Phacoceratidae Shimansky, 1962

Family Phacoceratidae Shimansky, 1962

Fig. 17 View Fig

Diagnosis

Family of the superfamily Subclymenioidea with a usually subinvolute conch; coiling rate usually extremely high; whorl overlap small to moderate. Adult whorl profile extremely compressed with flat flanks and an acute venter. Ornament with longitudinal ridges or lines in the juvenile stage, adult conch smooth or with delicate growth lines. Position of the siphuncle between the centre of the whorl profile and the venter. Suture line without or with a very small external lobe and a broadly rounded lateral lobe.

Included genera

Phacoceras Hyatt, 1884 (Viséan to Serpukhovian; 4 species).

Leuroceras Hyatt, 1893 (Viséan; 1 species).

Diorugoceras Hyatt, 1893 (Viséan to Serpukhovian; 3 species).

Phaceras Teichert & Glenister, 1952 [nomen nullum].

Epiphacoceras Turner, 1966 (Viséan; 1 species).

Askeatonoceras Turner, 1966 (Viséan; 1 species).

Pseudostenopoceras Shimansky, 1967 (Serpukhovian to? Moscovian; 3 species).

Remarks

The Phacoceratidae are one of the lesser-known families of Carboniferous nautiloids. Kummel (1964) did not accept the family and included Phacoceras and similar genera together with some Devonian genera ( Centroceras Hyatt, 1884 , Carlloceras Flower & Caster, 1935 , Homaloceras Whiteaves, 1891 , Strophiceras Hyatt, 1884 ) in the family Centroceratidae .

Turner (1966) devoted a special article to the Phacoceratidae and, in discussing their phylogeny, pointed out that some of the genera were monospecific, with type species known from only a few specimens or even only a single specimen. He also saw a close relationship between Phacoceras and the Devonian centroceratids and postulated an evolutionary lineage from Carlloceras (possibly Late Devonian) through Diorugoceras to Phacoceras . This scenario is mainly based on the idea that the inverted trapezoidal whorl profile of Carlloceras transformed into an oxyconic profile during evolution.

Another hypothesis was proposed by Dzik (1984: 167). He derived the Phacoceratidae , to which he also included the genera Aphelaeceras and Stenopoceras , from the stroboceratids. This phylogenetic hypothesis is quite plausible, because the morphology of the inner whorl of Phacoceras shows a morphology known from Catastroboceras , with the pronounced umbilical margin decorated by longitudinal ridges, the flattened flanks and the distinct ventrolateral shoulder ( Schmidt 1951: pl. 6 fig. 4). It is therefore not necessary to propose a long and hidden evolutionary lineage including survival through the Hangenberg Event, based on poor material.

Poor preservation of type material is a problem when it comes to family composition. The two genera Epiphacoceras and Askeatomoceras newly described by Turner (1966) and Diorugoceras are based on poorly preserved material and are therefore difficult to evaluate.