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Taxonomic diversity structure of Silurian crinoids: Stability versus dynamism
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Annales de Paléontologie 97 (2011) 87–98
Original article
Taxonomic diversity structure of Silurian crinoids: Stability versus dynamism La structure de la diversité taxinomique des crinoïdes siluriens : stabilité contre dynamisme Dmitry A. Ruban ∗ Division of Mineralogy and Petrography, Geology and Geography Faculty, Southern Federal University, Zorge Street 40, Rostov-na-Donu, 344090, Russian Federation Available online 12 November 2011
Abstract Taxonomic diversity structure of crinoids is measured as a number of genera in orders for each epoch of the Silurian. Changes in this structure are evaluated with two similarity indices, namely the Czekanowski’s Quantified Coefficient and the Gower Index. Dynamics of the taxonomic diversity structure of crinoids was slow in the Silurian. The relatively strong turnovers occurred at the Ordovician – Silurian and Silurian – Devonian transitions. In contrast, the Wenlock – Ludlow transition was marked by an outstanding stability. When assemblages of non-successive epochs are considered, a persistence of similarity in this structure and a possibility of its re-appearance also indicate a certain stability. Nonetheless, some patterns of dynamism are also reported. The noted stability persisted despite palaeoenvironmental changes and biotic crises (like the end-Ordovician mass extinction and the Ireviken event). This highlights the role of intrinsic (biological) factors in the Silurian evolution of crinoids. © 2011 Elsevier Masson SAS. All rights reserved. Keywords: Crinoidea; Diversity; Biotic events; Evolution; Silurian
Résumé La structure de la diversité taxonomique des crinoïdes est mesurée à partir du nombre de genres dans chaque ordre pour chacune des époques du Silurien. Les variations de cette structure sont évaluées avec deux indices de similarité, respectivement le Coefficient Quantifié de Czekanowski et l’indice de Gower. La dynamique de renouvellement de la diversité taxonomique des crinoïdes était lente au cours du Silurien. Des renouvellements relativement plus forts sont intervenus aux transitions Ordovicien – Silurien et Silurien – Devonien. À l’inverse, la transition Wenlock – Ludlow fut marquée par une remarquable stabilité. Quand on considère les assemblages d’époques non-successives, une persistance de la similarité de la ∗
Corresponding author. P.O. Box 7333, Rostov-na-Donu, 344056, Russian Federation. E-mail address: [email protected]
0753-3969/$ – see front matter © 2011 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.annpal.2011.09.001
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structure de la diversité et la possibilité de sa réapparition montrent également une certaine stabilité. Cette stabilité persiste malgré les changements paléoenvironnementaux et les crises biologiques (telles l’extinction de masse de la fin de l’Ordovicien et l’événement Ireviken). Cela souligne le rôle des facteurs intrinsèques (biologiques) dans l’évolution des crinoïdes au Silurien. © 2011 Elsevier Masson SAS. Tous droits réservés. Mots clés : Crinoïde ; Diversité ; Événements biotiques ; Évolution ; Silurien
1. Introduction Investigations of early Paleozoic crinoids (Ausich and Peters, 2005; Baumiller, 1993; Deline and Ausich, 2011; Foote, 1994; Peters and Ausich, 2008; Simpson, 2010) provide some clues for a deeper understanding of the evolution of all marine invertebrates and their responses to the changing environment. For instance, the influence of the end-Ordovinian mass extinction on these fossils is debating (Ausich and Peters, 2005; Deline and Ausich, 2011). Generally, the diversity of crinoids rose in the early Silurian and declined in the late Silurian (Fig. 1), which coincided with a cyclic change of the whole biodiversity established by Purdy (2008) and did not coincide with a gradual diversification recorded by the sample-standardized curve of Alroy et al. (2008). The present paper aims at recognition of new patterns in the diversity dynamics of Silurian crinoids. The idea of taxonomic diversity structure, which permits to reveal changes in the importance of higher-ranked taxa (e.g., orders) for the diversity of lower-ranked taxa (e.g., genera), is a useful tool for studies of linear and non-linear effects in the evolution of particular fossils groups (Ruban, 2009). Crinoids were rich in both orders and genera during the Silurian, and, thus, their diversity can be explored with the noted tool. Results from this study may permit also to discuss the possible influences of major biotic events (e.g., the end-Ordovician mass extinction or the Ireviken event) and sharp palaeoenvironmental changes on the evolution of Silurian crinoids. 2. Materials and methods For the purpose of this study, data on distribution of 367 crinoid genera belonging to six orders are extracted from the database of Sepkoski (2002) (also available on-line: http://strata.geology.wisc.edu/jack/start.php). The number of genera in each order is established for all four Silurian epochs (Table 1). The late Ordovician and early Devonian crinoids are also considered in order to measure turnovers at the boundaries of the Silurian period. The data compiled by Sepkoski (2002) are representative, but they deserve a certain criticism (Ausich and Peters, 2005). However, taxonomic and stratigraphic inaccuracies are inevitable, and the only voluminous in-depth revision of the knowledge on early Paleozoic crinoids (e.g., in the case of updating the “Treatise on Invertebrate Paleontology”) will permit to avoid them partly. It appears that updated datasets used in the analyses of Ausich and Peters (2005), Peters and Ausich (2008), and Deline and Ausich (2011) encompass only a part of the stratigraphic interval considered in this study. Theoretically, some update of information about all Silurian crinoids is possible with a compilation of numerous data dispersed in published papers, but an alternative (and easier) way is to follow some recommendations of Ruban and van Loon (2008) to avoid possible
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Fig. 1. Silurian chronostratigraphy (after Ogg et al., 2008), crinoid diversity (on the basis of data from Sepkoski, 2002; see Table 1), and global sea level changes (1: after Haq and Schutter, 2008; 2: after Johnson, 2006). Important biotic events discussed in the text (not necessarily important for crinoids) are also indicated. Chronostratigraphie du Silutien (d’après Ogg et al., 2008) ; diversité des crinoïdes (d’après les données de Sepkoski, 2002, voir Tableau 1), et variations globales du niveau de la mer (1 : d’après Haq et Schutter, 2008 ; 2 : d’après Johnson, 2006).
errors and misinterpretations. First, the time resolution of the data analysis can be reduced. This helps to increase the accuracy of data as proven by Ausich and Peters (2005). Thus, only epochs are considered in the present study. Second, possible errors can be accounted in interpretations of results. In the present study, such errors are supposed to be less (because of lower time resolution at some intervals) than suggested by Ausich and Peters (2005). These authors realized that the
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Table 1 Order-genus diversity structure of late Ordovician – early Devonian crinoids. The number of genera is indicated for each order in given epoch. Original data from Sepkoski (2002) (see also: http://strata.geology.wisc.edu/jack/start.php). Structure de la diversité des ordres et genres de crinoïdes pour l’intervalle Ordovicien Supérieur – Dévonien inférieur. Le nombre de genres est indiqué pour chaque ordre à chaque époque. Les données originales viennent de Sepkoski (2002) (voir aussi http://strata.geology.wisc.edu/jack/start.php). Orders
O3
S1
S2
S3
S4
D1
CLADIDA DIPLOBATHRIDA DISPARIDA HYBOCRINIDA MONOBATHRIDA SAGENOCRINIDA
20 26 40 4 11 3
8 17 16 1 27 4
22 15 19 0 33 19
19 12 18 0 28 19
11 2 11 0 20 8
41 17 25 0 45 9
Epoch abbreviations: O3: late Ordovician; S1: Llandovery; S2: Wenlock; S3: Ludlow; S4: Pˇridoli; D1: Early Devonian.
error in the database of Sepkoski (2002) is 44% at the substage level and 32% at the stage level. This permits to anticipate the error of about 20-30% (or even less) at the level of epoch. However, it should be pointed out that the importance of new palaeontological information for updating the earlier biodiversity reconstructions remains debating (Ausich and Peters, 2005; Nardin et al., 2005; Ruban, 2005), and it is not excluded that true errors may be somewhat less important than expected. The main tool employed in this study is the same as used by Ruban (2009) for brachiopods. The taxonomic diversity structure establishes how diverse was each major taxon of a given group in each considered interval of the geologic time. When such information is available for several time intervals (e.g., epochs), dynamics of the taxonomic diversity structure can be measured with similarity indices. The most suitable are the Czekanowski’s Quantified Coefficient (QC) (Sepkoski, 1974) and the Gower Index (G) (Gower, 1971). Their values vary between 0 and 1. The higher value means higher similarity between the taxonomic diversity structure, and, consequently, weaker turnover and weaker dynamics. The Rst index employed by Ruban (2009) is not used in the present study, because Silurian crinoids constituted only six orders, which is not sufficient for a correlation measurement. When QC and G are established for successive time intervals (e.g., the Wenlock and the Ludlow), this permits to recognize linear effects in the dynamics of the taxonomic diversity structure. But when establishing these indices is attempted for non-successive time intervals (e.g., the Llandovery and the early Devonian), this permits to recognize non-linear evolutionary effects (Ruban, 2009). Consideration of the both effects is important for conclusions about stability or dynamism of the taxonomic diversity structure. In the present study, the order-genera diversity structure of the late Ordovician – early Devonian crinoids (Table 1) is analyzed with the tool described above. Besides an analysis of the taxonomic diversity structure, “simple” turnovers among crinoid genera and orders are measured with the Jaccard similarity index (J) (Jaccard, 1901; Ruban, 2009). The value of this index also varies between 0 and 1, and higher values mark weaker turnovers. As in the case of QC and G, the J index can be applied to successive and non-successive time intervals. Further comparison of J dynamics with that of QC and G permits to judge about the importance of “simple” turnovers for changes in the taxonomic diversity structure (Ruban, 2009). In this paper, new chronostratigraphic developments made by the International Commission on Stratigraphy (Ogg et al., 2008; see also www.sratigraphy.org) are accounted when possible
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(Fig. 1). However, the time units used in the database of Sepkoski (2002) are also considered broadly. The present analysis is attached to the major chronostratigraphic units, namely epochs. It seems rather unnecessary to involve the absolute time units because of several reasons (cf. Ruban and van Loon, 2008). First, the palaeontological data are essentially justified according to the bioor chronostratigraphic scale, not absolute time scale. Second, the day’s and year’s length changed through the geologic history (e.g., Hinnov and Park, 1998; Nesterov, 1999), and, consequently, the duration of a given epoch estimated with methods of absolute dating does not match necessarily its “real” duration, i.e., the number of years included. Third, it is unclear whether this will be methodologically correct to transform the rate of dynamics of the taxonomic diversity structure established with similarity indices into the rate plotted against million of years. Nonetheless, the possible influence of the different absolute duration of epochs on results of the present study is specially addressed. 3. Results During the late Ordovician – early Devonian time interval, the taxonomic diversity structure of crinoids remained similar enough. The both QC and G values are higher than 0.5 for all successive epochs (Table 2). This means that changes in the entity of orders that dominated the generic diversity of this fossil group were relatively slow. These changes were the strongest at the Ordovician – Silurian and Silurian – Devonian transitions, whereas the Wenlock – Ludlow transition is marked by an outstanding stability (Fig. 2). Differences between the QC and G curves (Fig. 2) are minor in comparison with the possible error (Ausich and Peters, 2005; see also above), and, thus, they are insignificant. The above-mentioned linear effects can be well interpreted with the original dataset (Table 1). At the Ordovician–Silurian transition, the importance of Cladida and Disparida for the total generic diversity decreased, whereas that of Monobathrida, in contrast, increased. Generic diversity of Diplobathrida reduced, but this order remained important in the taxonomic diversity structure. At the Silurian – Devonian transition, Disparida and Sagenocrinida, despite radiation of the former and slight diversification of the latter, lost their significance for the total generic diversity (if even Disparida remained relatively important), whereas the significance Table 2 Similarity of crinoid assemblages, relevant to the Late Ordovician – Early Devonian epochs, measured for the order-genus diversity structure with the QC and G indices (QC above, G below). See Table 1 for epoch abbreviations. Similarités des assemblages de crinoïdes, pour les périodes successives s’échelonnant de l’Ordovicien Supérieur au Dévonien inférieur, mesurées pour la structure de la diversité ordre-genre avec le Coefficient Quantifié de Czekanowski (QC) et le Gower Index (G) (QC valeur du dessus, G valeur du dessous). Voir Tableau 1 pour les abréviations des époques.
O3 S1 S2 S3 S4
S1
S2
S3
S4
D1
0.63 0.73
0.64 0.53 0.77 0.63
0.63 0.54 0.79 0.63 0.94 0.93
0.49 0.52 0.72 0.62 0.65 0.66 0.70 0.69
0.63 0.52 0.69 0.59 0.80 0.78 0.74 0.73 0.55 0.62
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Fig. 2. Linear changes in the taxonomic diversity structure of Silurian crinoids. Variations linéaires de la structure de la diversité taxonomique des crinoïdes siluriens, estimées à partir du Coefficient Quantifié de Czekanowski (QC) et du Gower Index (G).
of Diplobathrida increased. The generic diversity of crinoid orders remained comparable during the Wenlock – Ludlow (Table 1). Consideration of non-successive epochs permits to outline, at least, three non-linear effects. First, the taxonomic diversity structure remained quite similar for all assemblages of the late Ordovician – early Devonian time interval (Table 2), which is an evidence of significant stability. Second, the Pˇridoli assemblage became especially dissimilar from the older assemblage than this occurred at the previous time intervals. This permits to hypothesize a more or less significant disruption in the history of crinoids. Their taxonomic diversity structure, apparently, lost some patterns inherited from the Ordovician. Third, the early Devonian assemblage is, again, quite similar to the older assemblages, except for that Pˇridoli. The highest values (∼ 0.8) of similarity are documented between the Wenlock and the early Devonian (Table 2). This is an evidence of that the taxonomic diversity structure, which persisted in the late Ordovician – early Silurian, re-appeared in the early Devonian, and the same crinoid orders (Cladida and Disparida), which dominated the total generic diversity before the Pˇridoli, re-gained their importance after this epoch. In other words, the Pˇridoli evolution of crinoids seems to be “anomalous”. The reappearance of the earlier-existed taxonomic diversity structure is an evidence of some evolutionary stability. As suggested by J values (Fig. 3), there were no “simple” turnovers in Silurian crinoid communities at the level of orders with the only exception of the Llandovery – Wenlock transition, when 1 order, namely Hybocrinida, disappeared (Table 1). In contrast, “simple” turnovers at the level of genera were strong. They were the strongest in the beginning and the end of the Silurian, but even in the mid-Silurian they remained strong (Fig. 3). An analysis of non-successive time intervals reveals stability at the level of orders and gradual decrease in similarity between the older and younger assemblages at the level of genera (Table 3). Comparing the values of J, QC, and G, it is possible to realize that the documented dynamics of the taxonomic diversity structure was unlikely produced by the only “simple” turnovers. However, the noted stability at the level of orders could contribute to the relative similarity of assemblages with respect to their diversity structure.
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Fig. 3. “Simple” turnovers in the evolution of Silurian crinoids. Renouvellements dans l’évolution des crinoïdes siluriens, estimés au niveau des ordres et au niveau des genres par l’indice de similarité de Jaccard (J). Table 3 Similarity of crinoid assemblages, relevant to the late Ordovician–early Devonian epochs, measured for orders (above) and genera (below) with the J index. See Table 1 for epoch abbreviations. Similarité des assemblages de crinoïdes, pour les périodes successives s’échelonnant de l’Ordovicien Supérieur au Dévonien inférieur, mesurées pour les ordres (valeur du dessus) et les genres (valeur du dessous) avec l’indice de Jaccard (J). Voir Tableau 1 pour les abréviations des époques.
O3 S1 S2 S3 S4
S1
S2
S3
S4
D1
1.00 0.10
0.83 0.06 0.83 0.23
0.83 0.04 0.83 0.17 1.00 0.46
0.83 0.03 0.83 0.12 1.00 0.26 1.00 0.47
0.83 0.02 0.83 0.07 1.00 0.13 1.00 0.20 1.00 0.29
4. Discussion During the Silurian, the global sea level experienced rises and falls of different frequency and magnitude (Johnson, 2006, 2008; Haq and Schutter, 2008) (Fig. 1). For instance, Haq and Schutter (2008) indicated major eustaic drops near the end of the Llandovery and at the Ludlow/Pˇridoli boundary. The climate changed, and the late Ordovician and early Wenlock glaciations occurred (Boucot, 2009; Brenchley et al., 2003; Delabroye and Vecoli, 2010; Ghienne, 2003; ˇ Le Heron and Craig, 2008; Lehnert et al., 2010; Nardin et al., 2011; Zigait˙ e et al., 2010). The oceanic environment underwent unprecedented and frequent perturbations throughout the entire Silurian (Aldridge et al., 1993; Jeppson et al., 1995; Jeppson and Aldridge, 2000; Jeppson and Calner, 2003). But despite of these significant palaeoenvironmental changes, the relative
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stability and the only weak dynamism of the taxonomic diversity structure of Silurian crinoids is documented. The latest Ordovician was characterized by a severe mass extinction, which stressed the entire marine biota (Eckert, 1988; Brenchley et al., 1995, 2003; Hallam and Wignall, 1997; Sheehan, 2001; Krug and Patzkowsky, 2007; Munnecke et al., 2010; Zhang et al., 2009). According to the data of Sepkoski (2002), the total generic diversity of crinoids reduced by 30%. Data interpretations by Ausich and Peters (2005) imply that the influence of this catastrophe on crinoids was not as significant as one could imagine. Earlier, on the basis of crinoid studies on the British Isles, Donovan and Harper (2003) also noted that some late Llandovery taxa are typical for the late Ordovician – Silurian interval, which is another evidence of weak influences of the end-Ordovician mass extinction, although this may be also explained by the location of the British Isles far from the areas affected by the end-Ordovician glaciation (Cocks and Torsvik, 2002). The present analysis of the taxonomic diversity structure suggests a turnover at the Ordovician–Silurian transition, which, however, was comparable in strength with the turnovers at other epoch transitions (Fig. 2). Moreover, the Silurian and even the early Devonian assemblages retained relatively high similarity with the taxonomic diversity structure of the late Ordovician assemblage (Table 2). If so, crinoids appear to be somehow resistive to this catastrophe. This differs partly from what is observed for brachiopods, which were evidently affected by the endOrdovician mass extinctions, although also not as strong as after later catastrophes (Ruban, 2009). In contrast, “simple” turnovers among crinoid genera measured with the J index were strong at the late Ordovician – Llandovery transition (Fig. 3), which is an evidence of the mass extinction influence. Surprisingly, some other echinoderms such as blastozoans (Nardin and Lefebvre, 2010) or stylophorans (Lefebvre et al., 2006) did not suffer strongly from the end-Ordovician mass extinction. This indicates a kind of homogeneity in the reaction of echinoderms to this catastrophe. The other significant biotic crisis was linked with the Ireviken event to occur near the Llandovery – Wenlock transition (Eriksson, 2006; Lehnert et al., 2010; Ruban, 2008). Results from the present analysis (Fig. 2, Table 2) do not indicate any outstanding linear or non-linear effects related to this transition. The G similarity dropped slightly (Table 2), but differences between the QC and G values are linked, probably, with the differences of the methods of their calculation themselves, and they are below the possible error (Ausich and Peters, 2005; see also above). Thus, the noted drop of G is unimportant. This does not mean, however, that the Ireviken event did not affect crinoids. As suggested by the available data (Table 1), none Hybocrinida crossed the Llandovery/Wenlock boundary, whereas Cladida and Sagenocrinida experienced a spectacular radiation. “Simple” turnovers among crinoid genera were also strong near this boundary (Fig. 3), which suggests a strength of the relevant biotic perturbation. The Mulde event occurred in the late Wenlock (Jeppson and Calner, 2003; Calner, 2005a). According to our results (Table 2, Fig. 2), it is very unlikely that it affected the taxonomic diversity structure of crinoids, which remained stable in the mid-Silurian. The Lau event took place in the late Ludlow (Calner, 2005a,b). It is not excluded that it was responsible for a decrease in the similarity of the Ludlow and Pˇridoli assemblages (Fig. 2) as well as for the dissimilarity of the Pˇridoli assemblage from some older assemblages (Table 2; see above). However, the values of the QC and G indices remain rather high, and the similarity between older and younger assemblages re-appeared in the early Devonian (Table 2). Thus, the possible influence of the Lau event on crinoids cannot be judged catastrophic with regard to the taxonomic diversity structure. “Simple” turnovers among crinoid genera were minimal when the Mulde and Lau events took place (Fig. 3).
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If none of the above-mentioned palaeoenvironmental factors and biotic events stressed crinoids so strong to provoke major changes in their taxonomic diversity structure, intrinsic (biological) mechanisms can be favored as the main evolutionary force of this group through the Silurian period. This is also supported by the above-mentioned homogenous resistivity of echinoderms to such an outstanding perturbation as the end-Ordovician mass extinction. Our results do not allow to reveal any especially strong turnover (“revolution”) in the Silurian history of crinoids, and the documented patterns resemble the phenomenon of coordinated stasis (Brett and Baird, 1992; Morris et al., 1995; Brett et al., 1996). However, the values of the similarity indices are only relatively high, and sometimes they are below 0.7 (Table 2), which indicates some gradual changes in the taxonomic diversity structure of crinoids through the Silurian (this echoes the note of Deline and Ausich (2011), who judged crinoid evolution as not static in the Silurian). But, on the other hand, the relatively high values of the both QC and G do not reject the idea on punctuated equilibrium advocated by Eldredge and Gould (1972) and Gould (2002, 2007). Better to say, our results coincide with the idea of the heterogeneity of the evolutionary dynamics through the geologic time (Hunt, 2008). One may hypothesize that the results of the present study are affected by the different absolute duration of the epochs. A longer epoch would give more possibilities for crinoids to evolve, and, consequently, the relevant assemblage will be more dissimilar from others than that of a short epoch. Such a hypothesis can be tested with two examples. The late Ordovician and the Llandovery were relatively long epochs, which lasted 17.2 Ma and 15.5 Ma respectively (Ogg et al., 2008). The similarity of the relevant crinoid assemblages was diminished, but just slightly (Fig. 3). The Pˇridoli epoch was relatively short and lasted just 2.7 Ma (Ogg et al., 2008). However, the relevant crinoid assemblage was dissimilar from those earlier and later existed by the same degree as was the late Ordovician assemblage (Fig. 3). Thus, it is unlikely that differences in the absolute duration of epochs affects the results presented above strongly. Finally, it should be noted that a low-resolution analysis is employed in this study. It seems to be appropriate for detecting some general trends, but the discussion of the influences of particular biotic events on crinoids requires further detailed studies. E.g., the noted Mulde and Lau events took place within the Wenlock and the Ludlow epochs respectively. It cannot be excluded that they disturbed the taxonomic diversity structure for only short terms. 5. Conclusions The present study of Silurian crinoids permits two important conclusions: • the taxonomic diversity structure of these fossils was characterized by a certain stability during the Silurian, although weak dynamism also took place; • Silurian crinoids might have been resistive to some palaeoenvironmental perturbations and biotic crises, which raises an importance of intrinsic, i.e., biotic, controls on their evolution. Results of many quantitative palaeobiological studies like the present one remain valid until new corrections in the fossil systemartics occur. Therefore, further taxonomic and stratigraphic improvements of the information on crinoids (e.g., together with the revision of the relevant volumes of “Treatise on Invertebrate Paleontology”) will allow to modify our conclusions and to increase the time resolution of the analysis of the taxonomic diversity structure of Silurian crinoids.
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Acknowledgements The author gratefully thanks D. Néraudeau (France) and an anonymous reviewer for their thorough consideration of this article, valuable suggestions, improvements, and French translations. Additionally, R.J. Aldridge (UK), M. Calner (Sweden), M.E. Johnson (USA), P. Novack-Gottshall (USA), W. Riegraf (Germany), A.J. van Loon (Netherlands/Poland), and other colleagues are acknowledged for useful communications and help with literature. References Aldridge, R.J., Jeppson, L., Dorning, K.J., 1993. Early Silurian oceanic episodes and events. Journal of the Geological Society, London 150, 501–513. Alroy, J., Aberhan, M., Bottjer, D.J., Foote, M., Fürsich, F.T., Harries, P.J., Hendy, A.J.W., Holland, S.M., Ivany, L.C., Kiessling, W., Kosnik, M.A., Marshall, C.R., McGowan, A.J., Miller, A.I., Olszewski, T.D., Patzkowsky, M.E., Peters, S.E., Viller, L., Wagner, P.J., Bonuso, N., Borkow, P.S., Brenneis, B., Clapham, M.E., Fall, L.M., Ferguson, C.A., Hanson, V.L., Krug, A.Z., Layou, K.M., Leckey, E.H., Nürnberg, S., Powers, C.M., Sessa, J.A., Simpson, C., Tomaˇsov´ych, A., Visaggi, C.C., 2008. Phanerozoic trends in the global diversity of marine invertebrates. Science 321, 97–100. Ausich, W.I., Peters, S.E., 2005. A revised macroevolutionary history for Ordovician–early Silurian crinoids. Paleobiology 31, 538–551. Baumiller, T.K., 1993. Survivorship analysis of paleozoic crinoidea: effect of filter morphology on evolutionary rates. Paleobiology 19, 304–321. Boucot, A.J., 2009. Early paleozoic climates (Cambrian-Devonian). In: Gornitz, V. (Ed.), Encyclopedia of paleoclimatology and ancient environments. Springer, Dordrecht (pp. 291–293). Brenchley, P.J., Carden, G.A.F., Marshall, J.D., 1995. Environmental changes associated with the “first strike” of the late Ordovician mass extinction. Modern Geology 20, 69–82. Brenchley, P.J., Carden, G.A., Hints, L., Kaljo, D., Marshall, J.D., Martma, T., Meidla, T., Nolvak, J., 2003. Highresolution stable isotope stratigraphy of upper Ordovician sequences: constraints on the timing of bioevents and environmental changes associated with mass extinction and glaciation. Geological Society of America Bulletin 115, 89–104. Brett, C.E., Baird, G.C., 1992. Coordinated stasis and evolutionary ecology of Silurian-Devonian marine biotas in the Appalachian Basin. Geological Society of America, Abstracts and Programs 24, 139. Brett, C.E., Ivany, L.C., Schopf, K.M., 1996. Coordinated stasis: an overview. Palaeogeography, Palaeoclimatology, Palaeoecology 127, 1–20. Calner, M., 2005a. Silurian carbonate platforms and extinction events - ecosystem changes exemplified from Gotland, Sweden. Facies 51, 603–610. Calner, M., 2005b. A late Silurian extinction event and anachronistic period. Geology 33, 305–308. Cocks, L.R.M., Torsvik, T.H., 2002. Earth geography from 500 to 400 million years ago: a faunal and palaeomagnetic review. Journal of the Geological Society, London 159, 631–644. Delabroye, A., Vecoli, M., 2010. The end-Ordovician glaciation and the Hirnantian stage: a global review and questions about late Ordovician event stratigraphy. Earth Science Reviews 98, 269–282. Deline, B., Ausich, W.I., 2011. Testing the plateau: a reexamination of disparity and morphologic constraints in early Paleozoic crinoids. Paleobiology 37, 214–236. Donovan, S.K., Harper, D.A.T., 2003. Llandovery Crinoidea of the British Isles, including description of a new species from the Kilbride formation (Telychian) of western Ireland. Geological Journal 38, 85–97. Eckert, J.D., 1988. Late Ordovician extinction of North American and British crinoids. Lethaia 21, 147–167. Eldredge, N., Gould, S.J., 1972. Punctuated equilibria: an alternative to phyletic gradualism. In: Schoft, T.J.M. (Ed.), Models in Paleobiology. Freeman Cooper, San Francisco (pp. 82–115). Eriksson, M.E., 2006. The Silurian Ireviken event and vagile benthic faunal turnovers (Polychaeta; Eunicida) on Gotland, Sweden. GFF 128, 91–95. Foote, M., 1994. Morphological disparity in Ordovician–Devonian crinoids and the early saturation of morphological space. Paleobiology 20, 320–344. Ghienne, J.-F., 2003. Late Ordovician sedimentary environments, glacial cycles, and post-glacial transgression in the Taoudeni Basin, West Africa. Palaeogeography, Palaeoclimatology, Palaeoecology 189, 117–145. Gould, S.J., 2002. The structure of evolutionary theory. Belknap Press, Cambridge (pp. 1433).
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Annales de Paléontologie 97 (2011) 87–98
Original article
Taxonomic diversity structure of Silurian crinoids: Stability versus dynamism La structure de la diversité taxinomique des crinoïdes siluriens : stabilité contre dynamisme Dmitry A. Ruban ∗ Division of Mineralogy and Petrography, Geology and Geography Faculty, Southern Federal University, Zorge Street 40, Rostov-na-Donu, 344090, Russian Federation Available online 12 November 2011
Abstract Taxonomic diversity structure of crinoids is measured as a number of genera in orders for each epoch of the Silurian. Changes in this structure are evaluated with two similarity indices, namely the Czekanowski’s Quantified Coefficient and the Gower Index. Dynamics of the taxonomic diversity structure of crinoids was slow in the Silurian. The relatively strong turnovers occurred at the Ordovician – Silurian and Silurian – Devonian transitions. In contrast, the Wenlock – Ludlow transition was marked by an outstanding stability. When assemblages of non-successive epochs are considered, a persistence of similarity in this structure and a possibility of its re-appearance also indicate a certain stability. Nonetheless, some patterns of dynamism are also reported. The noted stability persisted despite palaeoenvironmental changes and biotic crises (like the end-Ordovician mass extinction and the Ireviken event). This highlights the role of intrinsic (biological) factors in the Silurian evolution of crinoids. © 2011 Elsevier Masson SAS. All rights reserved. Keywords: Crinoidea; Diversity; Biotic events; Evolution; Silurian
Résumé La structure de la diversité taxonomique des crinoïdes est mesurée à partir du nombre de genres dans chaque ordre pour chacune des époques du Silurien. Les variations de cette structure sont évaluées avec deux indices de similarité, respectivement le Coefficient Quantifié de Czekanowski et l’indice de Gower. La dynamique de renouvellement de la diversité taxonomique des crinoïdes était lente au cours du Silurien. Des renouvellements relativement plus forts sont intervenus aux transitions Ordovicien – Silurien et Silurien – Devonien. À l’inverse, la transition Wenlock – Ludlow fut marquée par une remarquable stabilité. Quand on considère les assemblages d’époques non-successives, une persistance de la similarité de la ∗
Corresponding author. P.O. Box 7333, Rostov-na-Donu, 344056, Russian Federation. E-mail address: [email protected]
0753-3969/$ – see front matter © 2011 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.annpal.2011.09.001
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structure de la diversité et la possibilité de sa réapparition montrent également une certaine stabilité. Cette stabilité persiste malgré les changements paléoenvironnementaux et les crises biologiques (telles l’extinction de masse de la fin de l’Ordovicien et l’événement Ireviken). Cela souligne le rôle des facteurs intrinsèques (biologiques) dans l’évolution des crinoïdes au Silurien. © 2011 Elsevier Masson SAS. Tous droits réservés. Mots clés : Crinoïde ; Diversité ; Événements biotiques ; Évolution ; Silurien
1. Introduction Investigations of early Paleozoic crinoids (Ausich and Peters, 2005; Baumiller, 1993; Deline and Ausich, 2011; Foote, 1994; Peters and Ausich, 2008; Simpson, 2010) provide some clues for a deeper understanding of the evolution of all marine invertebrates and their responses to the changing environment. For instance, the influence of the end-Ordovinian mass extinction on these fossils is debating (Ausich and Peters, 2005; Deline and Ausich, 2011). Generally, the diversity of crinoids rose in the early Silurian and declined in the late Silurian (Fig. 1), which coincided with a cyclic change of the whole biodiversity established by Purdy (2008) and did not coincide with a gradual diversification recorded by the sample-standardized curve of Alroy et al. (2008). The present paper aims at recognition of new patterns in the diversity dynamics of Silurian crinoids. The idea of taxonomic diversity structure, which permits to reveal changes in the importance of higher-ranked taxa (e.g., orders) for the diversity of lower-ranked taxa (e.g., genera), is a useful tool for studies of linear and non-linear effects in the evolution of particular fossils groups (Ruban, 2009). Crinoids were rich in both orders and genera during the Silurian, and, thus, their diversity can be explored with the noted tool. Results from this study may permit also to discuss the possible influences of major biotic events (e.g., the end-Ordovician mass extinction or the Ireviken event) and sharp palaeoenvironmental changes on the evolution of Silurian crinoids. 2. Materials and methods For the purpose of this study, data on distribution of 367 crinoid genera belonging to six orders are extracted from the database of Sepkoski (2002) (also available on-line: http://strata.geology.wisc.edu/jack/start.php). The number of genera in each order is established for all four Silurian epochs (Table 1). The late Ordovician and early Devonian crinoids are also considered in order to measure turnovers at the boundaries of the Silurian period. The data compiled by Sepkoski (2002) are representative, but they deserve a certain criticism (Ausich and Peters, 2005). However, taxonomic and stratigraphic inaccuracies are inevitable, and the only voluminous in-depth revision of the knowledge on early Paleozoic crinoids (e.g., in the case of updating the “Treatise on Invertebrate Paleontology”) will permit to avoid them partly. It appears that updated datasets used in the analyses of Ausich and Peters (2005), Peters and Ausich (2008), and Deline and Ausich (2011) encompass only a part of the stratigraphic interval considered in this study. Theoretically, some update of information about all Silurian crinoids is possible with a compilation of numerous data dispersed in published papers, but an alternative (and easier) way is to follow some recommendations of Ruban and van Loon (2008) to avoid possible
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Fig. 1. Silurian chronostratigraphy (after Ogg et al., 2008), crinoid diversity (on the basis of data from Sepkoski, 2002; see Table 1), and global sea level changes (1: after Haq and Schutter, 2008; 2: after Johnson, 2006). Important biotic events discussed in the text (not necessarily important for crinoids) are also indicated. Chronostratigraphie du Silutien (d’après Ogg et al., 2008) ; diversité des crinoïdes (d’après les données de Sepkoski, 2002, voir Tableau 1), et variations globales du niveau de la mer (1 : d’après Haq et Schutter, 2008 ; 2 : d’après Johnson, 2006).
errors and misinterpretations. First, the time resolution of the data analysis can be reduced. This helps to increase the accuracy of data as proven by Ausich and Peters (2005). Thus, only epochs are considered in the present study. Second, possible errors can be accounted in interpretations of results. In the present study, such errors are supposed to be less (because of lower time resolution at some intervals) than suggested by Ausich and Peters (2005). These authors realized that the
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Table 1 Order-genus diversity structure of late Ordovician – early Devonian crinoids. The number of genera is indicated for each order in given epoch. Original data from Sepkoski (2002) (see also: http://strata.geology.wisc.edu/jack/start.php). Structure de la diversité des ordres et genres de crinoïdes pour l’intervalle Ordovicien Supérieur – Dévonien inférieur. Le nombre de genres est indiqué pour chaque ordre à chaque époque. Les données originales viennent de Sepkoski (2002) (voir aussi http://strata.geology.wisc.edu/jack/start.php). Orders
O3
S1
S2
S3
S4
D1
CLADIDA DIPLOBATHRIDA DISPARIDA HYBOCRINIDA MONOBATHRIDA SAGENOCRINIDA
20 26 40 4 11 3
8 17 16 1 27 4
22 15 19 0 33 19
19 12 18 0 28 19
11 2 11 0 20 8
41 17 25 0 45 9
Epoch abbreviations: O3: late Ordovician; S1: Llandovery; S2: Wenlock; S3: Ludlow; S4: Pˇridoli; D1: Early Devonian.
error in the database of Sepkoski (2002) is 44% at the substage level and 32% at the stage level. This permits to anticipate the error of about 20-30% (or even less) at the level of epoch. However, it should be pointed out that the importance of new palaeontological information for updating the earlier biodiversity reconstructions remains debating (Ausich and Peters, 2005; Nardin et al., 2005; Ruban, 2005), and it is not excluded that true errors may be somewhat less important than expected. The main tool employed in this study is the same as used by Ruban (2009) for brachiopods. The taxonomic diversity structure establishes how diverse was each major taxon of a given group in each considered interval of the geologic time. When such information is available for several time intervals (e.g., epochs), dynamics of the taxonomic diversity structure can be measured with similarity indices. The most suitable are the Czekanowski’s Quantified Coefficient (QC) (Sepkoski, 1974) and the Gower Index (G) (Gower, 1971). Their values vary between 0 and 1. The higher value means higher similarity between the taxonomic diversity structure, and, consequently, weaker turnover and weaker dynamics. The Rst index employed by Ruban (2009) is not used in the present study, because Silurian crinoids constituted only six orders, which is not sufficient for a correlation measurement. When QC and G are established for successive time intervals (e.g., the Wenlock and the Ludlow), this permits to recognize linear effects in the dynamics of the taxonomic diversity structure. But when establishing these indices is attempted for non-successive time intervals (e.g., the Llandovery and the early Devonian), this permits to recognize non-linear evolutionary effects (Ruban, 2009). Consideration of the both effects is important for conclusions about stability or dynamism of the taxonomic diversity structure. In the present study, the order-genera diversity structure of the late Ordovician – early Devonian crinoids (Table 1) is analyzed with the tool described above. Besides an analysis of the taxonomic diversity structure, “simple” turnovers among crinoid genera and orders are measured with the Jaccard similarity index (J) (Jaccard, 1901; Ruban, 2009). The value of this index also varies between 0 and 1, and higher values mark weaker turnovers. As in the case of QC and G, the J index can be applied to successive and non-successive time intervals. Further comparison of J dynamics with that of QC and G permits to judge about the importance of “simple” turnovers for changes in the taxonomic diversity structure (Ruban, 2009). In this paper, new chronostratigraphic developments made by the International Commission on Stratigraphy (Ogg et al., 2008; see also www.sratigraphy.org) are accounted when possible
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(Fig. 1). However, the time units used in the database of Sepkoski (2002) are also considered broadly. The present analysis is attached to the major chronostratigraphic units, namely epochs. It seems rather unnecessary to involve the absolute time units because of several reasons (cf. Ruban and van Loon, 2008). First, the palaeontological data are essentially justified according to the bioor chronostratigraphic scale, not absolute time scale. Second, the day’s and year’s length changed through the geologic history (e.g., Hinnov and Park, 1998; Nesterov, 1999), and, consequently, the duration of a given epoch estimated with methods of absolute dating does not match necessarily its “real” duration, i.e., the number of years included. Third, it is unclear whether this will be methodologically correct to transform the rate of dynamics of the taxonomic diversity structure established with similarity indices into the rate plotted against million of years. Nonetheless, the possible influence of the different absolute duration of epochs on results of the present study is specially addressed. 3. Results During the late Ordovician – early Devonian time interval, the taxonomic diversity structure of crinoids remained similar enough. The both QC and G values are higher than 0.5 for all successive epochs (Table 2). This means that changes in the entity of orders that dominated the generic diversity of this fossil group were relatively slow. These changes were the strongest at the Ordovician – Silurian and Silurian – Devonian transitions, whereas the Wenlock – Ludlow transition is marked by an outstanding stability (Fig. 2). Differences between the QC and G curves (Fig. 2) are minor in comparison with the possible error (Ausich and Peters, 2005; see also above), and, thus, they are insignificant. The above-mentioned linear effects can be well interpreted with the original dataset (Table 1). At the Ordovician–Silurian transition, the importance of Cladida and Disparida for the total generic diversity decreased, whereas that of Monobathrida, in contrast, increased. Generic diversity of Diplobathrida reduced, but this order remained important in the taxonomic diversity structure. At the Silurian – Devonian transition, Disparida and Sagenocrinida, despite radiation of the former and slight diversification of the latter, lost their significance for the total generic diversity (if even Disparida remained relatively important), whereas the significance Table 2 Similarity of crinoid assemblages, relevant to the Late Ordovician – Early Devonian epochs, measured for the order-genus diversity structure with the QC and G indices (QC above, G below). See Table 1 for epoch abbreviations. Similarités des assemblages de crinoïdes, pour les périodes successives s’échelonnant de l’Ordovicien Supérieur au Dévonien inférieur, mesurées pour la structure de la diversité ordre-genre avec le Coefficient Quantifié de Czekanowski (QC) et le Gower Index (G) (QC valeur du dessus, G valeur du dessous). Voir Tableau 1 pour les abréviations des époques.
O3 S1 S2 S3 S4
S1
S2
S3
S4
D1
0.63 0.73
0.64 0.53 0.77 0.63
0.63 0.54 0.79 0.63 0.94 0.93
0.49 0.52 0.72 0.62 0.65 0.66 0.70 0.69
0.63 0.52 0.69 0.59 0.80 0.78 0.74 0.73 0.55 0.62
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Fig. 2. Linear changes in the taxonomic diversity structure of Silurian crinoids. Variations linéaires de la structure de la diversité taxonomique des crinoïdes siluriens, estimées à partir du Coefficient Quantifié de Czekanowski (QC) et du Gower Index (G).
of Diplobathrida increased. The generic diversity of crinoid orders remained comparable during the Wenlock – Ludlow (Table 1). Consideration of non-successive epochs permits to outline, at least, three non-linear effects. First, the taxonomic diversity structure remained quite similar for all assemblages of the late Ordovician – early Devonian time interval (Table 2), which is an evidence of significant stability. Second, the Pˇridoli assemblage became especially dissimilar from the older assemblage than this occurred at the previous time intervals. This permits to hypothesize a more or less significant disruption in the history of crinoids. Their taxonomic diversity structure, apparently, lost some patterns inherited from the Ordovician. Third, the early Devonian assemblage is, again, quite similar to the older assemblages, except for that Pˇridoli. The highest values (∼ 0.8) of similarity are documented between the Wenlock and the early Devonian (Table 2). This is an evidence of that the taxonomic diversity structure, which persisted in the late Ordovician – early Silurian, re-appeared in the early Devonian, and the same crinoid orders (Cladida and Disparida), which dominated the total generic diversity before the Pˇridoli, re-gained their importance after this epoch. In other words, the Pˇridoli evolution of crinoids seems to be “anomalous”. The reappearance of the earlier-existed taxonomic diversity structure is an evidence of some evolutionary stability. As suggested by J values (Fig. 3), there were no “simple” turnovers in Silurian crinoid communities at the level of orders with the only exception of the Llandovery – Wenlock transition, when 1 order, namely Hybocrinida, disappeared (Table 1). In contrast, “simple” turnovers at the level of genera were strong. They were the strongest in the beginning and the end of the Silurian, but even in the mid-Silurian they remained strong (Fig. 3). An analysis of non-successive time intervals reveals stability at the level of orders and gradual decrease in similarity between the older and younger assemblages at the level of genera (Table 3). Comparing the values of J, QC, and G, it is possible to realize that the documented dynamics of the taxonomic diversity structure was unlikely produced by the only “simple” turnovers. However, the noted stability at the level of orders could contribute to the relative similarity of assemblages with respect to their diversity structure.
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Fig. 3. “Simple” turnovers in the evolution of Silurian crinoids. Renouvellements dans l’évolution des crinoïdes siluriens, estimés au niveau des ordres et au niveau des genres par l’indice de similarité de Jaccard (J). Table 3 Similarity of crinoid assemblages, relevant to the late Ordovician–early Devonian epochs, measured for orders (above) and genera (below) with the J index. See Table 1 for epoch abbreviations. Similarité des assemblages de crinoïdes, pour les périodes successives s’échelonnant de l’Ordovicien Supérieur au Dévonien inférieur, mesurées pour les ordres (valeur du dessus) et les genres (valeur du dessous) avec l’indice de Jaccard (J). Voir Tableau 1 pour les abréviations des époques.
O3 S1 S2 S3 S4
S1
S2
S3
S4
D1
1.00 0.10
0.83 0.06 0.83 0.23
0.83 0.04 0.83 0.17 1.00 0.46
0.83 0.03 0.83 0.12 1.00 0.26 1.00 0.47
0.83 0.02 0.83 0.07 1.00 0.13 1.00 0.20 1.00 0.29
4. Discussion During the Silurian, the global sea level experienced rises and falls of different frequency and magnitude (Johnson, 2006, 2008; Haq and Schutter, 2008) (Fig. 1). For instance, Haq and Schutter (2008) indicated major eustaic drops near the end of the Llandovery and at the Ludlow/Pˇridoli boundary. The climate changed, and the late Ordovician and early Wenlock glaciations occurred (Boucot, 2009; Brenchley et al., 2003; Delabroye and Vecoli, 2010; Ghienne, 2003; ˇ Le Heron and Craig, 2008; Lehnert et al., 2010; Nardin et al., 2011; Zigait˙ e et al., 2010). The oceanic environment underwent unprecedented and frequent perturbations throughout the entire Silurian (Aldridge et al., 1993; Jeppson et al., 1995; Jeppson and Aldridge, 2000; Jeppson and Calner, 2003). But despite of these significant palaeoenvironmental changes, the relative
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stability and the only weak dynamism of the taxonomic diversity structure of Silurian crinoids is documented. The latest Ordovician was characterized by a severe mass extinction, which stressed the entire marine biota (Eckert, 1988; Brenchley et al., 1995, 2003; Hallam and Wignall, 1997; Sheehan, 2001; Krug and Patzkowsky, 2007; Munnecke et al., 2010; Zhang et al., 2009). According to the data of Sepkoski (2002), the total generic diversity of crinoids reduced by 30%. Data interpretations by Ausich and Peters (2005) imply that the influence of this catastrophe on crinoids was not as significant as one could imagine. Earlier, on the basis of crinoid studies on the British Isles, Donovan and Harper (2003) also noted that some late Llandovery taxa are typical for the late Ordovician – Silurian interval, which is another evidence of weak influences of the end-Ordovician mass extinction, although this may be also explained by the location of the British Isles far from the areas affected by the end-Ordovician glaciation (Cocks and Torsvik, 2002). The present analysis of the taxonomic diversity structure suggests a turnover at the Ordovician–Silurian transition, which, however, was comparable in strength with the turnovers at other epoch transitions (Fig. 2). Moreover, the Silurian and even the early Devonian assemblages retained relatively high similarity with the taxonomic diversity structure of the late Ordovician assemblage (Table 2). If so, crinoids appear to be somehow resistive to this catastrophe. This differs partly from what is observed for brachiopods, which were evidently affected by the endOrdovician mass extinctions, although also not as strong as after later catastrophes (Ruban, 2009). In contrast, “simple” turnovers among crinoid genera measured with the J index were strong at the late Ordovician – Llandovery transition (Fig. 3), which is an evidence of the mass extinction influence. Surprisingly, some other echinoderms such as blastozoans (Nardin and Lefebvre, 2010) or stylophorans (Lefebvre et al., 2006) did not suffer strongly from the end-Ordovician mass extinction. This indicates a kind of homogeneity in the reaction of echinoderms to this catastrophe. The other significant biotic crisis was linked with the Ireviken event to occur near the Llandovery – Wenlock transition (Eriksson, 2006; Lehnert et al., 2010; Ruban, 2008). Results from the present analysis (Fig. 2, Table 2) do not indicate any outstanding linear or non-linear effects related to this transition. The G similarity dropped slightly (Table 2), but differences between the QC and G values are linked, probably, with the differences of the methods of their calculation themselves, and they are below the possible error (Ausich and Peters, 2005; see also above). Thus, the noted drop of G is unimportant. This does not mean, however, that the Ireviken event did not affect crinoids. As suggested by the available data (Table 1), none Hybocrinida crossed the Llandovery/Wenlock boundary, whereas Cladida and Sagenocrinida experienced a spectacular radiation. “Simple” turnovers among crinoid genera were also strong near this boundary (Fig. 3), which suggests a strength of the relevant biotic perturbation. The Mulde event occurred in the late Wenlock (Jeppson and Calner, 2003; Calner, 2005a). According to our results (Table 2, Fig. 2), it is very unlikely that it affected the taxonomic diversity structure of crinoids, which remained stable in the mid-Silurian. The Lau event took place in the late Ludlow (Calner, 2005a,b). It is not excluded that it was responsible for a decrease in the similarity of the Ludlow and Pˇridoli assemblages (Fig. 2) as well as for the dissimilarity of the Pˇridoli assemblage from some older assemblages (Table 2; see above). However, the values of the QC and G indices remain rather high, and the similarity between older and younger assemblages re-appeared in the early Devonian (Table 2). Thus, the possible influence of the Lau event on crinoids cannot be judged catastrophic with regard to the taxonomic diversity structure. “Simple” turnovers among crinoid genera were minimal when the Mulde and Lau events took place (Fig. 3).
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If none of the above-mentioned palaeoenvironmental factors and biotic events stressed crinoids so strong to provoke major changes in their taxonomic diversity structure, intrinsic (biological) mechanisms can be favored as the main evolutionary force of this group through the Silurian period. This is also supported by the above-mentioned homogenous resistivity of echinoderms to such an outstanding perturbation as the end-Ordovician mass extinction. Our results do not allow to reveal any especially strong turnover (“revolution”) in the Silurian history of crinoids, and the documented patterns resemble the phenomenon of coordinated stasis (Brett and Baird, 1992; Morris et al., 1995; Brett et al., 1996). However, the values of the similarity indices are only relatively high, and sometimes they are below 0.7 (Table 2), which indicates some gradual changes in the taxonomic diversity structure of crinoids through the Silurian (this echoes the note of Deline and Ausich (2011), who judged crinoid evolution as not static in the Silurian). But, on the other hand, the relatively high values of the both QC and G do not reject the idea on punctuated equilibrium advocated by Eldredge and Gould (1972) and Gould (2002, 2007). Better to say, our results coincide with the idea of the heterogeneity of the evolutionary dynamics through the geologic time (Hunt, 2008). One may hypothesize that the results of the present study are affected by the different absolute duration of the epochs. A longer epoch would give more possibilities for crinoids to evolve, and, consequently, the relevant assemblage will be more dissimilar from others than that of a short epoch. Such a hypothesis can be tested with two examples. The late Ordovician and the Llandovery were relatively long epochs, which lasted 17.2 Ma and 15.5 Ma respectively (Ogg et al., 2008). The similarity of the relevant crinoid assemblages was diminished, but just slightly (Fig. 3). The Pˇridoli epoch was relatively short and lasted just 2.7 Ma (Ogg et al., 2008). However, the relevant crinoid assemblage was dissimilar from those earlier and later existed by the same degree as was the late Ordovician assemblage (Fig. 3). Thus, it is unlikely that differences in the absolute duration of epochs affects the results presented above strongly. Finally, it should be noted that a low-resolution analysis is employed in this study. It seems to be appropriate for detecting some general trends, but the discussion of the influences of particular biotic events on crinoids requires further detailed studies. E.g., the noted Mulde and Lau events took place within the Wenlock and the Ludlow epochs respectively. It cannot be excluded that they disturbed the taxonomic diversity structure for only short terms. 5. Conclusions The present study of Silurian crinoids permits two important conclusions: • the taxonomic diversity structure of these fossils was characterized by a certain stability during the Silurian, although weak dynamism also took place; • Silurian crinoids might have been resistive to some palaeoenvironmental perturbations and biotic crises, which raises an importance of intrinsic, i.e., biotic, controls on their evolution. Results of many quantitative palaeobiological studies like the present one remain valid until new corrections in the fossil systemartics occur. Therefore, further taxonomic and stratigraphic improvements of the information on crinoids (e.g., together with the revision of the relevant volumes of “Treatise on Invertebrate Paleontology”) will allow to modify our conclusions and to increase the time resolution of the analysis of the taxonomic diversity structure of Silurian crinoids.
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