Acernaspis Campbell, 1967

Sandford, Andrew C. & Holloway, David J., 2006, Early Silurian phacopide trilobites from central Victoria, Australia, Memoirs of Museum Victoria 63 (2), pp. 215-255 : 219-220

Acernaspis Campbell, 1967

=Eskaspis Clarkson et al., 1977; = Murphycops Lespérance,

1968

Type species. Phacops orestes Billings, 1860 from the Jupiter Formation (Llandovery), Anticosti I., Quebec, by original designation .

Remarks. Chatterton and Ludvigsen (2004: 39) discussed the possible synonymy of Acernaspis with Portlockia McCoy, 1846 (type species P. sublaevis ) from the Wenlock of western Ireland and gave notice of their intention to apply to the International Commission of Zoological Nomenclature to suppress the latter generic name as a nomen oblitum. We would support such an application as being in the best interests of nomenclatural stability.

Chatterton and Ludvigsen (2004) considered Acernaspis mimica Lespérance and Letendre, 1982 to be a synonym of A. orestes , and also noted that A. superciliexcelsis Howells, 1982 differs only in very minor respects from orestes and might be considered synonymous. These conclusions are in accord with the cladistic analysis of Acernaspis presented by Ramsköld and Werdelin (1991, fig. 5), in which these three taxa are grouped together. Curtis and Lane (1998) considered a number of other species to be synonyms of orestes including A. quadrilineata ( Angelin, 1851) , A. konoverensis Männil, 1970 , A. elliptifrons ( Esmark, 1833) and A. sororia Ramsköld, 1985 ; however, Ramsköld and Werdelinʼs cladistic analysis shows these species and orestes grouped quite separately. Chatterton and Ludvigsen arrived at a similar conclusion, rejecting Curtis and Laneʼs synonymy and listing a number of characters distinguishing orestes and quadrilineata , emphasising minor differences in the lens formula and the proportions of the glabella.

Acernaspis rubicundula Ramsköld, 1985 , from the lower Wenlock of Sweden, was interpreted by Ramsköld as intermediate between Acernaspis and Ananaspis with respect to morphological trends that he recognised between his late Llandovery species Acernaspis sororia and Ananaspis stokesii ( Milne Edwards, 1840) from the British Wenlock. He emphasised the glabellar width, the short (exsag.) L2 and the glabellar tuberculation as characters of rubicundula typical of Ananaspis , and remarked that stokesii differs from rubicundula in few features of which only the strongly forwardly expanding glabella and deep lateral border furrow (on the fixigena) were mentioned. Neotenic changes significant in the evolution of Ananaspis from Acernaspis , including a decrease in the length (exsag.) of L2 and the development of glabellar tuberculation, were outlined by Ramsköld (1988). In this context he interpreted rubicundula as a sister-taxon to all other Acernaspis species, exhibiting the first of these neotenic changes. In their cladistic analysis of Acernaspis, Ramsköld and Werdelin (1991) discovered that their consensus tree was essentially unresolved with the inclusion of rubicundula but on its removal an almost fully resolved consensus tree was obtained.

Nevertheless they retained rubicundula in Acernaspis , and we agree that the majority of its characters clearly ally the species with that genus. However, in view of its unusual features, we assign rubicundula to Acernaspis with question, together with a somewhat similar species described below as A.? sp.