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Journal of Paleontology 91(3):444–466
of pallial line at its posteroventral corner, pallial sinus small and shallow, posterior adductor muscle scar indistinct, very close to posterior shell margin; interior of posterior part of shell with distinct radial grooves.
Etymology.—Refers to the geographic area that is the source of all studied material.
Materials.—One articulated specimen (MSF 1276) and nine isolated valves (MSF 1273, 1274, 1279, 2105–2109; MZB 60218); see Table 8 for measurements.
Remarks.—Most similar to “Pliocardia” italica n. sp. are two extant species from the central Indo-Pacific Ocean: “Pliocardia” solidissima (Prashad, 1932) and “Pliocardia” ticaonica (Dall, 1908). Both have a similarly large and prosogyrate umbo and broad posterior margin, but differ from the Italian Miocene “Pliocardia” italica n. sp. by having a more convex posterodorsal margin; in addition, P. ticaonica is shorter than “Pliocardia” italica n. sp. Among the taxa from the Caribbean Sea and the Gulf of Mexico, “Pliocardia” caribbea (Boss, 1967) has a broader anterior margin and has the anterior adductor muscle scar in a more dorsal position; “Pliocardia” cordata (Boss, 1968) is shorter and has a smaller and more pointed beak; and “Pliocardia” ponderosa (Boss, 1968) seems to have a steeper sloping posterodorsal margin.
Discussion
The present study is intended to provide a taxonomic baseline for future biogeographic and evolutionary studies. At first glance, the bivalve fauna of the ‘Calcari a Lucina’ seep deposits show a wide range of biogeographic links to places as distant as the Recent western Pacific(Bathymodiolus, Archivesica, and Meganodontia), the Recent Indian Ocean (“Pliocardia”), as well as the Miocene Caribbean seep faunas (Meganodontia). We identified 11 chemosymbiotic bivalve species from the middle to late Miocene ‘Calcari a Lucina’ methane-seep deposits in northern Italy, among them three mytilids, one thyasirid, two lucinids, and four or five vesicomyids. Apart from Bathymodiolus (s.l.) moroniae n. sp. and B.(s.l.)
miomediterraneus n. sp. (two species that resemble extant bathymodiolins) the taxonomic affinity of the new genus and species Samiolus iohannesbaptistae is unclear. This taxon might represent a rare case of a genus that occurs exclusively at deep- water methane seeps, but does not belong to the bathymodiolins. An interesting taphonomic aspect of the mytilids is the common
Table 8. Material of “Pliocardia” italica new species, all specimens from Ca’ Cavalmagra; H=height, L=length,W=width of two valves.
Specimen
MGGC 21909 (holotype) MSF 1275 (paratype) MSF 1276
MSF 1274 (LV)
MSF 2105 (RV) MSF 2106 (LV) MSF 2107 (RV) MSF 2108 (LV) MSF 2109 (RV)
L (mm) 59.6
56.6 55
60 60
50 35
H (mm) W(mm) 42.1
39.4 50
37 31 30
occurrence of B.(s.l.) moroniae n. sp. and Samiolus iohannesbap- tistae n. gen. n. sp. as disarticulated shells,which is in contrast to the articulated mode of occurrence of the vast majority of other fossil seep-associated bathymodiolins (Squires and Goedert, 1991; Amano et al., 2010; Kiel et al., 2010; Saether et al., 2010; Amano and Jenkins, 2011; Kiel and Amano, 2013). Articulated shells, including juveniles and giant specimens up to 133mm in length, indeed occur at Ca’ Piantè, aswell as inDeruta, but this is rather an exception than the rule. At thewell-sampled sites at Montepetra and Case Rovereti, where disarticulated mussel shells are common, the co-occurring vesicomyids and lucinids were found as articulated or semi-articulated shells. This excludes the possibility that the mussel shells disarticulated due to transport, but suggests in-situ disarticulation, either by the force of the ligament, or perhaps due to large scavengers. In addition to the seep-inhabiting bathymodiolins reported
here, a bathymodiolin named Adipicola apenninica Danise, Bertolaso, and Dominici, 2016 was recently described from a whale bone from the middle Miocene (Langhian) Pantano Formation in the vicinity of the town of Carpineti (Reggio Emilia province, northern Italy). Adipicola apenninica was reported as being very abundant, where the associated thyasirids and lucinids were rare (Danise et al., 2016). Most Cretaceous to extant lucinids at methane seeps belong
to either the Codakiinae or the Myrteinae (Taylor et al., 2011; Kiel, 2013). Thus, if the inclusion of Meganodontia in Pegophyseminae (cf., Taylor et al., 2011, 2014) is correct, it would be the third lucinid subfamily to have a considerable fossil history at methane seeps (Taylor et al., 2014; Kiel and Hansen, 2015). The taxonomy of the Vesicomyidae is still in flux, partially
due to the frequent convergence among their few shell characters (Krylova and Sahling, 2010). Amano and Kiel (2010) recently questioned Paleogene records of Archivesica, and Amano et al. (2014) identified the early Miocene Japanese A. sakoi Amano et al., 2014 as earliest member of Archivesica sensu strictu. The middle Miocene Archivesica strigarum n. sp. introduced here is thus among the earliest Archivesica species and is, just as A. sakoi, very elongate compared to extant Archivesica species.
Acknowledgments
This paper is a tribute to the late paleontologist M.A. Moroni for her intuition in the ‘60s that the Santa Sofia ‘Calcari a Lucina’ was a peculiar habitat of its own.We are grateful to P. Aharon, S. Conti, G.B. Vai, R. Barbieri, S. Cau, L. Angeletti, G. Bini, S. Gualtieri, for their cooperative work in recognizing and sampling ‘Calcari a Lucina’ limestones and their equivalents along the Italian Apennines over more than 25 years. Above all, we warmly thank M. Sami (Museo Civico di Scienze Naturali, Faenza) for the generous access to the vast collection of fossils from the ‘Calcari a Lucina’ deposits and for comments on various aspects of stratigraphy, and the collaborators of the Museo Civico di Scienze Naturali di Faenza, A. Benericetti, M. Diversi, and V. Liverani, for collecting part of the material. Thanks to D. Ormezzano (Museo Regionale di Scienze Naturali, Torino) for access to, and help with, the collection under his care, and S. Cavagna and G. Pavia for their cooperation.
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