Galeasteridae Minin et al., 2024

Family Galeasteridae Minin et al. View in CoL fam. nov.

( Fig. 1A View Figure 1 )

ZooBank LSID: urn:lsid:zoobank.org:act:557DEC84-05B3-4C7C-B5B1-81091BE59436 .

Diagnosis: Frontal furrow deepest at ambitus. Peristome at posterior end of furrow, facing slightly downwards, visible from below. Labral plate not separated from sternal plate. No rostral plate. Posterior gonopore-bearing plates disjunct from anterior gonopore-bearing plate by pair of intercalating plates; or all gonopore-bearing plates in contact. Marginal fasciole absent.

Genera included: Galeaster Seunes 1889 , Upper Cretaceous (Maastrichtian) to Palaeocene, from Europe to Kazakhstan.

Remarks: The fossil genus Galeaster has been attributed by Poslavskaya and Moskvin (1960) to the family Pourtalesiidae . This status was widely accepted in later studies ( Poslavskaya and Solovjev 1964, Solovjev 1994, 2005, Saucède et al. 2004, Smith 2004, Kroh and Mooi 2023a). The morphological differences discerning Galeasteridae from both Pourtalesiidae and Ceratophysidae are more substantial than the differences between the two latter families. Among these three families, Galeasteridae are unique in the orientation of the peristome, the shape of the frontal groove, and in plastron plating, while Ceratophysidae differ from Pourtalesiidae in plastron plating only.

In the family Galeasteridae , two morphological features show much greater variation than in both Pourtalesiidae and Ceratophysidae . The opening of the posterior gonopores is extremely variable in Galeaster . In the species Galeaster carinatus Ravn 1927 (Danian of Crimea, Mangyshlak, Ustyurt, and Denmark) and Galeaster dagestanensis Poslavskaya and Moskvin 1960 (Upper Danian—Montian of Crimea, Dagestan, and Mangyshlak), posterior gonopores can open either on the genital plates 1 and 4 (typical condition for Galeaster ) or on the anterior paired ocular plates ( Poslavskaya and Moskvin 1960: fig. 9d; Solovjev 1994: fig. 4d). Although in most Galeaster species the labral and sternal plates remain in full contact, in G. carinatus they are almost separated by the ambulacral plates Ia.2 and Vb.2 wedging in ( Poslavskaya and Moskvin 1960). Smith and Jeffery (2000) indicated that the differences in plastron plating might represent growth variability and proposed to synonymize all Galeaster species under the type species Galeaster bertrandi Seunes 1889 . Further detailed morphological comparisons are needed to confirm this synonymy.

The preceding stage of plastron disjunction recorded in G. carinatus Ravn 1927 and a migration of the posterior gonopores onto the anterior paired oculars in G. carinatus and G. dagestanensis Poslavskaya and Moskvin 1960 result in their morphological similarity to the Pourtalesiidae and Ceratophysidae . In the two latter families, posterior gonopore-bearing plates are pierced by ocular pores and adjacent posterolaterally to the anterior genital plate(s). Saucède et al. (2004) and Smith (2004) interpret the apical system of pourtalesiids and ceratophysids as a result of anterior migration of the posterior gonopores.