Schizocrania HALL et WHITFIELD, 1875

Mergl, Michal & Nolčová, Lucie, 2016, Schizocrania (Brachiopoda, Discinoidea): Taxonomy, Occurrence, Ecology And History Of The Earliest Epizoan Lingulate Brachiopod, Fossil Imprint 72 (3 - 4), pp. 225-238 : 226-227

Genus Schizocrania HALL et WHITFIELD, 1875

T y p e s p e c i e s.? Orbicula filosa HALL, 1847 ; Trenton

Group, Ordovician; New York State, USA.

R e m a r k s. In his emended diagnosis of Ptychopeltis PERNER, 1903 , and comment to this genus, Havlíček (1972) noted that there are two main differences between Ptychopeltis and Schizocrania HALL et WHITFIELD, 1875 : the very high dorsal valve and the narrow, parallel-sided pedicle notch of the former genus. Later, Havlíček and Vaněk (1996) stated that distinction between these genera needs further investigation.

The parallel-sided pedicle notch of Ptychopeltis is distinctly seen in newly sampled specimens of S. multistriata ( REED, 1905) (Pl. 1, Figs 11, 14) from the Barrandian area. Unfortunately, the outline of the pedicle notch is unknown in the majority of species formerly referred to Ptychopeltis and Schizocrania , from Bohemia and elsewhere. Only a few illustrated specimens ( Hall and Clarke 1892, Havlíček 1972, Lockley and Williams 1981, Bassett 1986, Sutton et al. 2000, Mergl 2006, 2010) indicate that the pedicle notch shape may vary from parallel-sided ( S. multistriata ), moderately triangular ( S. striata (J. DE C. SOWERBY, 1839) , S. verneuilii ( DAVIDSON, 1848) , Schizocrania sp. A sensu Basset (1986)) to very broad ( S. filosa , S. striata ). The shape of the pedicle notch can vary between individuals referred to the same species ( Sutton et al. 2000), and its outline can be changed by post-mortem process. Dorsally flexed margins of the pedicle notch in living specimens are easily broken off and compressed ( Mergl 2006) in deformed specimens. Thus, the broadly triangular pedicle notch can be an artifact of flexible compression. The broadly wide outline of the notch of S. filosa figured by Hall and Clarke (1892) could have been exaggerated by preservation, because shells of S. filosa are generally collapsed and flattened. An undeformed ventral valve of S. multistriata in a nodule retains the original parallel-sided outline ( Havlíček 1972, Mergl 2002).

High convexity of the dorsal valve is the second difference noted by Havlíček (1972), and is prominent in numerous specimens currently referred to Schizocrania (= Ptychopeltis incola , P. hornyi ) in Sandbian sandstone (Libeň and Letná formations) in the Barrandian area. The convexity of specimens preserved in shale varies widely, from low to highly conical. Lockley and Antia (1980: text-fig. 2A), Bassett (1986: pl. 8, fig. 15), Mergl (2006) figured originally strongly convex dorsal valves of Schizocrania , but the dorsal valves of S. verneulii ( Mergl 2006, 2010) are only gently arched. The shell convexity and nature of anterior commissure were influenced by the substrate and its relief, by mode of preservation, and by sediment compaction; they therefore have weak taxonomic value.

The observed absence of radial costellae on the ventral valve in some early species of Schizocrania clade is seemingly stable, and significantly different from the later Silurian and Devonian species. The latter regularly bear radial costellae on the ventral valve exterior ( Mergl 2006, 2010). The Ordovician species of Schizocrania are much complicated. Some species are erected only on the dorsal valves ( Reed 1917, Havlíček 1972, Harper 1984), and the ventral valve ornamentation is unknown. The figured ventral valves of Schizocrania multistriata REED, 1905 from the Middle Ordovician of Wales ( Lockley and Williams 1981) and the Darriwilian of Bohemia ( Havlíček 1972, Mergl 2002) lack radial costellation, but other specimens referred to the same species ( Sutton et al. 2000: pl. 14, fig. 4) show distinct radial costellation. The type species Schizocrania filosa is illustrated ( Hall and Clarke 1892) without radial costellation of the ventral valve, and the newly well-illustrated specimens referred to S. filosa (http://drydredgers.org/brachschizo.htm, accessed 20 November 2016) show only concentric ornamentation. In summary, because there is no distinct feature that might be useful for discrimination of Ptychopeltis from Schizocrania , the former is designated here as a junior synonym of Schizocrania .

The genus Orbiculothyris WOLFART, 1968 was synonymized with Schizocrania by Holmer and Popov (2000), but that needs further confirmation. A distinct feature is the posteriorly closed pedicle opening of Orbiculothyris , unknown in any species of Schizocrania , which indicates relationship to other trematids and supports the validity of Orbiculothyris .

Some occurrences of Schizocrania are poorly known, and relationship of the described shells need revision ( Barrett 1878, Douglas 1933, Newell and Tafur 1944, Hede 1951). Among the problematic species, Hurst (1979) described and illustrated Schizocrania hewardi from the Upper Ordovician, Onny Shale Formation (Early Katian; Streffordian) of Shropshire, England ( Cocks 2008). Due to its unusual ornamentation with subdued capillate ornamentation, absence of distinct larval shell, very small size (only 1.2 mm shell length), and highly convex dorsal valve at this growth size, we consider that figured specimens is not a trematid brachiopod. The figured specimens of Hurst (1979: figs 78–83) might be an ostracod or a primitive mollusc.

Species previously referred to genus:

Schizocrania asteroidea REED, 1917 ; Upper Ordovician, Katian (Rawtheyan); Girvan, Scotland.

Schizocrania filosa ( HALL, 1847) ; Upper Ordovician; NE of USA and Canada.

Schizocrania helderbergia HALL et CLARKE, 1892 ; Lower Devonian, Lower Helderberg Group; NE of USA.

Schizocrania hewardi HURST, 1979 ; Upper Ordovician, Katian; Shropshire, England (likely it is not a brachiopod).

Schizocrania multistriata ( REED, 1905) ; Middle Ordovician, Darriwilian; Pembrokeshire, Wales; Barrandian area, Bohemia.

Schizocrania schucherti HALL et CLARKE, 1892 ; Upper Ordovician, Hudson River Group; USA (likely junior synonym of S. striata ).

Schizocrania striata (J. DE C. SOWERBY, 1839) ; Silurian to Early Devonian, Ludlow to Lochkovian; Shropshire, England; Barrandian area, Bohemia; Gotland, Sweden; Podolia, Ukraine.

Schizocrania superincreta BARRETT, 1878 ; Lower Devonian, Oriskany Sandstone; USA.

Schizocrania ? turnbulli ( REED, 1905); Upper Ordovician , Katian; Pembrokeshire, Wales .

Schizocrania verneuilii ( DAVIDSON, 1848) ; Silurian, Wenlock; Herefordshire, England.

Schizocrania sp. ; Middle Ordovician, Darriwilian; Carmarthenshire, Wales ( Lockley and Antia 1980); Upper Ordovician; Quebec, Canada ( Harland and Pickerill 1987); Silurian to Lower Devonian, Pridoli to Lochkovian; North France ( Bassett 1986).