A Case Study of Eyeless, Dimorphic Devonian Trilobites from Poland

[Palaeontology, Vol. 59, Part 5, 2016, pp. 743–751]

ANCIENT ANIMAL MIGRATION: A CASE STUDY OF EYELESS, DIMORPHIC DEVONIAN TRILOBITES FROM POLAND

† by BŁAZEJ_ BŁAZEJOWSKI_ 1, CARLTON E. BRETT2,ADRIANKIN3, , † † ANDRZEJ RADWANSKI 4, and MICHAŁ GRUSZCZYNSKI 3, 1Institute of Palaeobiology, Polish Academy of Sciences, Twarda 51/55, 00-818, Warszawa, Poland; [email protected] 2Department of Geology, University of Cincinnati, Cincinnati, OH 45221-0013, USA; [email protected] 3‘Phacops’ – Association of Friends of Geosciences, Grajewska 13/40, Warszawa, 02-766, Poland 4Institute of Geology, University of Warsaw, Zwirki_ i Wigury 93, 02-089, Warszawa, Poland

Typescript received 15 February 2016; accepted in revised form 8 July 2016

Abstract: We report evidence of one of the oldest known have migrated periodically to shallow marine areas for mass animal migratory episodes in the form of queues of the eye- mating and spawning. The sudden death of the trilobites in less trilobite Trimerocephalus chopini Kin & Błazejowski,_ the queues may have been caused by excess carbon dioxide from the Late Devonian (Famennian) of central Poland. In and hydrogen sulphide introduced into the bottom water by addition, there is evidence for two morphs in this popula- distal storm disturbance of anoxic sediments. This study tion, one with nine segments and the other with ten. We demonstrates the potential for further research on the evolu- infer that these queues represent mass migratory chains tion and ecology of aggregative behaviour in marine arthro- coordinated by chemotaxis, comparable to those observed in pods. modern crustaceans such as spiny lobsters, and further suggest that the two forms, which occur in an approximately Key words: animal migration, eyeless trilobite, Phacopinae, 1:1 ratio, may be dimorphs. These ancient arthropods may Devonian, Holy Cross Mountains, Poland.

P ERIODIC mass migrations have been observed in many migratory aggregates. However, the forms and structures modern groups of vertebrate and invertebrate animals described are distinct from positions or behaviour known (Hoare 2009a, b). There are several possible functions of in trilobites, as brieﬂy noted earlier (Radwanski et al. such periodic migratory behaviour including reproductive 2009, p. 467). Linear clusters of trilobites are also known and trophic aggregations (Thollot et al. 1999; Crossin from the Ordovician of Morocco (Chatterton & Fortey et al. 2009). Recent studies on the processes controlling 2008) and that of Portugal (Gutierrez-Marco et al. 2009), mass migrations (Wilcove 2008) form an important but their relationship to migration or mating aggregation aspect of behavioural science, particularly migrations has not been pursued in detail. These instances in dis- related to human activity and rapid variations of climates, tantly related clades, however, may point to a more com- but very little is yet known about the evolutionary origin mon phenomenon among arthropods than previously and ecological signiﬁcance of mass migrations, which recognized. have rarely been recognized in the fossil record. Chain- New material from the Kowala Quarry (Holy Cross like clusters of the enigmatic early Cambrian arthropod Mountains, Central Poland) described herein has great (crustacean?) Synophalos xynos have been described before potential for the study of the nature of such migrations (Hou et al. 2008, 2009); in this example the organisms based on queues of the blind phacopide trilobite Trimero- were seemingly linked in that the caudal region of each cephalus. A previous study of these queues (Radwanski individual inserted into the anterior carapace of the one et al. 2009) concerned the structure (organization) of behind. The authors interpreted these chains as probable queues in relation to the interpreted ambient environment. Size frequency aspects and dimorphism, were not discussed in the former study. Thus, the present paper † Deceased aims to provide substantial further details and discussion

© The Palaeontological Association doi: 10.1111/pala.12252 743 744 PALAEONTOLOGY, VOLUME 59 of the signiﬁcance of these ﬁndings and a reinterpretation preserved specimen, examined in detail using x-ray tomo- of queues as evidence for migratory behaviour, probably graphy, revealed the oldest fully preserved moulting in association with reproduction in these blind trilobites. episode (exuvium plus soft-shelled trilobite) known from the fossil record (Błazejowski_ et al. 2015). All trilobite specimens have been prepared manually. MATERIAL AND METHOD The collected material is housed at the Institute of Palaeobiology, Polish Academy of Sciences in Warsaw All trilobites discussed herein occur in an approximately (ZPAL Tr.8). 25 m thick succession of marly shales in the early Famen- Carbon dioxide for isotopic analysis of matrix carbon- nian (approximately 365 Ma), at the Kowala Quarry near ate was extracted either manually using traditional vac- Kielce, Holy Cross Mountains, central Poland (Fig. 1) uum extraction lines or using an automated online (Radwanski et al. 2009). A total of 78 examples of rows or Isocarb reaction system. Carbon and oxygen stable iso- queues of the blind trilobite Trimerocephalus chopini Kin & tope data were obtained using a Finnigan MAT 251 mass Błazejowski,_ 2013, have been obtained, each with up to 19 spectrometer at the University of Liverpool. All isotopic aligned, articulated individuals. These trilobite queues are data are reported with reference to the VPDB interna- preserved within three horizons of marly shales with occa- tional standard; typical precisions of 0.1 & are reported sional small, calcareous concretions. Trilobites in the by this laboratory. Statistical calculations were performed queues are arranged in the same direction, one after the with PAST v. 3.01 (Hammer et al. 2001). other, in cephalon to pygidium (head to tail) manner, and frequently contact or overlap each other (Figs 2A, C; 3). All specimens are preserved in apparent life-positions and RESULTS those in queues fall into a relatively narrow size range rela- tive to those seen in other phacopine trilobites (7–19 mm The size-range of articulated trilobite exoskeletons within in length). Nearly all specimens are fully articulated and a the queues varies somewhat. Surprisingly, however, the few individuals preserve elements of locomotory appen- studied specimens in ten sample queues have an inconsis- dages (i.e. endo- and exopodites), indicating rapid, in situ tent number of thoracic segments, with nearly equal burial. They are directly associated with numerous small numbers of two morphs (Kin & Błazejowski_ 2013): 51% enrolled specimens of T. chopini. One exceptionally of individuals have nine segments, while 49% have ten

FIG. 1. A, map of Poland indicat- A C ing the location of the Holy Cross Mountains area (HCM; rectangle). B, generalized geological map of the Holy Cross Mountains and the location of Kowala Quarry (arrowed). C, map of Kowala Quarry with the location of the studied section exposed in the western part of the northern quarry wall (arrowed). Scale bars represent 100 km (A); 10 km (B); and 0.5 km (C).

B BŁAZE_ J O W S K I ET AL.: ANCIENT ANIMAL MIGRATION 745

A B

FIG. 2. Examples of periodic migratory queues occurring among the ancient and modern arthropods: trilobites and spiny lobsters. A, idealized reconstruction of Trimerocephalus chopini Kin & Błazejowski,_ 2013 queue showing trilobites about 365 million years ago, Kowala (Holy Cross Mountains, Poland). B, example of a relatively short migratory queue formed by six specimens of the spiny lobster Panulirus argus (Linnaeus), similar to that photographed by Herrnkind (1975, p. 828), Bimini (Bahamas). C, typical example of T. chopini queue (formed by over ten well-aligned specimens; Kow/Ta 60) from the third horizon. Scale bar in C represents 10 mm. Colour online.

(Table 1). In general, the 9-segmented morphs are smaller among spiny lobsters (Decapoda, Palinuridae) in pre- (N = 34; mean = 8.8 mm; range: 5–12 mm) than 10-seg- adult, moulting and reproductive phases of their life cycle mented forms (N = 36; mean = 14.5 mm; range: 7– (Herrnkind 1980) (Fig. 2B). Reproductive cycles are the 19 mm); the medians are significantly different (Mann- major causes of periodic mass migration in the spiny lob- Whitney U-test: U = 90; p = 6.669). ster Jasus edwardsi Hutton, from New Zealand; this spe- In addition, at least 119 enrolled specimens were found cies may migrate considerable distances, from offshore to in association with the queues at each of the three levels, inshore sites (Linnane et al. 2005). Another lobster spe- though typically a few millimetres above the bedding cies, Panulirus ornatus Fabricius migrates more than planes containing the queues. These specimens usually 500 km to shallow water sites for mass breeding (Moore range in cephalic size from 2 to 4 mm. & McFarlane 1982). The lobster breeding sites vary, but Geochemical and mineralogical investigations revealed are generally in shallow depths of a few metres and fre- very fine-grained calcareous silt with abundant limonitic quently occur close to coral reef buildups (Kanciruk & pseudomorphs after original pyrite, directly underneath Herrnkind 1978). There are many other examples of mass individual T. chopini skeletons from the trilobite queues. migration among Palinuridae including Panulirus argus Carbonate sediment underneath the individual T. chopini Latreille, Panulirus cygnus George, and Jasus verreauxi exuviae, yielded d13C values that oscillated around Milne-Edwards (Herrnkind 1980; Phillips 1983; Booth À1.5&, whereas values for the sediment underneath the 1997). trilobite skeletons in the queues are somewhat higher, Modern aquatic arthropods, such as crayfish and lob- being around À0.3& PDB (Fig. 4G–H). sters, are highly sensitive to specific chemical compounds present in minute amounts within the ambient water (Attema 1995), due to specialized chemoreceptor cells. DISCUSSION Urine-borne compounds are among the most common chemical signals, which stimulate both group behaviour Seasonal migrations of modern marine arthropods: a (e.g. scent paths) and individual interactions (e.g. court- modern analogue ship, dominance). For example, spiny lobsters of the species P. argus disseminate urine from special glands over Among modern marine arthropods, the phenomenon of distances of 1–2 m, which allows other individuals to mass seasonal migration has been observed, for example, sense the surrounding environment, especially under 746 PALAEONTOLOGY, VOLUME 59

usually during day to day movements, and hundreds or even thousands of individuals in seasonal migrations (Booth 1997).

Trilobite migratory queues controlled by chemotaxis

In the case of Trimerocephalus chopini queues, it is probable that trilobites moved one after another following chemosensory cues because T. chopini was eyeless and is regarded as having had no alternate photoreceptors. Therefore, T. chopini queues are recognized as the first fossil record of possible chemotaxis, and these lines are interpreted as the oldest known migratory queues. We suggest that the trilobites were undergoing mass migrations to spawning-grounds during reproductive periods. This hypothesis is supported by the relatively small but variably sized specimens forming queues, demonstrating putative dimorphism with approximately equal numbers of the two dimorphs, and by comparison with modern analogues. There is abundant evidence for possible aggregative moulting and mating behaviour in phacopid trilobites (Speyer & Brett 1985; Brett et al. 2011; Brett et al. 2012). Mass occurrences of Ordovician trilobites have also been explained as a result of mating behaviour (Karim & Westrop, 2002; Gutierrez-Marco et al. 2009). The repeated occurrences of species-segregated clusters of carcasses and moult ensembles, of similarly sized individuals in particular facies in the Middle Devonian Hamilton Group suggests that the trilobites migrated to particular breeding grounds prior to mass moulting and spawning as in certain extant peracarid crustaceans (Speyer & Brett 1985). Such mating grounds might also become trilobite ‘nurseries’ (Paterson et al. 2007). Modern analogues of this phenomena are represented by ‘nursery grounds’ of the horseshoe crabs (e.g. Tachypleus tridentatus Leach and Limulus polyphemus Linnaeus (Carmichael et al. 2003)). ‘Limulid nurseries’ are also described from the Upper Jurassic of Poland (Kin & Błazejowski_ 2014; Błazejowski_ 2015), and similar ‘nurseries’ observed in some echino- derms are widely discussed by Radwanski et al. (2012, 2014). The latter suggestion appears to be supported in the FIG. 3. Migratory queue of Trimerocephalus chopini Kin & case of the Kowala trilobites by the occurrence of a num- Błazejowski,_ 2013 from Kowala Quarry (Holy Cross Mountains). Schematic drawing of three marly shale surfaces exposing the ber of small, sphaeroidally or discoidally enrolled – longest T. chopini queue (formed by 19 specimens; Kow/Ta 3) T. chopini specimens (Fig. 4A F) in situ within each of as extracted from the first of studied horizon (left) and close- the three horizons with the queues. All of these enrolled ups of the best preserved fragments (right). Scale bar represents individuals represent meraspid stages with 3–7 thoracic 50 mm. Colour online. segments, i.e. juvenile forms of T. chopini. Thus, the juveniles may represent remains of a ‘trilobite nursery’ and conditions of significantly reduced visibility caused by the T. chopini queues formed by sexually mature individ- bottom water turbidity. The number of lobsters moving uals (i.e. holaspids in strict morphological terms) may in single queues along the scent path (= urinal trail) is represent migration to the spawning area during repro- variable, reaching a few to several dozen individuals duction cycles. It should be emphasized that the largest TABLE 1. Biometric measurements of Trimerocephalus chopini Kin & Błazejowski,_ 2013 forming 10 sample trilobite queues, that occur in a succession of marly shales of early Fame- nnian age, exposed in the Kowala Quarry (Holy Cross Mountains, central Poland).

Queue Kow/Ta 3 Kow/ Kow/ Kow/ Kow/ Kow/ Kow/ Kow/Ta 60 Kow/Ta 63 Kow/Ta 77 characteristics (Fig. 3) Ta 19 Ta 22 Ta 23 Ta 45 Ta 47 Ta 59 (Fig. 2C)

Total number of 19 5 5 5 5 6 7 10 10 13 individuals Individuals with 9 8 2 34 11 2 2 5 6 thoracic segments Length of 5/11/12/9/10/ 9/11 10/7/11 10/8/~11/10 11 ~59/~10 11/12 10/10/~12/9/10 9/10/8/~10/11/10 BŁA individuals with 9 ~10/9/11 EJOWSKI K S W O J ZE

thoracic segments _ (mm) Individuals with 10 4 3 21 35 3 6 4 5 thoracic segments Length of 15/17/14/~19 17/17/18 15/17 13 7/13/17 11/11/9/ 15/18/15 14/14/15/ 14/14/~15/17 13/14/17/14/~14 TAL ET individuals with 10 10/~16 ~14/12/17 thoracic segments (mm) MIGRATION ANIMAL ANCIENT .: Additional data for 5:3/~7:5/6:4/~5:3 & ––– 5:3 – 8:6 & 7:5 ~7:5 & single – 5:3 & ~5:3 incomplete three small to part of thorax individuals (width: semi-large partially length of cephalon; preserved thoraces mm) 747 748 PALAEONTOLOGY, VOLUME 59

FIG. 4. Two examples of juvenile enrolled trilobites representing a A BC meraspid stage, occurring in the upper parts of two horizons with Trimerocephalus chopini queues. A– C, example of a small, enrolled specimen found in horizon 1. D–F, slightly larger enrolled specimen from horizon 2; note the white arrow D EF marking siderite ﬁlling the enrolled specimen. G, backscattered electron image of the sediment just beneath the skeleton of one of the trilobites in the T. chopini queue with a place of composition analysis marked with arrow; note the amount of iron oxide (white) crystals pseudomorphs after pyrite and the stable carbon and oxy- G H gen isotope compositions for the sediment matrix. H, T. chopini exuvium; note that the d13C values and d18O isotope values for the sediment from underneath the exuviae are identical to those for the sediment adjacent to the trilobite queue. Scale bars represent 1 mm (A–C, G); 2mm(D–F); 4 mm (H). Colour online.

number of segments observed in the thorax of T. chopini number within a species, or different moult stages. Alter- was 11 (Kin & Błazejowski_ 2013). Variable numbers of natively, and more probably, the 9- or 10-segmented segments in holaspid trilobites is very rare, but have been individuals both represent holaspids, but male and female demonstrated in the unusual form Aulacopleura konincki dimorphs respectively. The fact that both 9- and 10-seg- (Hughes & Chapman 1995). In the latter, dimorphism is mented forms show a substantial and similar range of ruled out as the segment numbers vary from 18 to 22. variation (5 mm for 9-segmented forms and 7 mm in If the unique queues formed by T. chopini discussed 10-segmented forms) suggests that each morph is not sim- herein were strictly connected to periodic migrations for ply a different growth stage. Rather, each form occurs as breeding, it could be assumed that T. chopini reached both smaller and larger sized individuals, a range much maturity during the 9- or 10-segmented stage. It also larger than that found in a single meraspid instar of any means that the sexual maturity of T. chopini was not nec- known trilobite. Furthermore, the earlier meraspid forms, essarily combined with morphological stabilization of their up to stage 8, are represented by the minute enrolled spec- exoskeletons, as revealed essentially by the occurrence of a imens, so there was a very large size difference between stable number of trunk segments. It is associated with the meraspid stage 8 and stage 9 or 10, suggesting that once transition between two main ontogenetic phases: from one or the other of these segment numbers was achieved the morphologically variable larval (meraspid) period to it remained stable through a substantial size range, most the morphologically stable post-larval (holaspid) period, likely representing several moult episodes. i.e. from anamorphic to epimorphic phases (Hughes et al. Other forms of probable sexual dimorphism in trilo- 2006; Cronier^ 2011). Thus, the results of this study pro- bites have been documented, including the presence of vide a possible example of sexually mature trilobites, preglabellar bulbs, being possible brood pouches (Fortey forming unique migratory queues and represented by & Hughes 1998), and median cephalic bulbs and spines non-holaspid individuals (i.e. non ‘adult-type’ forms, in in raphiophorids (Knell & Fortey 2005). These morpho- terms of widely accepted morphological classification). logical features can be logically related to functional dif- The variation in thoracic segment number could simply ferences between sexes (e.g. brood pouches in females) or represent an unusual case of meristic variation of segment sexual selection. Conversely, it is difficult to conceive of BŁAZE_ J O W S K I ET AL.: ANCIENT ANIMAL MIGRATION 749 functional distinctions in forms with one more or less forming concretions contain abundant iron oxide pseudo- segment. Possibly, this was simply related to size differ- morphs after pyrite crystals; both also display d13C values ences between sexes, but, as yet, we have no more plausi- of ~À0.5&. The enrolled specimens of T. chopini suggest ble explanation. For this reason, we must be slightly that they had time to take defensive action, i.e. enrolling tentative in ascribing this dimorphism to sexual differ- the body to protect sensitive ventral respiratory surfaces, ences. Nonetheless, considering the cohort of trilobites so their death was not as sudden as that of trilobites in occurring in queues as a whole, the ratio of 9- to 10-seg- the queues. mented individuals is very nearly 1:1. While sex ratios in Any hypothesis for the preservation of these queues some organism populations are significantly different must explain the nearly synchronous mortality of many from unity, the preservation of 1:1 ratios of morphotypes individuals slightly prior to abrupt burial. We suggest is strongly suggestive of sexual dimorphism. Interestingly, that the initial affect of storms may have been to stir up the vast majority of contemporary arthropods become anoxic, sulphide-rich sediment slightly upramp from the sexually mature exactly after entering into the epimorphic site of mortality, producing thin suspensions of toxic, phase (Minelli 1992, 2003; Minelli et al. 2006). This turbid water. The most likely scenario for sudden death of occurrence of dimorphism in segment number is, to our both the adult and juvenile T. chopini involves suffocation, knowledge, unique among trilobites and its functional hypercapnia (excess CO2) and/or toxicity of sulphidic significance, if any, is not known, although differences in waters. Seemingly, these trilobites died literally in their mean size between sexes are common. tracks but the carcasses were then subject to minor, incipi- Sedimentary characteristics of the bottom sediment for ent post-mortem decay, without transport, prior to rapid the migratory queues of T. chopini indicate (see Rad- burial. We suggest that clouds of suspended sediments wanski et al. 2009) a low energy hydrodynamic regime. subsequently blanketed the seafloor, perhaps as a result of Typical parallel to low-angle cross lamination and occa- fallout of flocculated muds in the aftermath of the same sional hummocky cross stratification of the carbonate silt storm that had killed the animals. indicate a low energy setting affected by occasional Juvenile forms of T. chopini could attain fully enrolled storms. An abundance of remains of primitive plants (i.e. postures, perhaps because small forms were more tolerant psilophytes) provides evidence for close proximity to of anoxic/sulphidic conditions compared to adult individ- land; however, these remains may have been transported uals, as has been demonstrated in experiments (see Hol- offshore to settle out in relatively deep water, where dys- man & Hand 2009) carried out on ghost shrimp of the oxic conditions favoured their preservation. species Lepidophthalmus louisianensis Schmidt. Thus, these juveniles survived long enough to respond to the toxic stimulus by enrollment. Mass mortality of Trimerocephalus chopini

The most striking feature of the examined trilobite queues CONCLUSIONS is the preservation of the members of the queue in their life positions suggesting instantaneous mortality. A few Results of our research demonstrate that repetitive migra- trilobite individuals in the queues exhibit slightly flexed tory behaviour had already evolved among marine animals postures, suggesting incipient but incomplete enrollment, by at least the mid-Palaeozoic Era. We infer that the breed- a plausible defensive behaviour for trilobites. The occur- ing strategy for the blind trilobite Trimerocephalus chopini rence of fine-grained pyrite and slightly negative d13C val- Kin & Błazejowski_ 2013 included mass migration to ues for the sediment adjacent to trilobite queues, as well spawning-grounds; the mass migrations in queues were as that directly beneath the carcasses, strongly suggests likely coordinated by primary chemosensory phenomenon, bacterial decay of rapidly buried trilobite soft parts in the as in some modern crustaceans. An important argument sulphate reduction zone. Anaerobic degradation of related to periodic migration of T. chopini to the breeding organic matter underneath the trilobite skeletons may areas is the co-occurrence of numerous enrolled juveniles, have increased alkalinity sufficiently that pore fluids which occur in each queue horizon; these abundant juve- became supersaturated with respect to calcium carbonate niles could record ‘trilobite nurseries’. Moreover, individu- causing precipitation of cements. als of T. chopini forming migratory queues do not show a Small, enrolled T. chopini exoskeletons are filled either fixed number of thoracic segments. A plausible explanation by siderite (d13C = À1& and d18O = À2.5&), or sedi- for the 9- and 10-segmented variants, considering their ment of almost identical appearance to the sediment nearly 1:1 ratio, is that they represent sexual dimorphs in a beneath the skeletons of queuing trilobites, forming con- single species; these two morphotypes each show consider- cretionary infillings within the enrolled skeletons. Both able and overlapping size ranges, although the 10-segmen- the sediment inside the enrolled trilobites and that ted morph is generally larger. 750 PALAEONTOLOGY, VOLUME 59

As to causes of sudden death of the described blind BOOTH, J. D. 1997. Long-distance movements in Jasus ssp. trilobites, we suggest poisoning by hydrogen sulphide, and their role in larval recruitment. Bulletin of Marine which is very diffusive and highly toxic, and/or hypercap- Sciences, 61, 111–128. nia, owing to carbon dioxide released from sediment, BRETT, C. E., ALLISON, P. A. and HENDY, A. J. W. replacing oxygen in the bottom waters. Regardless, the 2011. Comparative taphonomy and sedimentology of small-scale mixed carbonate/siliciclastic cycles: synopsis of similar state of in situ preservation of these blind trilobites Phanerozoic examples. 107–199. In ALLISON, P. A. and suggests almost simultaneous death of all of the individu- BOTTJER, D. J. (eds). Taphonomy: process and bias through als (both enrolled juveniles and sexually mature adults time.2nd edn. Springer Science. arranged in the queues). Abrupt burial of T. chopini popu- -ZAMBITO, J. J., HUNDA, B. R. and SCHINDLER, E. lations followed very shortly after the mass mortalities, 2012. Mid Paleozoic trilobite Lagerst€atten: models of probably from fallout of flocculated muds stirred up by diagenetically enhanced obrution deposits. Palaios, 27, 326–345. storms (Type 1 assemblages of Brett et al. 2012). This CARMICHAEL, R. H., RUTECKI, D. and VALIELA, I. must have been a recurrent phenomenon in this area 2003. Abundance and population structure of the Atlantic during the Late Devonian because at least three occasions horseshoe crab Limulus polyphemus in Pleasant Bay, Cape 246 – of storm burial overlapped with reproductive periods Cod. Marine Ecology Progress Series, , 225 239. of T. chopini to produce similar queues at three distinct CHATTERTON, B. D. E. and FORTEY, R. 2008. Linear clusters of articulated trilobites from the Lower Ordovician horizons. (Arenig) strata at Bini Tinzoulin, north of Zagora, southern Finally, we conclude that chemotaxis may have func- Morocco. 73–78. In RABANO, I. GOZALO,R. and tioned as a long-term macroevolutionary process control- GARCIA-BELLIDO,D. (eds). Advances in trilobite research. ling reproductive behaviour of arthropods, the largest Instituto Geologico y Minero de Espana,~ Madrid. known phylum of animals on Earth. CRONIER,^ C. 2011. Varied development of trunk segmentation in three related Upper Devonian phacopine trilobites. Acknowledgements. Most of the work presented in this paper, Historical Biology, 22 (4), 341–347. including field and laboratory studies and the idea and overall CROSSIN, G. T., HINCH, S. G., COOKE, S. J., COOP- design of research, were initiated by Adrian Kin who passed away ERMAN, M. S., PATTERSON, D. A., WELCH, D. W., before the completion of the final draft of this paper. Błazej_ HANSON, K. C., OLSSON, I., ENGLISH, K. K. and Błazejowski,_ Carlton E. Brett and Andrzej Radwanski accepted FARRELL, A. P. 2009. Mechanisms influencing the timing responsibility for the validity of data and conclusions presented and success of reproductive migration in a capital breeding herein. Warmest thanks are expressed to Urszula Radwanska semelparous fish species, the sockeye salmon. Physiological & (University of Warsaw) for many helpful suggestions during the Biochemical Zoology, 82, 635–652. early phase of this investigation. Comments and criticism sup- FORTEY, R. A. and HUGHES, N. 1998. Brood pouches in plied by Tammie Gerke greatly improved the quality and content trilobites. Journal of Paleontology, 72 (4), 638–649. of this manuscript. We would like to thank Adam T. Halamski GUTIERREZ-MARCO, J. C., SA, A. A., GARCIA- (Institute of Palaeobiology, PAS) for assistance with statistical BELLIDO, D. C., RABANO, I. and VALERIO, M. 2009. analysis. 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