Ceratophysidae Minin et al., 2024

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

( Figs 1F–H View Figure 1 , 2F–I View Figure 2 , 3 View Figure 3 , 5–7 View Figure 5 View Figure 6 View Figure 7 , 8 View Figure 8 , 10 View Figure 10 )

ZooBank LSID: urn:lsid:zoobank.org:act:FFBDFC0D-66AF-45E4-8593-D3EA5FBCD941 .

Diagnosis: Frontal furrow deepest on oral side of test. Peristome at posterior end of deep furrow, facing forwards or slightly upwards, not visible from below. Labral plate large, symmetrical, separated from sternal plate by one or two symmetrical pairs of plates ( Fig. 1F–H View Figure 1 ). No rostral plate. All gonopore-bearing plates in contact. Marginal fasciole absent.

Genera included: Ceratophysa Pomel 1883 (type genus), Echinocrepis Agassiz 1879 , Echinogutta Mironov 1997 , Echinosigra Mortensen 1907 , Helgocystis Mortensen 1907 , Rictocystis Mironov 1996 , Solenocystis Mironov 2008 , and Spatagocystis Agassiz 1879 . The ceratophysids are unknown as fossils.

Remarks: The establishment of the family Ceratophysidae is well supported by both molecular and morphological data ( Fig. 3 View Figure 3 ). Representatives of the family demonstrate peculiar arrangement of plastron plating: well-developed labrum is always separated from the sternum by one or two symmetrical pairs of elongated ambulacral plates. This feature allows ceratophysids to be unambiguously distinguished from both pourtalesiids and galeasterids. Tuberculation of the plastronal area is different in Ceratophysidae and Pourtalesiidae . The intercalating ambulacral plates of the ceratophysids are densely covered with tubercles along the ventral midline. Dense tuberculation then stretches posteriorly onto the sternal plate and episternals. Intercalating ambulacral plates of the pourtalesiids lack dense tuberculation, which is observed along the ventral midline only on the sternal plate and episternals. Furthermore, the family Ceratophysidae is characterized by the prevalence of tridentate pedicellariae over other types, whereas in most pourtalesiid species, rostrate pedicellariae are the most common.

Based on morphological features, Mironov (2008) previously distinguished two groups among genera now belonging to the family Ceratophysidae . Each group is characterized by a unique type of plastron plating. In the first group ( Ceratophysa , Echinocrepis , Echinogutta , Echinosigra , and Helgocystis ) only one pair of ambulacral plates separates the labrum from the sternum (plastron plating type IV; Fig. 1F, G View Figure 1 ). The intercalating ambulacral plates carry long intra-plate lines. These lines extend meridionally and resemble the sutures between adjacent plates, but typically do not reach the plate border at one or both ends ( Fig. 1F, G View Figure 1 ). According to Mironov (2008), these structures might either be original elements related to the strengthening of the plates, or traces indicating incomplete plate coalescence. The anterior part of the test in the genera of the first group usually forms a so-called ‘neck’ ( Fig. 2G View Figure 2 ). The ‘neck’ is indistinguishable only in Ceratophysa and Echinocrepis , characterized by the broadest tests. Miliary spines are curved in their distal part in this group (with the exception of Helgocystis carinata ).

In the second group ( Solenocystis , Spatagocystis and, probably, Rictocystis ), the labrum is separated from the sternum by two pairs of ambulacral plates (plastron plating type V; Fig. 1H View Figure 1 ). The latter lack long intra-plate lines. The ‘neck’ is absent in this group. Since Rictocystis is known only by test fragments, the type V plastron plating is inferred for this genus based on morphological similarity of its spines and pedicellariae with those of Solenocystis and Spatagocystis . Miliary spines in all three genera of this group are curved in the median part.

Molecular phylogenetic reconstructions reveal two major sub-clades within Ceratophysidae ( Fig. 3 View Figure 3 ); however, the genera characterized by a differentiated ‘neck’ and plastron, disrupted by one pair of ambulacral plates, are found within both of them. Each of these sub-clades includes a genus, characterized by the miliary spines curved in the median part (either Solenocystis or Helgocystis ). No morphological features distinguishing these sub-clades from each other were found. However, only one genus with two pairs of intercalating ambulacral plates ( Solenocystis ) was included in the analysis. Molecular data on two other genera, Spatagocystis and Rictocystis , are needed to test the monophyletic status of this morphological grouping.

The rank of the subgenus Echinosigra (Echinogutta) is raised to a genus rank. Phylogenetic reconstructions show that the genera Echinosigra and Echinogutta belong to different, very distant lineages within the family Ceratophysidae ( Fig. 3 View Figure 3 ). Although Echinosigra was represented in the analysis only by the type species Echinosigra phiale , we propose that the three other species with a long ‘neck’ and two gonopores (diagnostic features of the nominal subgenus) should remain within the genus. These species are Echinosigra mortenseni , Echinosigra porrecta , and Echinosigra vityazi . The genus Echinogutta thus should accommodate the species with the short ‘neck’ and four gonopores ( Echinogutta amphora , Echinogutta antarctica , Echinogutta fabrefacta , Echinogutta valvaedentata , and, provisionally, Echinogutta sp. A ). In the phylogenetic reconstructions presented here, the putative new species Echinogutta sp. A and Echinogutta amphora belong to the same lineage, but do not group together ( Fig. 3 View Figure 3 ). This indicates the possibility that Echinogutta is paraphyletic. Further reconstructions with broader taxon sampling are needed to clarify the phylogeny and taxonomic composition of Echinosigra and Echinogutta .

The only species of the genus Rodocystis , R. rosea , was initially described as a Pourtalesia species ( Agassiz 1879, 1881). The majority of subsequent studies attributed it to the genus Cerathophysa based on the presence of the prominent subanal rostrum and umbrella-shaped ophicephalous pedicellaria ( Clark 1917, 1925, Mortensen 1950, McCauley and Carey 1967, Smith and Kroh 2011, Kroh and Mooi 2023b). The subgenus Pourtalesia (Rodocystis) was erected by Lambert and Thiéry in 1924 to accommodate this species. Mironov (1975, 1978a) promoted the subgenus to a genus level and noted substantial differences in apical disc structure and fascioles between Rodocystis and pourtalesiids. The monotypic genus Rodocystis was recently transferred to the family Calymnidae based on the presence of the marginal fasciole ( Mironov et al. 2015).

The presence of the marginal fasciole, passing over plates 5.a.4/5.b.5 beneath the periproct, is the main diagnostic feature of the family Calymnidae ( Smith 2004) . Among all recent Urechinina , the marginal fasciole is recorded only in the genera Calymne , Rodocystis , and Sternopatagus . Molecular evidence supports the grouping of genera Rodocystis and Sternopatagus ( Fig. 3 View Figure 3 ). However, molecular data on Calymne are needed to confirm that these genera should be attributed to the family Calymnidae with certainty.