The fossil record of the earliest tetrapods (vertebrates with limbs rather than paired fins) consists of body fossils and trackways. The earliest body fossils of tetrapods date to the Late Devonian period (late Frasnian stage) and are preceded by transitional elpistostegids such as Panderichthys and Tiktaalik that still have paired fins. Claims of tetrapod trackways predating these body fossils have remained controversial with regard to both age and the identity of the track makers. Here we present well-preserved and securely dated tetrapod tracks from Polish marine tidal flat sediments of early Middle Devonian (Eifelian stage) age that are approximately 18 million years older than the earliest tetrapod body fossils and 10 million years earlier than the oldest elpistostegids. They force a radical reassessment of the timing, ecology and environmental setting of the fish-tetrapod transition, as well as the completeness of the body fossil record.

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Integrated biostratigraphical, microfacial and geochemical studies of the Lower Kellwasser Event in the Płucki succession (southern Poland) provide details about redox conditions during the deposition of this horizon in the deep-shelf Łysog ory basin of the Holy Cross Mountains. The environment is characterized by calm sedimentation and soft, muddy carbonate substrate. However, microfacies changing from wackestones to grainstones, the presence of crushed or current-oriented nautiloid shells and the occurrence of redeposited material from shallow-water Dyminy Reef environments (such as calcispheroids, algae and girvanellid cyanobacteria) suggest episodes of a higher-energy regime. Uranium/thorium ratios indicate that bottom-water redox conditions changed periodically from being mainly anoxic in the middle part of the Lower Kellwasser Horizon to dysoxic in the lower and upper parts. During a short-term episode of bottom-water ventilation, the seafloor was rapidly colonized by a dense assemblage of opportunistic buchiolid bivalves, which suffered mass mortality upon the return to anoxic conditions. A very rich concentration of cephalopods and homoctenids may be regarded as reflecting a bloom of high-density populations during high-productivity events. Similarly, they suffered mass mortality when episodically increasing anoxia/euxinia reached the upper part of the water column. The Late Frasnian inorganic carbon isotope records in the Płucki section show a positive shift with a maximum amplitude of 3&. This enrichment in d 13 C can be correlated with the deposition of the Lower Kellwasser Horizon and reflects the expansion of anoxic and probably high-productivity regimes. □ Anoxia, Holy Cross Mountains, Late Devonian, Lower Kellwasser Event, redox conditions. Michał Rakoci nski [michal.rakocinski@us.edu.pl], Faculty of Earth Sciences, University of Silesia, Be z dzi nska 60, 41-200 Sosnowiec, Poland; Agnieszka Pisarzowska [ndpisarz@ cyf-kr.edu.pl],

The placoderm sand stone (Emsian, Holy Cross Moun tains) ex posed in the aban doned quarry at Pod³azie Hill was re vis ited and ex ca vated dur ing field work con ducted in 2011-2013. Bone-bear ing brec cias were stud ied in de tails for the first time at this site and sub jected to taphonomic anal y sis. Ver te brate re mains are dom i nated by heterostracans, while true placoderms com pose less than 20% of the to tal ver te brate as sem blage. The high de gree of frag men ta tion of the bones and low de gree of abra sion in di cate that the re mains were re worked and trans ported be fore fi nal burial. This is con sis tent with the mixed charac ter of the bone ac cu mu la tions, which com prise both open-shelf forms (ac an tho dians, chondrichthyans) as well as those re lated to mar ginal-ma rine en vi ron ments (placoderms and sarcopterygians). The bone-bear ing suc ces sion has been sub divided into five depositional fa cies at trib uted to a coastal la goon in flu enced by stormy, pos si bly tidal con di tions. The oc currence of the in ver te brate trace fos sil Ilmenichnus sp. ac com pa nied by Lockeia and Monomorphichnus sup ports this in ter pre ta tion.

Opinions differ on whether the evolution of tetrapods (limbed vertebrates) from lobe-finned fishes was directly linked to terrestrialization. The earliest known tetrapod fossils, from the Middle Devonian (approximately 390 million years old) of Zachełmie Quarry in Poland, are trackways made by limbs with digits; they document a direct environmental association and thus have the potential to help answer this question. However, the tetrapod identity of the tracks has recently been challenged, despite their well-preserved morphology, on account of their great age and supposedly shallow marine (intertidal or lagoonal) depositional environment. Here we present a new palaeoenvironmental interpretation of the track-bearing interval from Zachełmie, showing that it represents a succession of ephemeral lakes with a restricted and non-marine biota, rather than a marginal marine environment as originally thought. This context suggests that the trackmaker was capable of terrestrial locomotion, consistent with the appendage morphology recorded by the footprints, and thus provides additional support for a tetrapod identification.