Marylyreidinae, Van Bakel & Guinot & Artal & Fraaije & Jagt, 2012

Van Bakel, Barry W. M., Guinot, Danièle, Artal, Pedro, Fraaije, René H. B. & Jagt, John W. M., 2012, A revision of the Palaeocorystoidea and the phylogeny of raninoidian crabs (Crustacea, Decapoda, Brachyura, Podotremata) 3215, Zootaxa 3215 (1), pp. 1-216 : 85-86

Subfamily Marylyreidinae n. subfam.

Type genus. Marylyreidus n. gen.

Diagnosis. Carapace elongated, widest anterior to mid-length, weakly arched in longitudinal cross section, convex in transverse cross section. Anterolateral margin short, with a single small spine. Posterolateral margin gently sinuous, with vertical anterior portion, divergent posterior portion; margins rounded in cross section. Posterior margin concave. Orbitofrontal margins straight, convergent, conspicuously wide (~80 % maximum carapace width), with 2 distinct orbital notches. Rostrum subtrapezoidal, bifid, with 2 small subdistal teeth. Branchiocardiac grooves short, cervical groove medially present, interrupted between gastric pits. Carapace surface covered with flattened hexagonal caps (constructed by fungiform nodes; Waugh et al. 2009). Pterygostome elongated, moderately vaulted, with low, blunt ridge, posterior corner without distinct recess. Exhalant channels well developed. Mxp3 elongated, oxystomian condition; coxae rather large, intercalated between thoracic sternum, pterygostome; exopod, endopod ischium, endopod merus of equal length. Branchiostegite reduced; pleurites 5‒7 exposed. Sternite 3 crown shaped; sternite 4 slightly wider, squarish, not connected to pterygostome; episternite 4 large, widened; sternite 5 proportionally broad, extended between P1, P2, connected to exposed pleurites, episternite 5 with short, hook-like projections, distally with double peg; sternites 5, 6 with lateral ridge; sternite 6 narrow, no connection sternum/exposed pleurites between P2, P3; absence of medial line at sternites 1‒6. Distal articles of pereiopods flattened. Abdomen narrow, somites 2, 6 long, somite 3 with blunt node.

Remarks. The Marylyreidinae n. subfam. is referred to Raninoidea based on the presence of exposed pleurites 5‒7 (gymnopleurity). The distinctly widened sternite 5, which is connected to the exposed pleurites, and the anterolateral margin with but a single tooth, excludes it from Palaeocorystoidea . Placement in Lyreididae is based on the presence of an abdominal holding system, hence having the abdomen held against the body. The abdominal-holding system of Marylyreidinae n. subfam. consists of hook-like projection arising from episternite 5, distally with a double peg, here considered homologous to the long hook ending in two curved teeth of Lyreidinae . Presence of an abdominal-holding system is the most notable feature of Lyreididae ; it is lost in all other Raninoidea . It consists of a double peg, similar to that seen in Palaeocorystoidea , but placed on a stout ( Marylyreidinae n. subfam.) or elongated ( Lyreidinae ) hook-like projection, a feature unique amongst brachyurans. All other raninoids lack an abdominal holding system (assumed to have been lost), and keep their (shortened) abdomen behind the carapace.

Several characters distinguish Marylyreidinae n. subfam. from Lyreidinae . In Marylyreidinae n. subfam. sternite 4 is not expanded anteriorly; there is a sternum/pterygostome junction, the mxp3 coxae being intercalated between the thoracic sternum and the pterygostome. Sternite 6 is not widened; there is no junction with the exposed pleurites between P2 and P3, which is present in Lyreidinae . Sternites 5 and 6 have a lateral ridge, which is absent in Lyreidinae . The hook-like projection arising from episternite 5 are short, stout, but long, curved projection in Lyreidinae . The carapace of Marylyreidinae n. subfam. also shows characters that are unusual for Lyreididae . The orbital margin is conspicuously wide, its relative size being more similar to that of Palaeocorystidae than that of extant lyreidids. Of fossil representatives, only the Eocene lyreidine Rogueus shows an equally wide orbitofrontal margin. The cuticle microstructure of Marylyreidinae n. subfam. is remarkable, consisting of fungiform nodes ( Haj & Feldmann 2002; Waugh et al. 2009), in contrast to the cuticular microstructure of Lyreidinae which comprises upright nodes and setal pits. Several palaeocorystid genera (e.g., Cretacoranina , Ferroranina n. gen. and Eucorystes ) exhibit similar cuticle structures, as does one raninoid genus, Symethis .

Some characters of Marylyreidinae n. subfam. may be regarded as intermediate between those of Palaeocorystidae and Lyreididae . There is no connection joining the thoracic sternum to the pterygostome, a feature that is typical of Lyreidinae and all other Raninoidea , whereas the condition is ‘normal’ in Palaeocorystidae . In Marylyreidinae n. subfam. the mxp3 coxae are intercalated between the thoracic sternum and pterygostome, thus the presence of Milne-Edwards openings and a ‘normal’ repiratory system as in Palaeocorystidae . Sternite 4 is neither very narrow anteriorly (as in Palaeocorystidae ), nor distinctly widened (as in Lyreididae ). This is the more primitive condition found in Raninoidea , similar to that of Palaeocorystoidea . However, the presence of exposed pleurites (gymnopleure condition, synapomorphy of Raninoidea ) in Marylyreidinae n. subfam. justifies its inclusion in Raninoidea . The character state shown by Marylyreidinae n. subfam. is part of a series of changes in the evolutionary modification of the respiratory system. Sternite 4 is not widened anteriorly, episternite 4 is remarkably large, together with the widened sternite 5. No medial line is present, at least on sternites 1‒6. Projections of episternite 5 are present (absent in Palaeocorystidae ); however, they are short, which contrasts to the long projections in Lyreidinae . We hypothesise that this condition is intermediate in an evolutionary line of shortening and posterior migration of the abdomen (see Abdominal holding below).