Boucot et al. (1968) report the first citation of a micritic orthoceratid limestone in the entire circum-Mediterranean Silurian, later followed by Vai (1978). According to Kríz (1984), Ferretti and Kríz (1995) and Kríz and Serpagli (1993) the cephalopod limestone biofacies in the Prague Basin, southwestern Sardinia, Carnic Alps, and Montagne Noire represents a useful hard substratum for the settlement of the Cardiola Community Group. This faunal assemblage is mostly composed of epibyssate forms. Kríz (1984, pp. 191-192) writes, "The accumulation of empty cephalopod shells on the sea floor created good conditions for the attachment of organisms in this environment, with constant water flow providing a sufficient food supply. The origin and distribution of this biofacies was closely related to surface currents carrying empty floating shells of cephalopods before they were deposited on the relatively shallow sea floor." Many bivalve community groups are linked to nautiloid cephalopod tapho-communities that represent the remnant of living cephalopods inhabiting relatively shallow shelves or platforms (see Crick 1990 and Kríz 1999).
Another important environmental factor is that "The persistence in space and recurrence in time of the "Orthoceras" limestone facies are probably linked to both eustatic-climatic sea-level changes affecting the CCD, and periodic reactivation of the rifting system." (Vai 1991, pp. 234-237). According to the same author, the effect of a very shallow carbonate compensation depth (CCD), reinforced by high non-calcareous productivity (e.g., graptolites, acritarchs, and chitinozoans), constituted a crucial event in the Palaeozoic. "Eustatic sea-level falls may explain synchronous recurrence of widespread "Orthoceras"-limestone horizons, which can be traced as key beds for more than 1200 kilometers from Mali to Algeria, as observed by the Silurian-Devonian Boundary Subcommission during their 1971 field-trip" (see Kríz (1984). According to Vai (1991) this is attributable to differing regimes of oceanic and atmospheric circulation due to a warmer, more uniform climate than at present. Frakes (1979, p. 107), who provides an exhaustive study of climates throughout geologic time, writes: "A widespread marine transgression in Early Silurian reflects the melting of polar ice in Africa and South America, and several features combine to indicate generally warm and possibly dry climates during the Silurian and Devonian periods. Carbonates in general and reefs in particular seem to be more abundant than in the earlier Palaeozoic." Frakes continues (p. 106) "In summary, the paleolatitudinal distributions of several climatic indicators fall generally within the latitudinal ranges for their modern counterparts. This, while establishing the credibility of the paleomagnetic reconstruction for the Silurian and Devonian, also suggests that early Paleozoic temperature gradients and humidity patterns did not differ greatly from those of today. There is some faunal indication of warming in the early Silurian and gentle cooling until the middle Devonian, followed by a warm late Devonian. Overall, aridity seems to have typified Silurian and Devonian climates, except for the late Silurian and early late Devonian."
Before entering into any palaeobiogeographic and palaeoecological speculations based on nautiloid cephalopod assemblages, Crick's (1990, p. 147) comment on nautiloid paleoecology is worth noting; "The distribution of fossil nautiloids illustrated here (Late Cambrian to end of Devonian), and those of Flower (1976) and Crick (1980), indicates that these nautiloids were not truly part of the nekton capable of oceanic dispersal, but were members of the shallow-shelf vagrant benthos and were thus capable of dispersal only along continuous shelves or over shallow stretches of open ocean. For these reasons, simple distance and the depth of water separating shallow shelf seas were capable of restricting the dispersal of nautiloid cephalopods until such time as the physical environment removed these barriers." According to Crick, taking into account various magnitude events controlling in general the biogeography of nautiloid cephalopods and in particular those of the first magnitude such as geotectonic, it is possible to separate nautiloid biogeography into four episodes: (1) Late Cambrian; (2) Ordovician; (3) Silurian through Early Devonian; and (4) Middle through Late Devonian. The third episode is the concern of this article.
Holland et al. (1994) documented the commonly high concentration of specimens, characterizing the cephalopod limestone biofacies, describing Ordovician to Devonian examples from widely separated parts of the world. According to these authors different environmental causes could explain the high concentration of fossils. They also consider Doyle and MacDonald's (1993, p. 67) suggestions regarding so-called `belemnite battlefields' as most likely caused by post-spawning mortality. In conclusion, however, they write: "Thus we are left with catastrophic mass-mortality as the most likely explanation of the Palaeozoic concentrations. It must be said that this is adequate but that its precise nature may be difficult or impossible to discover." (Holland et al. 1994, p. 96).