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Queues of Blind Phacopid Trilobites Trimerocephalus : a Case of Frozen Behaviour of Early Famennian Age from the Holy Cross Mountains, Central Poland

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Queues of Blind Phacopid Trilobites Trimerocephalus : a Case of Frozen Behaviour of Early Famennian Age from the Holy Cross Mountains, Central Poland Acta Geologica Polonica , Vol. 59 (2009), No. 4, pp. 459 –481 Queues of blind phacopid trilobites Trimerocephalus : A case of frozen behaviour of Early Famennian age from the Holy Cross Mountains, Central Poland ANDRZEJ RADWAŃSKI 1, ADRIAN KIN 2 AND URSZULA RADWAŃSKA 3 1,3 Institute of Geology, University of Warsaw, Al. Żwirki i Wigury 93; PL-02-089 Warszawa, Poland E-mail : [email protected] 2Geoscience Friends Association ‘Phacops’, Targowa 29, PL-90-043 Łódź, Poland ABSTRACT : Radwański, A., Kin, A. and Radwańska, U. 2009. Queues of blind phacopid trilobites Trimerocephalus : A case of frozen behaviour of Early Famennian age from the Holy Cross Mountains, Central Poland. Acta Geologica Polonica , 59 (4), 459 –481 . Warszawa. An assemblage of blind phacopid trilobites of the genus Trimerocephalus McCoy , 1849, representing either the species Trimerocephalus mastophthalmus (Reinhard Richter , 1856 ) or its allies (possibly, a new species) , from an Early Famennian (Early Marginifera Zone) marly sequence of the Holy Cross Mountains, Central Poland, is composed of well organized single-file queues. The trilobites in the queues appear almost intact, having been preserved in the position they kept when forming the queues, and are interpreted showing migratory behaviour known in various present-day arthropods, but unreported from the fossil state. This queuing was induced by en - vironmental stress caused by a dramatic drop in sea level, temporarily leading to emersion. The preservation of the queues at omission horizons is thus ascribed to a mass mortality event, caused by subaerial exposure. The trilobites were suffocated and fossilized in a mortal spasm , and finally blanketed by calcareous ooze when in - undated at a highstand. The assemblage of trilobite queues represents a unique example of frozen behaviour and a snapshot of the geological past. Key words: Trilobites; Frozen behaviour; Migratory queues; Eco-taphonomy; Mass mortality; Famennian; Holy Cross Mountains; Poland. INTRODUCTION specimens and their state of preservation are quite dis - similar to any of the hitherto reported cases of trilobite The aim of the present paper is to describe unique mass-occurrences all over the world. aggregates of Upper Devonian (Lower Famennian) To characterize the aggregates, the term queue is blind phacopid trilobites of the genus Trimerocephalus used herein, instead of pod which was earlier used by McCoy , 1849, organized in single-files of reasonable Warner (1977, p. 117) for irregular or spherical heaps length comprising up to 1 3 specimens lined up unidi - formed by king crabs. On the other hand, the term rectionally. These are preserved in two horizons of a queue has already been used by Herrnkind ( 1969, 1975, thick sequence of almost unfossiliferous marlstones 1983, 1985) to describe the migratory behaviour of the from the Holy Cross Mountains, Central Poland (see present-day American Spiny Lobster, Panulirus argus Text-figs 1-2) . Both the mode of aggregation of the (Linnaeus ). 460 ANDRZEJ RADWAŃSKI ET AL. Former reports on mass-aggregated trilobites, espe - be the lower one ( 1 in Text-fig. 2B ) which yielded the cially the important studies by Speyer and Brett (1985) majority of the well-organized queues (see Text-fig .3 and Speyer (1987, 1990), concerned patch-like accu - and Pls 1 –5), collected within an about 2 to 3 cm inter - mulations, either of ‘fully ’ preserved specimens (?live, val of the marlstone sequence. The upper horizon (ar - or dead), or of moults. The single-file queues described rowed as 2 in Text-fig. 2B ) yielded only some herein represent live trilobites entombed while migrat - disarranged queues (see Text-fig. 8). ing , and frozen by rapid burial. Environmental condi - Berkowski (1991) documented two horizons of tions under which the trilobites lived/migrated and died mass-aggregated Trimerocephalus exuvia, interpreted as will be considered, because comparisons with the mi - having been left by infaunal (burrowing) specimens. gratory behaviour of extant arthropods (decapod crus - These two horizons are located within the boundary beds taceans, xiphosurans, freshwater amphipods, insects) of Unit J and K (solely in K according to Berkowski do not offer satisfactory analogues. 2002, p. 8) , about 35 –4 0 metres above the beds with the The trilobite queues, discovered by one of us (AK) trilobite queues. Neither the behavioural nor the tapho - in the late 1990s , became a subject of considerable ed - nomic interpretation given by Berkowski (1991) is com - ucational interest (Radwańska 1999a, pl. 19, fig. 3 ; see patible with those apparent from the present study of the also Kin and Radwański 2008 , p. 44). queues occurring in the lowest part of Unit I. LOCATION DATA THE QUEUE MATERIAL The Famennian sequence that yields the trilobite All the trilobite queues are distributed on the top queues is exposed within the Kowala Quarry in the vil - surface of the marlstone layers. As the queue-bearing lage of Kowala , situated in the western part of the Holy marlstones are more or less compact and brittle, having Cross Mountains, Central Poland, some 10 km south of been densely cracked by a complex joint pattern, the the town of Kielce (see Text-fig. 1). The Kowala Quarry collected material consists of relatively small slabs that is immediately adjacent to the classic Kowala railway vary in size from small pieces up to those as large as a section , both of which were thoroughly studied by Szul - tile. Of the slabs collected , almost all contain queues czewski (1971 ; updated in 1995, 1996, 1998). that reach the two opposite edges of the slab. The largest The queues occur near the base of the local lithos - illustrated specimens (Pl. 3, Fig. 1; Pl. 4, Figs 1 –2; Pl. tratigraphical Unit I of Berkowski (1991), to which 5, Figs 1–2 ) thus represent only parts of supposedly Szulczewski (1995, 1996, 1998) assigned an Upper much longer queues. Marginifera Zone age. An analysis of the conodont In view of the limited exposure , it is impossible to fauna performed for this study by Professor Zdzisław attempt to decipher the orientation of the queues in Bełka (University of Poznań) clearly shows that the space, that is over a temporary seafloor . queue-bearing part of Unit I actually belongs to the Some of the trilobites are also present on the top sur - Lower Marginifera Zone. face of calcareous nodules, where they occur either Unit I is composed of pale grey marlstones, at some singly, or in ‘imperfectly ’ organized queues (see Text- horizons replete with calcareous nodules, all weathering fig. 3). with a yellowish tint. The unit is about 20 m thick, and Taking into account the fact that most of the queues its macrofossil content was claimed to be limited to the studied were obtained from a thin marlstone interval presumably pelagic bivalve Guerichia (see Szulczewski (Horizon 1 in Text-fig. 2B), they are thought to repre - 1995; Berkowski 2002, p. 8) . All the abundant faunas sent either one, or several successive populations of that are well known from this quarry (see review by trilobites showing the same behaviour and burial con - Berkowski 2002) came from lithostratigraphic intervals ditions. Consequently, the collected material of these other than Unit I. queues is treated as one entity in the following descrip - The marlstones composing Unit I are exposed in tion. Some specimens of disorganized queues (see Text- part of the northern margin of the quarry (see Text-figs fig. 8) from Horizon 2 are treated separately. 1–2), along the wall parallel to the strike of beds dipping The material under study is housed in the Collec - towards that wall, and in its lower part are covered by tion of the Geoscience Friends Association ‘ Phacops ’ talus (see Text-fig. 2). Such an exposure provides very in Łódź , and registered under Catalogue Numbers limited access to the trilobite-bearing strata in which the Kow TA 1-78; these registration numbers refer to the queues were recognized in two distinct horizons, about sampled slabs regardless of the number of trilobite in - 1.2 metres apart. Of these, the more prolific appeared to dividuals aggregated in queues . MIGRATORY QUEUES IN DEVONIAN TRILOBITES TRIMEROCEPHALUS 461 PRESERVATION OF THE TRILOBITE SKELE - In some specimens, where a part of the exoskeleton TONS has become chipped away from the pleurae, the ap - pendages (exo- and/or endopodites ) may be detected The trilobite exoskeletons in the majority of speci - (e.g. Pl. 2, Fig. 2b at left ); these are still in need of care - mens are not fully exposed on the sediment surface. ful preparation to avoid damage of the queues during This is due to the overlapping of specimens in pairs further study . (e.g., Pl. 1, Figs 3 –7) and shorter or longer queues (e.g., Pl. 3, Fig. 2 and Pl. 5, Fig. 1), or forming jams to a vari - able extent (e.g., Pl. 2, Fig. 5 and Pl. 4, Fig. 2), and even TRILOBITE TAXONOMY a stacked heap in an extreme case (Pl. 3, Fig. 3). Apart from the occipital fracture, along which the The trilobites forming the queues are regarded as cephalon may be partly displaced (e.g., Pl. 1, Fig. 3; Pl. monospecific, in analogy to the present-day crabs that 2, Figs 1a and 4b), almost all the specimens bear either tolerate only conspecific individuals when they aggre - the pygidium alone, or together with last posterior tho - gate (see Warner 1977, p. 117). A problem arises, how - racic segment(s) turned under the body. As a result, ever, in determining the species in question . there are only a few straight specimens available for The trilobite exuvia recognized in the Kowala se - counting the number of segments. The maximum num - quence by Berkowski (1991) were classified by him as ber of 11 segments is observed rarely ( e.g.
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