Discussion

The identification of Eovolutilithes miria n. sp. from the Maastrichtian Miria Formation refines the earliest Australian record of Volutidae, showing it to represent a distinct endemic species. Compared with E. subsemiplicatus, E. miria n. sp. is larger and more elongate, with opisthocline ribs that extend farther toward the base of the last whorl. Holzapfel (1888–1889) reported E. subsemiplicatus as somewhat smaller (max shell length of 83 mm in that study; Fig. 2N) than the new species, which is estimated to reach at least 135 mm in length (Fig. 2I).

Darragh (1997) noted that the Maastrichtian Miria Formation shows no direct taxonomic relationship with the Palaeocene Pebble Point assemblage, except possibly a species referred to Fusinus? (Darragh & Kendrick 1994). This absence of continuity indicates a substantial faunal turnover across the Cretaceous–Palaeogene boundary in Australia, with the Pebble Point fauna representing a distinct, newly established Palaeocene assemblage. We found no evidence in the literature to suggest that Eovolutilithes survived the K–Pg boundary anywhere globally.

We interpret Costopeplum rhytidatum as part of the Palaeocene cosmopolitan expansion noted by Darragh (1997), yet it is morphologically distinct from any known non-Australian genus. The Palaeocene record of volutes is limited and uneven, and few assemblages outside Australasia and Antarctica preserve comparable material (Beu & Maxwell, 1990; Stilwell, 2016; Dockery, 1993; Stilwell, Zinsmeister & Oleinik, 2004). Given this incomplete record, the absence of close relatives elsewhere cannot be taken as evidence of provincial restriction. We therefore regard Costopeplum as a taxonomically conservative solution pending better global coverage of Palaeocene Volutidae.

Developmental mode and dispersal

Friend et al. (2021) study on Athleta showed that non-planktotrophic forms were absent during the Palaeocene and early Eocene, first appearing in the middle Eocene and becoming dominant by the late Eocene as planktotrophic lineages declined. Complementary evidence from the North American Pacific Slope indicates that this developmental transition was already underway by the latest Cretaceous: members of the Volutoderma–Longoconcha–Retipirula complex exhibit progressive reduction in protoconch size and whorl count, tracing a change from multispiral to paucispiral protoconchs and hence from planktotrophic to non-planktotrophic development (Olsson 1930; Saul & Squires 2008). These warm-temperate taxa demonstrate that developmental-mode evolution within Volutidae began before the K–Pg boundary and continued globally through the early Cenozoic.

As such, the multispiral protoconch of C. rhytidatum suggests that the retention of the ancestral planktotrophic mode may be considered feasible for this species (Fig. 3H, I), consistent also with the likely arrival of this species with a Palaeocene pulse of cosmopolitan arrivals into Australia (Darragh 1997). If a planktotrophic condition was indeed the cause for C. rhytidatum, the paucispiral protoconch of the Eocene N. protorhysum (Fig. 3J, K) may thus suggest a transition toward non-planktotrophy at some point in the Late Palaeocene–Eocene, paralleling the irreversible loss observed in Athleta during the Eocene (Friend et al., 2021). This global shift from multispiral to paucispiral protoconchs reflects the wider neogastropod trend in which planktotrophy was ancestral and non-planktotrophy evolved repeatedly and independently in multiple Cenozoic lineages (Bandel 1993).

Austral–Antarctic connections

The occurrence of morphologically comparable volutes in both New Zealand and Australia (Fig. 3) suggests that these regions shared an element of their respective volutid faunas already diversifying by the Late Palaeocene (Darragh 1997; Stilwell 2016). In addition to Teremelon onoua, Stilwell (2016) described Palaeocene volutids from the mid-Danian Wangaloa Formation (Wangaluta henaconstricta, Alcithoe s.l. wangaloaensis) and refigured Voluta neozelanica Finlay & Marwick 1937 (= Wangaluta? neozelanica). The affinity of these taxa to Costopeplum/Teremelon remains unknown and warrants further study.

Palaeocene volutes are also known from Antarctica, albeit largely appearing to be morphologically rather distinct from C. rhytidatum/T. onoua: Palaeomelon apheles Stilwell, Zinsmeister & Oleinik (2004) occurs in the Danian Sobral Formation on Seymour Island, showing that Volutidae reached Antarctic waters soon after the K–Pg event. The earliest recorded South American volutes appear later, in the Eocene (Del Río & Martínez 2006).

Biogeographic context

Australian molluscan lineages through the Palaeogene show limited interchange with Asia, consistent with the persistence of deep-water barriers across the Sunda–Sahul region until the Miocene (23–5.3 Ma; Hall 2012). The Tethyan–Indo-Pacific biogeographic signature identified by Darragh (1985) strengthens markedly only from the Miocene, when shallow marine corridors developed between the Australian and Southeast Asian plates.

In contrast, southern dispersal pathways linking Australasia, Antarctica, and South America were already well established during the latest Cretaceous and early Palaeogene, forming the Weddellian Biogeographic Province (Zinsmeister 1982; Crame 2015). Shallow, temperate seas between these regions likely facilitated faunal interchange until progressive Eocene rifting severed connections. In addition to the volute emergence and expansion in the Southern Hemisphere from the Palaeocene (see previous), the family Struthiolariidae also illustrates this continuity: originating in the Late Cretaceous of New Zealand (Conchothyra parasitica Hutton, 1877), the group occurs in the Palaeocene of Antarctica (Perissodonta austerocallosa Wilckens, 1911; refigured by Stilwell, Zinsmeister & Oleinik, 2004) and in Eocene–Miocene strata of South America (Struthiolaria Lamarck, 1816; Perissodonta E. von Martens, 1878 spp.) (Beu 2009; Crame 2015). Early Cenozoic Antarctic assemblages contain several neogastropod lineages allied to Austral taxa, supporting continued southern interchange before isolation (Darragh 1988; Beu 2009).

Conclusion

The recognition of Eovolutilithes miria n. sp. from the Maastrichtian of Western Australia provides the earliest record of Volutidae on the continent, representing an isolated Cretaceous occurrence rather than a precursor to later lineages. The subsequent appearance of Costopeplum rhytidatum n. gen. & n. comb. in the Palaeocene Pebble Point Formation documents a separate phase of volutid history, consistent with post–K–Pg faunal renewal and possible larval dispersal via planktotrophy during a period of cosmopolitan exchange. Together, these taxa bracket a major turnover in Australian marine faunas and offer rare stratigraphic reference points for the early evolution of Volutidae in the Southern Hemisphere. Although the record remains sparse, their combined evidence suggests that southern temperate shelves played an important role in the establishment and subsequent diversification of the family after the K–Pg event.