Aechmella Canu and Bassler, 1917

Taylor, Paul D., Martha, Silviu O. & Gordon, Dennis P., 2018, Synopsis of ‘ onychocellid’ cheilostome bryozoan genera, Journal of Natural History (J. Nat. Hist.) 52 (25 - 26), pp. 1657-1721 : 1665-1667

Genus Aechmella Canu and Bassler, 1917

( Figure 2 )

Type species

Aechmella filimargo, 1917 , by original designation. Eocene, late Jacksonian (= Priabonian), Sepulga River west bank, Escambia County, Alabama, USA .

Diagnosis

Colony encrusting. Autozooids subhexagonal ( Figure 2 (a–b)), rounded distally; zooidal boundaries raised with a medial groove. Cryptocyst extensive, seemingly granular. Gymnocyst lacking. Opesia terminal, occupying about one-third of frontal surface; bell shaped with lateral constrictions at approximately mid-length, lacking opesiular indentations, proximal edge concave. Ovicell hyperstomial, ooecium?cryptocyst-like ( Figure 2 (d)). Avicularia interzooidal, symmetrical, about half the width of an autozooid; opesia pear shaped; rostrum acutely triangular ( Figure 2 (a)). Ancestrula similar in morphology to budded autozooids but smaller ( Figure 2 (c)).

Remarks

The type species of Aechmella , A. filimargo , was described as rare by Canu and Bassler (1917, p. 30), who gave the range of the genus as Cenomanian to Miocene. Scanning electron micrographs of the type material of this species were kindly prepared for us by JoAnn Sanner of the USNM. The genus was named for the lance-like shape of the avicularia which, in contrast to the type and most other species of Onychocella , is symmetrical. Subsequently, Canu and Bassler (1920, p. 233) redescribed A. filimargo , introduced another species from the Eocene – A. crassimargo Canu and Bassler, 1920 – and referred a further nine existing species to Aechmella , the majority of which were originally described from the Late Cretaceous of Europe. Indeed, many of the species subsequently assigned to Aechmella are of Cretaceous or Paleocene age. There has been a tendency to use the genus for encrusting onychocellids with relatively small opesiae, wider than long, often almost crescent-shaped and sometimes with opesiular indentations but no lateral constrictions, and possessing small, symmetrical avicularia (e.g. Voigt 1959; Berthelsen 1962; Taylor and McKinney 2006; Taylor 2008). Based on the morphology of the type species, the generic identities of these species must be revised and we introduce the new genus Aechmellina below to accommodate them.

Range

Eocene to Miocene.

Figures

Figure 2. Aechmella filimargo Canu and Bassler; Eocene, late Jacksonian (= Priabonian), Sepulga River west bank, Escambia County, Alabama, USA; syntypes USNM 62585. (a,b) Syntype 2 showing autozooids and four avicularia marked by arrows. (c) Early astogeny in syntype 2 with ancestrula arrowed. (d) Syntype 1 with broken ovicell arrowed. Scale bars = 200 µm.