living to display their appreciable Instantaneous diversity can be plotted against the time scale turnover. Accumulated experience (DMITRIEV & ZHERIKHIN 1988, (Fig. 478a) to display directly the temporal dynamics of taxonomic RASNITSYN 1988b, I989a, JARZEMBOWSKI & Ross 1993, 1996, DMITRIEV diversity. Otherwise it can be plotted against the cumulative number of et al. 1994a,b, 1995, LABANDEIRA & SEPKOSKI 1993, LABANDEIRA 1994, taxa in the group up to the given time (Fig. 478b). The advantage of JARZEMBOWSKI & Ross 1996) confirms that insect families best satisfy this method is that the resulting chart, though ignoring duration of the the above conditions given the present state of our knowledge. stratigraphic units, displays separately both the gain and loss dynam- To select the appropriate calculation method of the diversity dynam- ics. The line connecting the succeeding time moments shows the num- ics, a larger discussion is necessary. The first methods coming to mind ber of gains (first appearances of taxa) when measured along the is just to compare the number of taxa registered at the succeeding abscissa, and the difference between the number of gains and losses stages of the fossil record, for different stratigraphic intervals vary (first and last appearances of taxa) when measured against the ordi- widely in completeness of their palaeontological study. Therefore, the nate. As a result, in case of pure losses the line will be directed straight number of taxa recorded at a time interval depends on both their past downwards, in case of pure gains – 45° obliquely upwards, in other diversity, their taphonomic properties (chance to enter the fossil cases – in intermediate directions. record, see Chapter 1.4), and general state of knowledge of respective Other calculation methods used employ numbers of gains and losses sections of the fossil record, and these different influences are hardly plotted separately against the time scale (Figs. 479a,b), and the share separable. In addition, the fossils are usually recorded after their strati- (per cent) of extinct families in the total insect assemblage (Fig. 480), graphic position in a particular stratigraphic unit (period, epoch, etc.), otherwise known as inverted Lyellian curves (ZHERIKHIN 1978, rather than by indication of their absolute age in millions of years. RASNITSYN 1988b, 1989a). This method displays how a group The stratigraphic units differ in their absolute time duration, so the approaches its contemporary taxonomic composition. longer unit is expected to have the higher number of taxa, an effect The present calculations are based on the fossil record database deserving to be counterbalanced. The stratigraphic data vary widely in in progress at the Arthropoda Laboratory, Palaeontological Institute, their level of detail and subdivision: one taxon might be recorded just Russian Academy of Sciences. 1049 families are considered, less as the Cretaceous, while another as a particular zone within the than the 1261 referred to by LABANDEIRA (1994): the extra 212 families Hauterivian, one of several stages of the Early Cretaceous. These are considered synonyms in our list. The stage is taken as the sources of uncertainty result in problems that are not easy to overcome. stratigraphic unit of the family duration. The families whose first/last Further, dealing with just the number of taxa recorded results in appearance is dated less precisely are not ignored but interpolated considerable loss of information compared to the separate use of the first in proportion of the precisely dated families. E.g. if an epoch consists of and last appearances of taxa (substitute for their origin and extinction. 3 stages with 10, 1, and 40 properly dated families, and 5 more 325 HISTORY OF INSECTS 326 GENERAL FEATURES OF INSECT HISTORY families attributed simply to this epoch, the total distribution taken for respectively. Long-term deviations in the rate of this sort are clearly calculation is 11, 1, 44. apparent, but the short term tendencies are difficult to appreciate cor- Figure 478 shows the dynamics of the instantaneous number of the rectly because of errors in identification of the duration of the taxo- insect families plotted against the time (a) and the cumulative number nomic and stratigraphic units involved. of family appearances (b). Both curves have a rather simple form. The The curves in Fig. 479 are of a simple form as well. Interesting is the family number (a) is growing slowly in the Carboniferous – Triassic near the linear part of the first appearance curve from its beginning resulting in the near-horizontal pace of the curve. From the Jurassic until the Middle Eocene. In contrast, the last appearance curve (b) is until the mid-Eocene, the growth is more rapid but almost linear, essentially convex throughout implying the largely decreasing extinc- although the norm of diversity accumulation (b) is not constant: it is tion rates – to nearly 0 in the Cainozoic. Details of the two curves higher in the Jurassic, but becomes slower near the Jurassic– shed additional light to the curves in Fig. 478. Low diversity in Cretaceous boundary, and then grows again to reach 1 in the the Palaeozoic is connected with extinction rates which are Cainozoic. The incomparably full record of Baltic amber fossils gives distinctly high compared to those of the Mesozoic. Lower diversity a high step in the curve (a), then the growth decreases towards the combined with high tempo of origination and extinction means a present time, though it is still very high in the Oligocene. In contrast, high rate of the taxonomic turnover resulting in the appearance of the Cainozoic growth of (b) is linear. numerous short-lived families: these are very characteristic of the Figure 479 gives further explanation of the above curves. It displays Palaeozoic insect fauna. the number of the first (a) and last appearances (b) of families since Since the Triassic, or maybe since the latest Permian, both origination the Serpukhovian Stage (latest Early Carboniferous). The slope and extinction rates decrease appreciably, and since near the beginning of the curves depends on the rates of family origin and extinction, of the Triassic, the family number curves show the start of the Fig. 478 Insect diversification: a – time versus instantaneous number of families, b – total number of families appearing at a given time versus instantaneous num- ber of families. In figures 478–482, the standard East European stratigraphic scale is used. Dots within the stratigraphic intervals denote, left to right, beginning of the stages (marked along the horizontal axis by the first letter of their names): in the Carboniferous – Serpukhovian, Bashkirian, Moscovian and Kasimovian; in the Permian – Asselian, Sakmarian, Artinskian, Kungurian, Ufimian, Kazanian and Tatarian; in the Triassic – Indian, Olenekian, Anisian, Ladinian + Karnian (because the richest Madygen deposits are not dated within that interval), Norian and Rhaetian. Jurassic material is often dated not precisely enough to permit calculating events on the strict stage boundaries: instead, the figures and respective dots are attributed here to imprecisely defined early, middle and late part of each the Early, Middle, and Late Jurassic. For the Early Cretaceous, dots represent beginnings, left to right, of the lower and upper halves of the Neocomian, Aptian and Albian, for the Late Cretaceous – beginnings of the Cenomanian, Turonian, Coniacian, Santonian, Campanian and Maastrichtian, for the Palaeogene – beginnings of the Danian, Montian, Thanetian, Ypresian, Lutetian, Priabonian, Rupelian and Chattian, in the Neogene – beginnings of the Early, Middle and Late Miocene and Pliocene. The right-most dot denotes the present time. Because of too limited records, dots representing some boundaries may be omitted or, in Fig. (b) coincident 327 HISTORY OF INSECTS long-term Meso-Cainozoic growth. Though, towards the end of the with the minimum diversity near the Perm-Triassic boundary, it delim- Triassic it has not surpassed the Permian value. The near linear growth its the distinct Palaeozoic stage, as opposed to the Mesozoic one, of of family number with time (Fig. 478a), in the scale of the accumu- insect taxonomic turnover. The basic pattern in other organisms: in lated first appearances (Fig. 478b), breaks down into several character- marine animals as a whole and their various subgroups, in freshwater istic parts. The curves (Fig. 479a,b) reveal the structure of this process. animals (DMITRIEV et al. 1995), in non-marine tetrapods (ALEKSEEV In the Early and Middle Jurassic, extinction rale decreases sharply, and et al.