Austrominius Buckeridge 1983

Buckeridge, John S. & Newman, William A., 2010, A review of the subfamily Eliminiinae (Cirripedia: Thoracica: Austrobalanidae), including a new genus, Protelminius nov., from the Oligocene of New Zealand, Zootaxa 2349, pp. 39-54 : 44-45

Genus Austrominius Buckeridge 1983

Elminius (Austrominius) Buckeridge, 1983a: 354 .

Diagnosis: Four-plated elminiines with thin-walled parietes; paries smooth internally, lacking “an inflected basal margin”; scutum without adductor ridge, articular ridge moderately prominent, adductor pits for depressor muscles absent; tergum with articular furrow wide, spur confluent with basi-rostral angle.

Distribution and age: Littoral, low energy environments. Recent: Southwest Pacific.

Type: Elminius modestus Darwin, 1854 . Recent: Australia, New Zealand [A common fouling species; introduced to southern England during the 1940s in war-time shipping convoys and more recently from Dunstaffnage, western Scotland (JB pers. obs., 1999)]. The distribution along the Atlantic seaboard is now fairly extensive, e.g. O’Riordan and Ramsay (1999), who recorded it from the Portuguese coast. They mention its occurrence in the Mediterranean (Zibrowius, pers. com.), but it is apparently known only from a very restricted distribution at Thau Lagoon, French Mediterranean (JB pers. obs., 1996, courtesy H. Zibrowius, and Zibrowius in Streftaris et al., 2005). The Thau population is small and may depend on yearly imports of oysters from the Atlantic coast of France (Zibrowius, pers. com). More recently, A. modestus has been report attached to drifting plastic as far north as the Shetland Islands ( Barnes and Miller, 2005).

Remarks: In the introduction we documented, as best we could, how the subgenus Austrominius became recognized at the full generic status, because it occurred more or less spontaneously rather than formally. This was done for historical rather than practical concerns, for Article 43.1 (Principle of Co-ordination) of the International Code of Zoological Nomenclature (1999) states that “a name established for a taxon at either rank in the genus group is deemed to have been simultaneously established by the same author for a nominal taxon at the other rank in the group; both nominal taxa have the same type species, whether it was fixed originally or subsequently.” Therefore, even though it is unnecessary, we consider it appropriate to formally endorse the full generic status of Austrominius , especially since it is being done in the same paper in which † Matellionius is being formally elevated.

Until Bayliss (1988, 1994), Austrominius had been a relatively small genus, but curiously his proposal of four new species did not attract attention at the time. His proposed species, Elminius adelaidae , E. eburescens , E. flindersi and E. placidus , all from the intertidal of South Australia, were then assignable to the subgenus Austrominius , which is now recognized as a full genus and, henceforth, they are so referred to herein ( Figures 1 , 2; Table 2). Populations of all four of Bayliss’s taxa, along with Austrominius modestus , are known to occur within 50 km of each other ( Bayliss, 1994: Figure 3 ). Bayliss (1994: 123) produced a key, in which differentiation of taxa is determined primarily on the basis of shell colour (although all of Bayliss’s taxa tend to be white or pale colours) and on pages 117 and 122, noted that it may be difficult to distinguish specimens that have worn or stained shells. Bayliss also used rather subtle variations in opercular valve morphology, minor variations in the colour patterns of the tergo-scutal flaps and slight differences in cirrial counts, but we fail to see how these could be considered specifically significant. Recent papers, such as Marchinko (2007), demonstrate that cirral counts in Balanus glandula Darwin, 1854 vary considerably in response to spatial and temporal differences in wave exposure; we suspect that ecotypic variation explains Bayliss’s forms, since they could be expected within a single species. There are numerous incidents in cirripedes where taxonomic splitting has been shown to have been premature: Van Syoc (1992) demonstrated that Amphibalanus saltonensis (Rodgers, 1949) (from the Salton Sea) was not a valid taxon, as specimens transferred to Mission Bay in California quickly established themselves as Amphibalanus amphitrite ( Darwin, 1854) ; Ross (1999) split Tetraclita rufotincta Pilsbry, 1916 into three species, which were shown in Appelbaum et al. (2002) to be only two species that nonetheless showed considerable “phenotypic plasticity”; thus we are not confident about Bayliss’ proposed species. Indeed, there is clearly a place for molecular techniques to clarify these taxa – and ideally this should be part of a genetic analysis of the Elminiinae as a whole. This will be the subject of a further paper2.

Hutton (1879) described two species of Austrominius from New Zealand: as Elminius sinuatus (from Wellington) and as Elminius rugosus (from The Bluff). Foster (1978) synonymised the former with E. modestus and confirmed the latter as a junior synonym of Epopella plicata . A further “species”, Austrominius australis , is listed in a table in Claessens (2005). This material was removed from the surface layers of a core sample collected in west Auckland, New Zealand and as the author provides no rationale for the name, it is here concluded that the use of “ australis ” is a mistake, as the material is almost certainly Austrominius modestus .

Late Eocene Miocene Holocene

Figures

FIGURE 1. Distribution of the Elminiinae and early Tetraclitoidea. The earliest record of the Elminiinae is the four plated Protelminius pomahakensis from the Rupelian (early Oligocene) of New Zealand. Assuming a six-plated ancestor to this taxon, parsimony requires a Hexaminius - type first occurring in eastern Australia during the Eocene, from which the four-plated form evolved and dispersed across a proto-Tasman Sea to New Zealand. If Hexaminius had occurred in New Zealand, it would not be unreasonable to expect it in the present warmer northern waters. It was most likely during the Oligocene to early Miocene that Elminius dispersed to South America (shaded arrow); with colonization of the Falkland Islands and the eastern seaboard of South America being facilitated by the opening of the Drake Passage, which had occurred by that time (Scher and Morris, 2006). The dotted line shows the very recent distribution path for Austrominius modestus from Australasia to Europe; although there is a report of single specimen of A. modestus from South African waters – a 1949 record in Sandison (1950), there have been no further records from Africa since then, and we can only assume that it has failed to colonise in the area. Uncertainty about the high Austrominius diversity in a restricted region in South Australia necessitates that these taxa be grouped. A further arrow (dotted and shaded), extending from the Antarctic to what would have been the proto-Tasman Sea, delineates a possible westward dispersal route for the tetraclitoid genus Austrobalanus; which has the earliest records from the Bartonian (middle Eocene) of the Antarctic Peninsula (Buckeridge, 2000). Oceanic patterns at and immediately prior to that time (see Nelson and Cooke, 2001) provided an opportunity for Austrobalanus to have dispersed to southern New Zealand (where it is first recorded from the Rupelian (Oligocene). Extinct genera marked †.

FIGURE 3. Sessilia divergence time chronogram. Nodes determined on the basis of the fossil record (figures in nonitalics in MYBP) and molecular and morphological data (figures in italics; after Pérez-Losada et al., 2007). The lack of fossil evidence for sessilian barnacles prior to the Cretaceous boundary is compelling, especially as there is a very diverse and widespread record of Cretaceous scalpelliformes (Newman and Ross, 1976; Buckeridge, 1996). We adopt a parsimonious approach to this, concluding that the Sessilia evolved very quickly during the period following the Cretaceous-Tertiary extinction event, such that all major balanomorph taxa had appeared between the early Palaeocene and the late Eocene. This is a much tighter timeframe than envisaged by Pérez-Losada et al., 2008.