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Non-Dino Mesozoic stuff: poison seas, belemnites, mammal skull bones

From: Ben Creisler

Apologies if this stuff is not strictly vertebrate- or
dinosaur-related. However, these recent papers may be of interest to
some for insights into Triassic paleobiology and early mammal

Sylvain Richoz, Bas van de Schootbrugge, Jörg Pross, Wilhelm Püttmann,
Tracy M. Quan, Sofie Lindström, Carmen Heunisch, Jens Fiebig, Robert
Maquil, Stefan Schouten, Christoph A. Hauzenberger & Paul B. Wignall
Hydrogen sulphide poisoning of shallow seas following the end-Triassic
Nature Geoscience (advance online publication)

The evolution of complex life over the past 600 million years was
disrupted by at least five mass extinctions, one of which occurred at
the close of the Triassic period. The end-Triassic extinction
corresponds to a period of high atmospheric-CO2 concentrations caused
by massive volcanism and biomass burning; most extinction scenarios
invoke the resulting environmental perturbations in accounting for the
loss of marine and terrestrial biodiversity. Here we reconstruct
changes in Tethyan shallow marine ecosystems and ocean redox chemistry
from earliest Jurassic (Hettangian)-aged black shales from Germany and
Luxemburg. The shales contain increased concentrations of the
biomarker isorenieratane, a fossilized pigment from green sulphur
bacteria. The abundance of green sulphur bacteria suggests that the
photic zone underwent prolonged periods of high concentrations of
hydrogen sulphide. This interval is also marked by the proliferation
of green algae, an indicator of anoxia. We conclude that the redox
changes in the entire water column reflect sluggish circulation in
marginal regions of the Tethys Ocean. We suggest that the resultant
repeated poisoning of shallow epicontinental seas—hotspots of Mesozoic
biodiversity—with hydrogen sulphide may have slowed the recovery of
marine ecosystems during the Early Jurassic.


Yasuhiro Iba, Shin-ichi Sano, Jörg Mutterlose and Yasuo Kondo (2012)
Belemnites originated in the Triassic—A new look at an old group.
Geology (advance online publication)
doi: 10.1130/G33402.1

Belemnites (order Belemnitida), a very successful group of Mesozoic
cephalopods, provide an important clue for understanding Mesozoic
marine ecosystems and the origin of modern cephalopods. Following
current hypotheses, belemnites originated in the earliest Jurassic
(Hettangian, 201.6–197 Ma) with very small forms. According to this
view their paleobiogeographic distribution was restricted to northern
Europe until the Pliensbachian (190–183 Ma). The fossil record is,
however, biased by the fact that all the previous studies on
belemnites focused on Europe. Here we report two belemnite taxa from
the Hettangian of Japan: a new species of the Sinobelemnitidae and a
large taxon of the suborder Belemnitina. The Sinobelemnitidae, which
may be included in the future in a new suborder, have also been
recorded from the Triassic of China, specimens so far poorly
understood. The presence of a very large rostrum attributed to the
Belemnitina suggests in addition that a diverse belemnite fauna
evolved earlier than previously thought. Our new findings therefore
(1) extend the origin of the belemnites back by ~33 m.y. into the
Triassic, (2) suggest that this group did not necessarily originate in
northern Europe, and (3) imply that belemnites survived the
Triassic–Jurassic extinction, one of the five big mass extinctions in
the Phanerozoic. Since belemnites provided a considerable amount of
food as prey, the origination of belemnites is probably an important
event also for the evolution of their predators, such as marine
reptiles and sharks.


Daisuke Koyabu, Wolfgang Maier, and Marcelo R. Sánchez-Villagra (2012)
Paleontological and developmental evidence resolve the homology and
dual embryonic origin of a mammalian skull bone, the interparietal.
Proceedings of the National Academy of Sciences (advance online publication)
doi: 10.1073/pnas.1208693109

The homologies of mammalian skull elements are now fairly well
established, except for the controversial interparietal bone. A
previous experimental study reported an intriguing mixed origin of the
interparietal: the medial portion being derived from the neural crest
cells, whereas the lateral portion from the mesoderm. The evolutionary
history of such mixed origin remains unresolved, and contradictory
reports on the presence or absence and developmental patterns of the
interparietal among mammals have complicated the question of its
homology. Here we provide an alternative perspective on the
evolutionary identity of the interparietal, based on a comprehensive
study across more than 300 extinct and extant taxa, integrating
embryological and paleontological data. Although the interparietal has
been regarded as being lost in various lineages, our investigation on
embryos demonstrates its presence in all extant mammalian “orders.”
The generally accepted paradigm has regarded the interparietal as
consisting of two elements that are homologized to the postparietals
of basal amniotes. The tabular bones have been postulated as being
lost during the rise of modern mammals. However, our results
demonstrate that the interparietal consists not of two but of four
elements. We propose that the tabulars of basal amniotes are conserved
as the lateral interparietal elements, which quickly fuse to the
medial elements at the embryonic stage, and that the postparietals are
homologous to the medial elements. Hence, the dual developmental
origin of the mammalian interparietal can be explained as the
evolutionary consequence of the fusion between the crest-derived
“postparietals” and the mesoderm-derived “tabulars.”

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