Abertellidae Durham, 1955

Mooi, Rich, Martínez, Sergio A., Del Río, Claudia J. & Ramos, Maria Inês Feijó, 2018, Late Oligocene - Miocene non-lunulate sand dollars of South America: Revision of abertellid taxa and descriptions of two new families, two new genera, and a new species, Zootaxa 4369 (3), pp. 301-326 : 316-317

Family Abertellidae Durham, 1955

Emended diagnosis. Scutelliforms with shallow to very deep, usually well-defined notch at ambitus in posterior interambulacrum; all oral interambulacra discontinuous, sometimes widely disjunct; interambulacral basicoronals large, usually more than twice length of ambulacrals. In addition, the following plesiomorphic features that are useful in that they are found in virtually all members of the family: two to five distinct trailing tube feet at end of each column of respiratory tube feet; pressure drainage channels absent, but ambulacral regions between main branches of food grooves populated by spines slightly shorter than interambulacral basicoronals; posterior interambulacral column typically narrowing towards ambitus and into posterior notch; periproct on oral surface between second, or second and third postbasicoronals.

Description and remarks. Previous diagnoses of the family ( Durham 1953, 1955, 1966) relied on combinations of easily discerned features otherwise not unique within the scutellines. Durham (1955) provided the following description for his new family: "Medium-sized to large, flattened; internal supports well developed; with broad ambulacral and anal indentations of margin; petals well defined, nearly closed; outer member of pore-pair greatly elongated, few primary pore-pairs outside petals; all interambulacra discontinuous on oral surface; basicoronal interambulacral plates considerably larger than ambulacral plates; periproct on oral surface; ambulacral food grooves bifurcating just outside basicoronal row; 4 genital pores." Unfortunately, not a single one of these features, taken on their own, is unique to the Abertellidae , and do not provide an unequivocal diagnosis in the modern concept of a diagnosis, which should consist of autapomorphies and, in some cases, distinctive plesiomorphic features unique to the taxon being diagnosed. In part because it is an entirely extinct group, this remains a challenge for the Abertellidae , and even the diagnosis provided above does not contain synapomorphies entirely unique to the abertellids.

However, when all these features are taken in combination, the family is well circumscribed. Durham (1955) noted both the discontinuous oral interambulacra and the pronounced posterior notch. Some other South American non-lunulate taxa possess this notch as well, but are not like abertellids in other ways, including the pattern of interambulacral discontinuity. Therefore, in considering such taxa, we are left with a choice of considerably broadening the concept of the Abertellidae to include these species, or establish new supraspecific taxa. We decided to restrict the concept of the Abertellidae to include only those forms that have all the oral interambulacra discontinuous, as discussed above in the sections dealing with the excluded forms such as Placatenella and Camachoaster .

The diagnosis of Abertellidae is therefore emended here to include scutellines with a shallow to very deep notch at the ambitus in the posterior interambulacrum, in combination with having all oral interambulacra strongly discontinuous in all adult specimens. Studies of specimens of the type species, A. aberti , from the Smithsonian Institution (NMNH 438168, 438169) permits inclusion of characters concerning spine morphology and distribution that seem unique to the family. Although the spines are differentiated and distributed in fields of aboral club-shaped and miliary types as in most scutelliforms, the oral surface is populated by fields of locomotory and geniculate spine types similar to those seen in mellitids, but not as strongly differentiated. There are no well-differentiated pressure drainage channels, but ambulacral regions between the main branches of the food grooves are populated by spines slightly shorter than interambulacral basicoronals. Each main branch of the food grooves beyond the primary bifurcation at the ends of the ambulacral basicoronals is always strongly developed in abertellids, but the degree of secondary branching is variable. The periproct is always situated distinctly on the flat portion of the oral surface, surrounded by the second pair of postbasicoronal interambulacral plates, or sometimes in contact with one (rarely both) of the third postbasicoronals. Features of the petals, which are lyrate, almost closed, but large (one half to three-quarters the length of the corresponding aboral ambulacrum), do not serve to distinguish abertellids from other scutelliforms. Trailing tube feet at the end of each column of respiratory tube feet are always large and distinct in abertellids, and can number up to five, but this also does not separate abertellids from other forms. The type species, A. aberti , can attain a TL of well over 150 mm, but the smallest known species, such as A. palmeri , are not known to exceed 60 mm TL. As noted and figured by Clark & Twitchell (1915), the microcanal system is well developed, as it is in several other scutelliform groups including lunulates such as the monophorasterids.

Type genus. Abertella Durham, 1953