Porambonites, PANDER, 1830

Mergl, Michal, 2013, Porambonites Havliceki Sp. Nov., A New Brachiopod From The Šárka Formation (Darriwilian) From Bohemia And Its Contribution To Early History Of The Porambonitidae, Acta Musei Nationalis Pragae Series B 69 (1 - 2), pp. 87-92 : 90

Origin of Porambonites

Unique feature of the majority of porambonitid species is a pitted (=fenestrate) ornament. Similar ornament (subcircular pits of various shape and depth intercalated between capillae) are present also in punctolirinids, but this subfamily is strophic, with sessile spondylium and saccate ventral mantle canal system ( Carlson 2002). Finely pitted ornament is a feature known in more porambonitid genera ( Noetlingia , Porambonites ), but this ornament has been assumed as missing in the early Ordovician representatives of Porambonites and the related Eoporambonites ( Popov et al. 2005) . A syntrophioiod genus Tetralobula and related genus Poramborthis display a similar fenestrate ornament ( Havlíček 1977, Popov et al. 2005, Mergl 2011). The ornament of porambonitids, with fine costellae separated by narrow interspaces having a row of small pits (= fenestrae) indicates that the tetralobulids are the best evolutionary candidates for origin of the porambonitid clade. Carlson (2002) listed necessary changes for derivation of pentameridines from the orthide-like ancestor: commissure change from rectimarginate to uniplicate, decrease of hinge line width, reduction of interareas, dental plate change from divergent to convergent, lost of cardinal process, and weak impression of muscles.

All these advanced features are present in the Porambonites PANDER, 1830 , a genus likely derived from Tetralobula or a similar ancestor. Unlike Tetralobula , the genus Poramborthis lacks distinctly uniplicate commissure, has strophic shell with narrow hinge line and weakly impressed muscle fields, diplays distinct cardinal process, and possesses subparallel dental plates and same digital pallial canal mantle system as Porambonites . Multicostellate ornament of Poramborthis is likely a synapomorphy with Porambonites , although some of the earliest members of the Porambonitidae are described as possessing only multicostellate ornament without pits or have shell devoid of distinct costellation ( Popov et al. 2005). Poramborthis was likely confined to the Tremadocian of Western Gondwana ( Mergl 2011), with several stratigraphically subsequent or overlapping species in present-day Bavaria, Bohemia and Spain ( Havlíček 1949, 1950, 1977, Sdzuy 1955, Havlíček and Josopait 1972, Sdzuy et al. 2001). The earlier presence of Poramborthis in West Gondwana shelves indicates, that the porambonitid fauna of Baltica, having the first occurrence in the Floian, may be derived from the West- Gondwanan Tremadocian-aged ancestors. Poramborthis is therefore assumed the missing link to the Late Cambrian tetralobulids, the earliest member of the family Porambonitidae , and also the evolutionary forerunner of Porambonites .