Abstract
Pyritized internal moulds of articulated shells of the Early Jurassic brachiopod taxon <i>Nannirhynchia pygmaea</i> were found in beds closely below the early Toarcian oceanic anoxic event in the <i>Polymorphum</i> Zone in Portugal. The material allows a detailed study of the outline of the muscle fields, the length and direction of the crura, and the orientation of the cardinalia, which are hitherto undescribed. Three-dimensional reconstructions of articulated shells of <i>N. pygmaea</i> occurring in a single horizon were produced to show the orientation and length of arcuiform crura. The preservation of internal moulds together with the three-dimensional reconstruction of the internal shell morphology allow a more precise description of the internal morphology of this taxon than it is possible with articulated shells and serial sections. <br><br> doi:<a href="http://dx.doi.org/10.1002/mmng.201200005" target="_blank">10.1002/mmng.201200005</a>
Highlights
Species of Nannirhynchia Buckman, 1918 are abundant in the stratigraphic record of Early Jurassic age outcropping in the Lusitanian Basin, where they represent important biostratigraphic tools, especially where ammonites are missing or are not well preserved
Three taxa of Nannirhynchia occur in the stratigraphic record of different palaeoenvironments in Portugal: (1) external platform: N. pygmaea (Morris, 1847), Polymorphum Zone (Davidson & Morris 1847), (2) subsiding areas: N. cotteri Choffat, 1995, Speciosum Zone (Almras et al 1995), and
The internal morphology of N. pygmaea is only known from two-dimensional serial sections
Summary
Species of Nannirhynchia Buckman, 1918 are abundant in the stratigraphic record of Early Jurassic age outcropping in the Lusitanian Basin, where they represent important biostratigraphic tools, especially where ammonites are missing or are not well preserved. Almras et al (1995) revised the genus with special attention on Portuguese material and established the first Nannirhynchia-based biostratigraphy for the Lusitanian Basin. Even though they pointed out that a high percentage of N. pygmaea in the S. Gio Formation (Barbosa et al 1988) is preserved as pyritized internal moulds, no description of muscle scars is given. We present a detailed description of ventral and dorsal muscle scars and the internal shell morphology according to threedimensional reconstructions after digitized serial sections and latex casts of internal moulds of Nannirhynchia pygmaea to allow future comparative studies of different kinds of preservation of this fossil group. The pyrite content in this time interval occurs only in the Tethyan but not in the Boreal Realm, where the Early Toarcian mass extinction event is described by the onset of a phase of black shale deposition (Hallam 1986; Little & Benton 1995; Wignall et al 2005)
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