Palaemon monsdamarum n. sp., 2018

Pasini, Giovanni & Garassino, Alessandro, 2018, Palaemon monsdamarum n. sp. (Crustacea, Decapoda, Palaemonidae) from the late Miocene of Mondaino (Rimini, Emilia-Romagna, Italy), Natural History Sciences 5 (1), pp. 7-10 : 8-10

Palaemon monsdamarum n. sp.

( Fig. 1 )

Diagnosis: Carapace subrectangular; rostrum elongate, with single tip and 3-4 dorsal granule-shaped spines; P1-P2 chelate; uropodal exopod without diaeresis.

Etymology: from Mons Damarum (= Fallow’s deer mount), ancient name of Mondaino, fossiliferous locality where the studied specimen was collected.

Holotype: MSNM i29208 .

Type locality: Mondaino (Rimini, Emilia-Romagna).

Geological age: late Miocene (Messinian).

Material and measurements: One specimen in lateral view (lcxp: 7 mm, excluding rostrum; wcxp: 4 mm).

Description: Small-sized caridean with thin, smooth exoskeleton. Carapace – Carapace subrectangular, elongate, narrowing slightly anteriorly due to slight curvature of the ventral margin; dorsal margin nearly straight; ridged posterior margin, gently sinuous and slightly concave in upper third; ventral margin curvilinear more inflated posteriorly; dorsal margin with thin longitudinal dorsal carina extending into long pointed, dorsally serrate rostrum; rostrum with a short rim of very small-sized granular suprarostral teeth decreasing in size towards the tip; ventral margin not observable; ocular incision narrow, shallow; antennal spine present; hepatic spine absent; no traces of grooves, carinae, or spines on carapace lateral surface. Cephalic appendages – Cephalic appendages incomplete; antennulae triflagellate; laminar scaphocerite present. Pleon – Pleon with s1, s3-s5 subtriangular ventrally, pointed and similar in length; s2 pleura subrounded, partially overlapping those of s1 and s3; s6 subrectangular, elongate, with curved ventral margin, narrowing posteriorly, longer than the others; anterior margin of s1, s3-s5 slightly sinuous; s4-s5 posteriorly projecting; tail fan longer than telson; triangular telson, poorly preserved; uropodal exopod without diaeresis, with thin longitudinal medial carina. Thoracic appendages – Mpx3 short, narrow, and spineless; P1-P5 similar in length; P1-P2 chelate; P1-P2 chelae very small and thin; P3-P5 achelate. Pleonal appendages – Pleopods not preserved.

Discussion. The fossil record of carideans from the Cenozoic deposits are very scarce. According to Schweitzer et al. (2010) only four genera are known to date from the Cenozoic, Bechleja Houša, 1956 , Propalaeomon Woodward, 1903, Pseudocaridinella Martins-Neto & Mezzalira, 1991 , and Palaemon Weber, 1795 .

Bechleja Houša, 1956 is known with four species, as follows: B. inopinata Feldmann, Grande, Birkhmer , Hannibal & McCoy, 1981 (Eocene, Green River Fm., Wyoming, USA); B. bahiaensis ( Beurlen, 1950) and B. robusta Martins-Neto & Mezzalira, 1991 (Oligocene, Brazil); B. inopinata Houša, 1956 (Oligocene-Miocene, Czech Republic) ( Beurlen, 1950; Houša, 1956; Feldmann et al., 1981; Martins-Neto & Mezzalira, 1991). According to Houša (1956) and Martins-Neto & Mezzalira (1991) the main diagnostic characters for the genus are the dorsally and ventrally serrate rostrum, a single filament antennule (instead two filament antennule according to Feldmann et al., 1981), P1 with short chela, and P2 with elongate chela. However, we raise doubt about the diagnosis of the genus. Indeed, Houša (1956) described B. inopinata as a palaemonid with dorsally serrate rostrum, Feldmann et al. (1981) reported B. rostrata as a palaemonid with ventrally serrate rostrum, whereas Martins-Neto & Mezzalira described B. robusta as a palaemonid with short spineless rostrum. Though these characters fit more or less with the diagnosis made by the above-mentioned authors, it would be important to revise the type species ( B. inopinata ) of Bechleja in order to establish its real diagnostic characters, avoiding to consider it as a “basket” genus.

Though the studied specimen shares two diagnostic characters for the genus, such as the dorsally serrate rostrum and the P1 with short chela, we can exclude its belonging to this genus not only for the triflagellate antennule, but also because Bechleja is known as freshwater genus, whereas the studied specimen inhabited a stricly marine environment.

Propalaeomon Woodward, 1903 is known with two species, as follows: P. osborniensis Woodward, 1903 (late Eocene, Isle of Wight, Great Britain) and P. longispinata Martins-Neto & Mezzalira, 1991 (Oligocene, Brazil) (Woodward, 1863; Woods, 1925; Martins-Neto & Mezzalira, 1991). Woodwad (1903) described also P. minor , including small specimens from the late Eocene of Isle of Wight that he considered more convenient to treat as a distinct species though they could be juvenile forms of P. osborniensis . According to Gaudant & Quayle (1988) P. minor has to be considered as a juvenile synonymous with the type species P. osborniensis . According to Gaudant & Quayle (1988) the main diagnostic characters for the genus are the dorsally serrate rostrum with bifid tip, short hepatic spine, P1 and P2 chelate with P2 longer than P1.

Based upon the main characters for the genus, we can exclude the belonging of the studied specimen to Propalaemon in having a rostrum with a single tip and lack of hepatic spine.

Pseudocaridinella Martins-Neto & Mezzalira, 1991 is known with two species from the Oligocene of Brazil, as follows: P. tremembeensis ( Beurlen, 1950) and P. roxoi ( Beurlen, 1950) ( Beurlen, 1950; Martins-Neto & Mezzalira, 1991). According to Martins-Neto & Mezzalira (1991) the main diagnostic characters for the genus are: rostrum smooth, P2 carpus excavated, and moderate dilatation of s2 base.

Based upon the main characters for the genus, we can exclude the belonging of the studied specimen to Pseudocaridinella in having dorsally serrate rostrum.

According to Schweitzer et al. (2010) Palaemon Weber, 1795 is known with six species, as follows: P vesolensis Bravi, Coppa, Garassino & Patricelli, 1999 and P. antonellae Garassino & Bravi, 2003 (Late Cretaceous, Italy); B. bahiaensis Beurlen, 1950 (Oligocene, Brazil); P. mortuus Smirnov, 1929 (Miocene, Caucasus), P. fabricii Michelotti, 1861 (Miocene, Italy); and P. exul Fritsch, 1872 (Miocene, Boemia) (Michelotti; 1861; Fritsch, 1872; Smirnov, 1929; Beurlen, 1950; Bravi et al. 1999; Garassino & Bravi, 2003). However, according to Martins-Neto & Mezzalira (1991) P. bahiaensis has to be considered a species of Bechleja , whereas P. fabricii has to be considered a species of Penaeus vanzii Beschin & Garassino, 1999 according to Beschin & Garassino (1999). Finally, Houša (1956) attested that P. exul should not be included in this genus “ because it differs from it by several important characters ” that, however, that author did not clarify why. According to Glaessner (1969) the main diagnostic characters for the genus are elongate dorsally and ventrally serrate rostrum; antennal and branchiostegal spines present; hepatic spine absent; antennule triflagellate; and telson with four apical spines.

The studied specimen, sharing some diagnostic characters for the genus, such as the dorsally serrate rostrum, the antenal spine, the lack of the hepatic spine, and the antennule triflagellate, is assigned to Palaemon .

The genus is known from the Miocene fossil record with two species, P. mortuus and P. exul . Palaemon monsdamarum n. sp. differs from the type species, P. mortuus , in having a rostrum with 3-4 granule-shaped dorsal spines (vs. rostrum with 4-5 strong dorsal spines in P. mortuus ). The bad quality of the drawing reported by Fritsch (1872) of P. exul hampers the comparison with the new species. Indeed the rostrum appears broken and the carapace seems to have a complex groove and carina system. As already pointed out by Houša (1956) a new genus should be erected to accomodate this species.

Based upon these observation, P. monsdamarum n. sp. is the first report of carideans from the Miocene of Italy.

Figures

Fig. 1 - Palaemon monsdamarum n. sp., Holotype, MSNM i29208.A) General view (x 5.7). B) Line drawing.