About one hundred goniatite beaks (jaws) and five radulae from the Late Mississippian (Early Carboniferous) of Arkansas were studied with light and scanning electron microscopy (SEM). Four beaks were found within the body chamber of the goniatite Girtyoceras. Owing to the three-dimensional preservation, these oldest known beaks could be studied in detail and compared with those in living coleoids. The beaks are univalved, and the lower one is larger than the upper. Each beak consists of an organic outer and inner lamella; only the rostrum is weakly calcified. In the lower beak the outer and inner lamellae are about the same length, but in the upper beak the outer lamella is considerably shorter than the inner lamella. The goniatite beaks resemble those in living coleoids in the relative length of the outer and inner lamellae in the upper beak, which probably indicates similarity in muscle insertion. Concerning the length of the inner and outer lamellae, the lower beak is similar to that in Varnpyroteuthis and the pelagic octopod Trernoctopus. Late Mississippian goniatite beaks dealt with here are similar to those of Carboniferous and Permian goniatites in general morphology, but differ from those of Mesozoic ammonoids. In the latter ammonoids, the lower beak has a long outer lamella and a short inner lamella, whereas both lamellae have about equal length in the goniatites. Goniatite radulae remain stable during ammonoid evolution and demonstrate a more or less distinct similarity with those in living coleoids.
N. 2014: Embryonic shell structure of Early-Middle Jurassic belemnites, and its significance for belemnite expansion and diversification in the Jurassic. Lethaia, Vol. 47, Early Jurassic belemnites are of particular interest to the study of the evolution of skeletal morphology in Lower Carboniferous to the uppermost Cretaceous belemnoids, because they signal the beginning of a global Jurassic-Cretaceous expansion and diversification of belemnitids. We investigated potentially relevant, to this evolutionary pattern, shell features of Sinemurian-Bajocian Nannobelus, Parapassaloteuthis, Holcobelus and Pachybelemnopsis from the Paris Basin. Our analysis of morphological, ultrastructural and chemical traits of the earliest ontogenetic stages of the shell suggests that modified embryonic shell structure of Early-Middle Jurassic belemnites was a factor in their expansion and colonization of the pelagic zone and resulted in remarkable diversification of belemnites. Innovative traits of the embryonic shell of SinemurianBajocian belemnites include: (1) an inorganic-organic primordial rostrum encapsulating the protoconch and the phragmocone, its non-biomineralized component, possibly chitin, is herein detected for the first time; (2) an organic rich closing membrane which was under formation. It was yet perforated and possessed a foramen; and (3) an organic rich pro-ostracum earlier documented in an embryonic shell of Pliensbachian Passaloteuthis. The inorganic-organic primordial rostrum tightly coating the protoconch and phragmocone supposedly enhanced protection, without increase in shell weight, of the Early Jurassic belemnites against explosion in deepwater environment. This may have increased the depth and temperature ranges of hatching eggs, accelerated the adaptation of hatchlings to a nektonic mode of life and promoted increasing diversity of belemnoids. This study supports the hypothesis that belemnite hatchlings were 'a miniature of the adults'. □ Belemnites, embryonic shell, expansion, inorganic-organic shell matter, Jurassic.
BackgroundPreservation of original organic components in fossils across geological time is controversial, but the potential such molecules have for elucidating evolutionary processes and phylogenetic relationships is invaluable. Chitin is one such molecule. Ancient chitin has been recovered from both terrestrial and marine arthropods, but prior to this study had not been recovered from fossil marine mollusks.Methodology/Principal FindingsOrganics consistent with β-chitin are recovered in cuttlebones of Mississaepia mississippiensis from the Late Eocene (34.36 million years ago) marine clays of Hinds County, Mississippi, USA. These organics were determined and characterized through comparisons with extant taxa using Scanning Electron Microscopy/Energy Dispersive Spectrometry (SEM/EDS), Field Emission Scanning Electron Microscopy (Hyperprobe), Fourier Transmission Infrared Spectroscopy (FTIR) and Immunohistochemistry (IHC).Conclusions/SignificanceOur study presents the first evidence for organics consistent with chitin from an ancient marine mollusk and discusses how these organics have been degraded over time. As mechanisms for their preservation, we propose that the inorganic/organic lamination of the cuttlebone, combined with a suboxic depositional environment with available free Fe2+ ions, inhibited microbial or enzymatic degradation.
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