Lakes and rivers of the younger Paleozoic and as well in the Triassic could not only house tetrapod and bony fish vertebrate dwellers but also selachian predators, in particular the Xenacanthida, well known for their characteristic neck spines, and the Hybodontiformes, which display a pair of lateral head spines and characteristic fin spines. The latter are distinct from those of the Acanthodii (popularly also referred to as "spiny sharks"), a group of basal vertebrates that ocurred in freshwater environments as well, but became extinct before the beginning of the Triassic.
Complete shark specimens are seldom recorded, the same is true for complex finds comprising a couple of skeletal elements from the same individual - taxonomists often have to deal with assemblages of individual scales, spines, and teeth and systematics heavily relies on tooth characteristics (e.g. Schneider 1988 for the Xenacanthida, Rees 2008 for hybodont sharks).
Like the recent bullhead sharks (Heterodontus) at least some of the Carboniferous to Triassic freshwater sharks were oviparous - different types of spiral egg capsules not quite unlike those capsules of Heterodontus occur in different types of freshwater environments, e.g. marginal lake sediments or low-energy river banks; often they appear unrelated to skeletal remains. This has been interpreted as being indicative for a separation between the actual habitats of the sharks and their spawning grounds (about the facial aspects: see Schneider & Reichel 1989). To what extent the occurrence of xenacanth and hybodont sharks in freshwater deposits is indicative for a marine influence is currently a matter of debate.
In the first descriptions of the 19th century fossil egg capsules were misinterpreted as cone-like fructifications of some kind of plant. This was due to the rhomboidal pattern the egg capsule impressions often display as consequence of taphonomic flattening (and the consequent overlap of the spiral patterns on the front and back sides). Two types of shark egg capsules have been recovered from the Madygen Formation during fieldwork in 2007 (Fischer et al. 2007) - more on that later.
Jan Fischer, featured in the last FPhotW, who is working on the Madygen chrondrichthyans (as soon as they appear) and isotope paleontology of shark teeth/spines, is thanked here for supplying me with literature. (Hopefully I can convince Jan to write a guest contribution).
Maisey, J.G. (1982): The Anatomy and Interrelationships of Mesozoic Hybodont Sharks. - American Museum Novitates 2724: 1- 48; New York.
Schneider, J. W. & Zajic, J. (1994): [Xenacanths (Pisces, Chondrichthyes) of the middle European Upper Carboniferous and Permian - revision of the originals of GOLDFUSS 1847, BEYRICH 1848, KNER 1867 and FRITSCH 1879-1890.] - Freiberger Forschungshefte, C 452: 101-151; Leipzig.
On tooth systematics:
Schneider, J.W. (1988): [Basics of the morphogeny, taxonomy, and biostratigraphy of isolated xenacanth teeth (Elasmobranchii)]. - Freiberger Forschungshefte, C 419: 71- 80; Leipzig.
Rees (2008): Interrelationships of Mesozoic hybodont sharks as
indicated by dental morphology – preliminary results. - Acta Geologica Polonica 58 (2): 217-221.
On egg capsules:
Schneider, J.W. & Reichel, W. (1989): [Chondrichthyan egg capsules from the Rotliegend (Lower Permian) of Middle Europe - conclusions regarding the palaeobiogeography of palaeozoic freshwater sharks.] - Freiberger Forschungshefte, C 436: 58- 69; Leipzig.
Fischer, J., Voigt, S. & Buchwitz, M. (2007):
First elasmobranch egg capsules from freshwater lake deposits of the Madygen Formation (Middle to Late Triassic, Kyrgyzstan, Central Asia). - Paläontologie, Stratigraphie, Fazies (15), Freiberger Forschungshefte, C 524: 41-46; Freiberg.
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