On the systematic placement of Pyreneplax Ossó, Domínguez & Artal, 2014 (Decapoda, Brachyura, Vultocinidae)

Examination of the thoracic sternum and pleonal elements of a new male specimen of Pyreneplax basaensis Ossó, Domínguez & Artal, 2014 from the upper Eocene of northern Spain confirms its assignment to the family Vultocinidae Ng & Manuel-Santos, 2007 and reveals the presence of an old lineage ( Pyreneplax and possible allies) that appeared during the Eocene, persisting to the present day.


Introduction
A new specimen of Pyreneplax basaensis with complete sternal and pleonal features and remains of ambulatory legs reveals additional features that were not seen in the type series. It has allowed observation of additional similarities to the extant species Vultocinus anfractus Ng & Manuel-Santos, 2007 and a more detailed comparison between both taxa (see Domínguez & Ossó, 2019).
The genus Pyreneplax Ossó, Domínguez & Artal, 2014 was erected to accommodate P. basaensis from the Priabonian of the south Pyrenean basins of Spain. Subsequently, other, closely similar species have been recorded from the Eocene of the Atlantic Coast of North America and from northern Italy. On the basis of dorsal carapace morphology, fossil crab species such as Lobonotus saundersi (Blow & Manning, 1997) from South Carolina (USA) and L. granosus  and L. sommarugai Beschin, Busulini & Tessier, 2009 from northern Italy, have lately been transferred to the genus Pyreneplax (see Ossó, Domínguez & Artal, 2014;De Angeli, 2014).
Dorsal and ventral (thoracic sternum, pleon) features of Pyreneplax basaensis confirm its family relationship with Vultocinus anfractus, in spite of the time span that separates both spe-cies, placing the origins of the family Vultocinidae in the late Eocene. Furthermore, new DNA molecular studies of Vultocinus anfractus have revealed that, "it comes out as a long branch inside the Heterotremata, far away from the rest of the Goneplacoidea" (Ng & Tsang, pers. comm., June/2019).
The studied material is housed at Museo de Ciencias Naturales de la Universidad de Zaragoza (Spain), under acronym MPZ.
Remarks: At the time, the dorso-ventral similarities highlighted by Ossó et al. (2014, pp. 36-38) were considered sufficient to place the Late Eocene Pyreneplax within the extant family Vultocinidae (Fig. 1); the additional sternal and pleonal features observed in the new specimen (Fig. 2) confirm this course of action (Domínguez & Ossó, 2019, pp. 70-72). Indeed, in the new specimen, a male, the sternopleonal cavity is deep and relatively narrow, almost reaching the end of sternite 4, as in Vultocinus (Ng & Manuel-Santos, 2007, p. 43, figs. 12A, 9A). The position of the press-button in Vultocinus, considered important by Ng & Manuel-Santos (2007, p. 42), cannot be observed in the new specimen due to preservation; however, in view of the position of the pleon,  In addition, the posterior edge of episternite 7 in Pyreneplax basaensis is, similar to Vultocinus anfractus, "strongly produced posteriorly to form a spur which reaches coxa of P5" (Ng & Manuel-Santos, 2007, p. 44). This character, considered "unusual" by Ng & Manuel-Santos (2007), which together the lateral expansion of pleonal somite 3 covers the penis completely, differentiates the Vultocinidae from other goneplacoid families (Ng & Manuel-Santos, 2007, pp. 44, 49, figs. 13A-B;Castro et al., 2010). It is also seen in Pyreneplax (Fig. 2E).
In this respect, the Eocene Martinocarcinus ickeae Böhm, 1922(family Martinocarcinidae Schweitzer, Feldmann & Bonadio, 2009 within the Goneplacoidea) also deserves attention. Indeed, Schweitzer et al. (2009, p. 4) already pointed out the striking similarities in sternal and pleonal features of Martinocarcinus and Vultocinus (Schweitzer et al., 2009, pl. 1, fig. 4; pl. 2, figs. 1-5), but in view of the substantial differences in dorsal carapace and chelae, they did not conclude that there was a close relationship between them. Ossó et al. (2014, p. 38) also noted the sternal and pleonal similarities between Martinocarcinus and Pyreneplax but argued against a family rela-tionship, based on differences of dorsal carapace morphology. However, in view of the new pieces of evidence provided by the new specimen of Pyreneplax, regarding penis protection, a re-examination of the holotype of Martinocarcinus ickeae would appear to be interesting, in particular to see whether or not it has the same pattern of penis protection as in Vultocinus anfractus and P. basaensis; this cannot be seen in the illustrations of Schweitzer et al. (2009). This character may well connect these three taxa phylogenetically.

Conclusions
Molecular studies carried out recently, using several mitochondrial and nuclear genes, have demonstrated that Vultocinus sits in its own deep lineage within the Heterotremata and is not related to any of the known goneplacoids, including those without living relatives (Ng & Tsang, pers. comm., October/2019). These results are consistent with what had already been stated in the original paper by Ng & Manuel-Santos (2007, p. 40) that, "Vultocinus, new genus, possesses a suite of unusual characters that make its precise affinities difficult to ascertain".
The fossil evidence suggests that we are dealing with a case of an extinct family with an ex- tant representative rather than an extant family with fossil representatives. The dorsal morphology of Pyreneplax is relatively common in numerous Eocene genera, but only preserved sternal and pleonal features can establish the relationship among these (Ossó et al., 2014, p. 41;Jagt et al., 2015, p. 883). The persistence of this dorsal carapace pattern is interpreted either as evolutionary success or an example of convergence. In the case of the Vultocinidae and in view of the preserved ventral features now observed in Pyreneplax basaensis, this indicates that the unusual penis protection structures are a successful adaptation and hence persisted over time. Future works and new discoveries will be expected to shed light on the suprafamily relationships of this family and their possible allies.