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Fish Like Me National Research Institute, Warsaw Dii and Placoderms

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Fish Like Me National Research Institute, Warsaw Dii and Placoderms Research in Progress Paleontology PLACODERMS Dr. Piotr Szrek from the aquatic vertebrates as we know them today (bony and cartilaginous fishes), the planet’s seas and National Geological Institute, oceans were also home to two other classes: Acantho- Fish Like Me National Research Institute, Warsaw dii and placoderms. Both are now extinct, but their fossils have been studied extensively. Placoderms first appeared in the Early Silurian as small demersal fishes in shallow, brackish waters. Placoderms are the iconic prehistoric fish from the They were a highly diverse group – hardly a surprise, given that they existed for around 70 million years. Devonian. Recent 3D scans have revealed their he Devonian – the geological era spanning They were present all over the globe and inhabited the period of between 420 and 360 million almost all marine ecological niches; they ranged from astonishing anatomical similarities with us humans. Tyears ago – is frequently dubbed the Age of Fish. Apart active predators several meters in length, to creatures between one and two meters long feeding on inverte- brates, to bottom feeders just a centimeter or so long. Piotr Szrek, PhD Their evolution shows clear signs of an increasing is a paleontologist. He drive to inhabit marine environments, and in late conducts research into Devonian, large predators reached the top of the ma- Paleozoic vertebrates, rine food pyramid. Placoderms also made a second- mainly Devonian fish ary move back to more brackish environments, and and early quadrupeds. representatives of Ptyctodontida even found their way His interests lie in fossils and other traces into freshwater rivers. left by extinct animals. Placoderms’ heads and thoraxes were covered He is the author of by articulated armored plates. These plates, which around sixty academic served as the animals’ exoskeleton, are the most com- and popular science monly fossilized remains. In ostracoderms (armoured publications. Since jawless fish from the Paleozoic, predating placo- 2017 he has been derms), such armor had protected the animals and a member of the served as store of mineral salts, but its role changed National Geographic significantly during the course of evolution. Apart Explorers team.. from the armor, the bodies of placoderms were some- [email protected] times covered with scales of all shapes, sizes and or- namentations. However, apart from a few species in the Antiarcha and Rhenanida orders, the majority of placoderms were naked. For a long time, evolutionary biologists were not especially interested in placoderms. The fish were widely regarded as a primitive evolutionary dead-end whose morphology made little or no contribution to current vertebrate structure and anatomy. It was only the influence of placoderms on the ecosystems of De- vonian oceans which remained undisputed – until the publication of paleontological discoveries in China. They originate from the Yunnan province during the late Silurian, when different kinds of aquatic verte- brates were emerging. The advent of jawed fishes was one of the most important turning points in vertebrate evolution. Armed with newly fangled jaws, fish rapidly became alpha predators in their food chain. Placoderms dom- inated almost all aquatic environment in the Devo- nian and created the first vertebrate superpredator Dunkleosteus, which could reach up to ten meters in length. However, fossil remains of placoderms orig- inating from the preceding Silurian are incredibly rare. The Yunnan fossils date back to that period; the earliest articulated bony fish, known as the Gui- yu oneiros, and the Entelognathus primordialis plac- THE MAGAZINE THE MAGAZINE OF THE PAS 48 49 OF THE PAS 4/56/2017 4/56/2017 Research in Progress Paleontology PLACODERMS as in the majority of placoderms? If the first jaws those from the Lower Devonian have a rather more 3D scan of a snout imprint of were indeed complex, then the earliest known com- complicated history. the Osculichnus tarnowskae plete placoderms were more advanced than all later The first placoderm fossils were discovered in the lungfish found in Lower placoderms. Phylogenetic analyses place the Entelo- Holy Cross Mountains by Georg Gürich, German ge- Devonian deposits in the Świętokrzyskie Mountains; gnathus as a sister group to all higher gnathostomata ologist from Wrocław who made a huge contribution lower jaw marked in green, (jawed vertebrates), to certain placoderms and all to geology research in Poland. In 1896, he introduced upper jaw in blue higher gnathostomata, or a direct sister group to the term “placoderm sandstone.” He described some bony fishes. of the formations of the Lower Devonian found in The recent discovery of the Qilinyu placoderm in the Holy Cross Mountains, rich in vertebrate fossils, China, made by the team led by Min Zhu, has shed with his search focusing on placoderms. Writing about some light on the problem. In contrast to the Entelog- Devonian formations in the mountain range (based nathus, the Qilinyu’s lower jaw was a single bone while on research near Bieliny and Łagów), he described the upper comprised two bones, as is the case in oth- placoderm sandstones in the lower layers, in which Fossil imprint made by er placoderms. The paper describes them as maxilla preserved fauna comprised “only remains of placo- Osculichnus tarnowskae and premaxilla, reflecting the similarity to bony fish- derms, less frequently ganoids representing families (lungfish), with es; in humans the maxilla and premaxilla are fused including Drepanaspidae, Coccosteidae, Asterolepi- a reconstructed visualization with the cranium and there is a single mandible. In dae, Hybodontidae (...) and Machaeracanthus polon- of the likely culprit. Qilinyu we see the next stage of the evolution of jaws icus”. Gürich made no illustrations of the specimens in placoderms, with the external bony layer starting to he described, and since his papers were all lost during strengthen the jaw (Meckel cartilage). The tendency the two world wars, it is now impossible to offer any is most advanced in the Entelognathus. The simplicity meaningful discussion on his comments. For some of placoderm jaws is regarded as a primitive trait for reason, representatives of jawless and cartilaginous vertebrate because they didn’t have an external bony fishes and acanthodians were all described as a single covering. Qilinyu offers a good explanation why oth- group even though they could be classified more pre- er placoderms lost their complex jaw structure and cisely using existing taxonomy. This was likely done how jaws evolved in bony fishes. This improves the for simplicity’s sake. homology between elements of jaws of placoderms and bony fishes. The Qilinyu cheek also reveals the Telling imprints lacrimal bone, found in the Entelognathus and early bony fishes but absent in all other placoderms. Qilinyu Placoderm sandstone is an informal name for Silicone casts of elements oderm are regarded as key in solving evolutionary also provides the first hard evidence that early placo- quartz-bearing sandstone comprising vertebrate of placoderm skeletons puzzles. Both fossils are an anatomical mosaic linking derms had paired ventral fins – the precursor to hind fossils. However, only empty spaces where those re- from Lower Devonian extinct placoderms and bony fish living today. In cla- limbs in quadrupeds. Recent research reveals that they mains were once contained remain until the present placoderm sandstones found distical terms, the latter group also includes humans. were also the first vertebrates with external genitals. day, reflecting the morphology immediately after the in the Holy Cross Mountains The Entelognathus jaw, or more specifically the struc- Some much later placoderms were also the first to organism was buried in sediment. How do we go about (A, B, C) with sketched ture and composition of the bones, is a mystery for give live birth. studying these voids? Until recently, researchers filled interpretations (D and E). evolutionary biologists. the gaps with a silicone mass, which, once set, provid- Mountain discoveries ed an accurate representation of the original fossil. Jaw-dropping The main limitation of this method is that some of the The extensive literature on placoderms includes sev- imprints can only be accessed by breaking up the rock momentum of late, 50 years after their discovery. After Further reading: So which came first: the complex structure of the eral important publications on fossils discovered in and thus reducing the number of casts which can be a series of studies and excavations, a number of issues Long J. (2016). The first jaws. first jaws comprising several bones – as in Entelog- Poland. While Upper Devonian placoderms have been made. Tomography is a far more effective, non-inva- have been understood concerning basic questions of Science 354 (6310): 280‒281. nathus and bony fishes – or simple single-bone jaws described on numerous occasions (e.g. Szrek 2008), sive method gaining widespread use. As well as mak- their genesis and taxonomic composition. The Low- Szrek P., Dupret V. (2017). ing it possible to examine all imprints, it also means er Devonian placoderm sandstones represent an ac- Placoderms from the Early their spatial arrangement can be studied, which helps cumulation of fossils of storm genesis. They cannot, Devonian “placoderm establish how the remains were accumulated. Another therefore, be used to identify precise age. It has also sandstone” of the Holy Cross Mountains, Poland with major advantage
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