Palaeobenthesicymus libanensis ( Brocchi, 1875 ) Audo & Charbonnier, 2013

Audo, Denis & Charbonnier, Sylvain, 2013, Late Cretaceous crest-bearing shrimps from the Sahel Alma Lagerstätte of Lebanon, Acta Palaeontologica Polonica 58 (2), pp. 335-349 : 340-348

Palaeobenthesicymus libanensis ( Brocchi, 1875) comb. nov.

Figs. 2–9 .

1875 Penaeus libanensis sp. nov.; Brocchi 1875: 609, 610, pl. 21 (see

Fig. 2A for original lithography). 1886? Penaeus libanensis Brocchi ; Dames 1886: 554, 555. 1922 Penaeus libanensis Brocchi ; Balss 1922: 131. 1929? Penaeus libanensis Brocchi ; Glaessner 1929: 310. 1930 Penaeus libanensis Brocchi ; Van Straelen 1930: 5. 1940 Penaeus libanensis Brocchi ; Van Straelen 1940: 3. 1940 Penaeus smyrnacus sp. nov.; Van Straelen 1940: 4, 5; fig. 3. 1944 Penaeus libanensis Brocchi ; Roger 1944: 848. 1945 Benthesicymus libanensis (Brocchi) ; Glaessner 1945: 645; fig. 4. 1946 Penaeus libanensis Brocchi : Roger 1946: 23–26 partim (not pl. 1,

3; figs. 15–17). 1965 Benthesicymus libanensis (Brocchi) ; Glaessner 1965: 117; fig. 6.

1967 Benthesicymus libanensis (Brocchi) ; Wolfart 1967: 98.

1969 Benthesicymus libanensis (Brocchi) ; Glaessner 1969: R447; fig. 253.2.

1975 Benthesicymus libanensis (Brocchi) ; Brugnoli Gioffredi et. al. 1975: 6, 7.

1994 Benthesicymus libanensis (Brocchi) ; Garassino 1994: 13, table 4.

1996 Benthesicymus libanensis (Brocchi) ; Briggs and Wilby 1996: 665.

2004 Penaeus libanensis Brocchi ; Larghi 2004: 528.

2005 Penaeus libanensis Brocchi ; Bracchi and Alessandrello 2005: 4.

2009 Benthesicymus libanensis (Brocchi) ; Feldmann 2009: 373.

2010 Benthesicymus libanensis (Brocchi) ; Schweitzer et al. 2010: 9.

Type material: Holotype of Brocchi (1875) presumably lost; neotype:

MNHN.F.A30593 (Arambourg collection), herein designated.

Type locality: Sahel Alma Lagerstätte , Lebanon, Middle East .

Type horizon: Upper Cretaceous, upper Santonian (Nannofossil biozone UC13 Burnett, 1998 —UC17 pro−parte Sissingh, 1977).

Material.—MNHN.F.A30582–A30587, A30589–A30599, A30601, A30602, A305605–A305609, A30705–A30715, A30717–A30719, A30735, A30739–A30741, A30805– A30808, A32428, B18862, SHA545, SHA1532, SHA 1852dg, SHA2981.

Diagnosis.—As for genus.

Synthetic description.—Large−sized shrimp with thin and slightly mineralized cuticle. Cephalothorax with obliquely triangular rostral crest, very thin, with a thickened dorsal ridge. Rostrum with blunt anterior end and without teeth. Cervical groove very narrow; branchiocardiac groove curved; postcervical groove weakly marked; inferior groove well developed sub−parallel to the ventral margin. Medio−dorsal ridge well−marked. Posterior margin with semi−circular curve in the inferior part. Pleon strong with short and high s1 to s5 all identical, s6 longer than the others. Compound eyes, ovoid, with median narrowing. Antennular peduncle with basal stylocerite; antennulae with two long flagella thick basally and tapering distally; antennal peduncle with well−developed scaphocerite (oval in outline and very thin) and long articulated flagellum. Third maxilliped short, with spines on the carpus. Pereiopods increasing in length posteriorly, P1 with short chelae, P2 and P3 with long and slender chelae; P4 and P5 very thin and achelate. Pleopods well developed, strong. Telson with pointed extremity and strong median ridge. Uropods with strong marginal ridge, uropodal exopod with semi−circular diaeresis, uropodal endopod longer than the telson. Intestine observable in the pleonal region. Description summarized after Brocchi (1875), Glaessner (1945), and Roger (1946) pro parte.

New observations.—Subrectangular cephalothorax ( CH = 1/3 CL; Fig. 2B ). Rostrum very short with rounded distal extremity and smooth margins ( Fig. 3A ). Anterior margin with shallow ocular and antennal incisions. Anterior dorsal margin with hypertrophied rostral crest ( Figs. 3B , 5A 1 ), very thin, compressed laterally, obliquely triangular, with thickened

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marginal ridge, triangular area with anterior angle narrower than posterior one, connection between crest and cephalothorax smooth, without ridge or carina ( Fig. 3B ). Posterior dorsal margin with small median ridge; posterior margin straight dorsally and semi−circular ventrally, bearing small ridge. Ventral margin poorly preserved, probably very thin. Cervical groove very narrow, slightly inclined, intercepting dorsal margin at an angle of ca. 35 °, posterior to the rostral crest. Short antennal groove. Postcervical groove very weak, inclined, subparallel to cervical groove, joining the branchiocardiac groove at an angle of ca. 30 °. Branchiocardiac groove well−developed, anteriorly curved and flanked by two parallel ridges, joining the hepatic groove anteriorly and the small ridge of the posterior margin. Hepatic groove short, slightly curved. Lateral ridge well−marked, longitudinally elongate in the middle height of the cephalothorax, slightly convex, subparallel to the ventral margin and joining the branchiocardiac and hepatic grooves ( Figs. 2B 2 , 4A 2 , 5A 2 ). Ornamentation of cephalothorax smooth, with very small, uniformly distributed pores.

Pleon with subrectangular somites; s1 slightly shorter than the others; s2–5 uniform in length; s6 subrectangular, longer than the other somites. Terga and pleurae of s1–6 smooth, with proximal margin reinforced by small ridge; terga with dorsal carina weak on s1–3 and well−marked on s5–6; pleurae with rounded ventral margin ( Fig. 6A 1 , B, C). Intestinal canal parallel to the pleonal axis ( Fig. 4A 2 ), crossing s 1–4 in the upper part of the pleon and s5 and s 6 in lower part with a strong curvature from s5 ( Fig. 6B ). Telson triangular, with thick dorsal ridge and fringed lateral margins ( Figs. 2B 2 , 6D ).

Compound eye, elongated longitudinally, bilobed with anterior and posterior lobes separated by a dorsal narrowing ( Figs. 3A , 5A 1 , 7A 4 ); visual surface preserved as a quadrate ommatidia network (Fig. 8A 3); ommatidia from the anterior lobe slightly smaller than those of the posterior lobe; ocular peduncle poorly preserved.

Cephalic appendages well preserved. Antennulae (a1) composed of antennular peduncle bearing two long multi−articulated flagella (length: ca. CL), very thick proximally and tapering distally ( Fig. 4B–D ); dorsal margin of antennular peduncle with small depression to accommodate the stalked eyes; triangular and pointed stylocerite attached to proximal margin of this depression. Antenna (a2) composed of antennal peduncle with three segments (distal segment is the shortest) and very long multi−articulated flagellum (length: ca. TL); lamellar and almost ovoid scaphocerite ( Figs. 2B 2 , 4A, B ), with fringed adaxial margin and opposite margin strengthened by a ridge separating from the lamellar part to form a pointed distal extremity ( Fig. 4C ); lamellar part rounded distally and reinforced along the two−thirds of its length by an oblique thickening. Scaphognathite with slender and crenulated posterior extension.

Thoracic appendages well preserved ( Figs. 2B 2 , 5). First and second maxillipeds with multi−articulated exopodites composed of numerous short segments. Exopodite of maxilliped 2 well developed ( Figs. 4A 2 , 5A 2 , E). Third maxilliped relatively long (ca. 40% CL), composed of slender segments decreasing in length distally, ending in a dactylus. Pereiopods 1–5 slightly increasing in length from first to fifth; P1–3 chelate, with outer surfaces reinforced by two longitudinal ridges ( Figs. 2B 2 , 5B); P1, P2, and P3 chelae long and slen− der with occlusal opening narrow and occlusal margins of index and dactylus straight with a slight indentation; merus and carpus elongate (length: ca. 40% CL); P2 merus with distal spine above its articulation with carpus; P3 merus with denticulate ventral margin ( Figs. 2B 2 , 5A 3 ); P4 and P5 thin and elongate (length: ca. 1.4 CL), both achelate, with outer surfaces reinforced by one longitudinal ridge; slender dactylus.

Abdominal appendages well preserved ( Figs. 6 , 7). Pleopods biramous, very long, equal in length, with two long multi−articulated flagella (length: ca. 33% CL) connected to well−developed protopodite; flattened and rounded protopodite, bearing transversally two opposite pairs of convex ridges ( Fig. 7A 1 , A 2 ); petasma (modified endopodite of the first pleopod in copulatory appendage in a male decapod crustacean) with two distinct units ( Fig. 6A ): unit 1 composed of two thin elements joined on their distal extremity and unit 2 composed of one subtriangular scale with very rounded posterior margin and small parallel longitudinal furrows. Uropods with short coxopodite and large, stocky basipodite; very long uropodal endopod (ca. 45% CL) reinforced by longitudinal median ridge, rounded distal margin, inner lateral margin finely fringed; elongate uropodal exopod (ca. 60% CL) with well−marked semi−circular diaeresis, distal margin rounded, inner lateral margin finely fringed, outer margin thickened by a straight ridge ( Figs. 3C , 5A 1 , A 2 , 7A 3 , B).

Discussion.—After new observations of the Lebanese material from the MNHN, Paris, we do not agree with Brocchi (1875), Van Straelen (1930) and Roger (1946) who hypothesized that the rostral crest corresponded to the scaphocerite extended posteriorly. We agree with the observations of Glaessner (1945) and confirm that the rostral crest is clearly attached to the carapace. The descriptions proposed by Van Straelen (1930) and Roger (1946) are based on composite material that mixed different penaeid shrimps. Both authors described a well−developed rostrum bearing dorsal teeth. These characters are unknown in the crest−bearing shrimps and do not correspond to Brocchi’s description. In fact, they probably correspond to another Cretaceous fossil shrimp from Sahel Alma: Penaeus natator Glaessner, 1945 . Moreover, Roger (1946) confused the inferior groove with a well−marked lateral ridge, subparallel to the ventral margin.

After comparisons with the arguments and the line drawing proposed by Glaessner (1945: 696, fig. 1), we question his generic attribution to the extant Benthesicymus (see generic discussion above). According to the description of Palaeobenthesicymus gen. nov., we proposed the new combination Palaeobenthesicymus libanensis ( Brocchi, 1875) .

In addition, we question the validity of Penaeus smyrnacus Van Straelen, 1940 for several reasons: (i) the proposed description follows that of Brocchi’s species, (ii) the illustration ( Van Straelen 1940: pl. 1: 3) clearly shows a

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specimen with rostral crest, bilobed eyes and cephalothoracic grooves (e.g., curved branchiocardiac groove limited by two ridges) very similar to those of the Brocchi’s species, and (iii) the specimen is preserved on a limestone slab very similar to those collected in the Sahel Alma Lagerstätte. Moreover, the holotype of Penaeus smyrnacus (specimen I.G. n ° 12747), originally housed in the collections of the Royal Belgian Institute of Natural Sciences, Brussels, Belgium is not accessible (lost?); the type locality (Izmir, western Anatolia, Turkey) and the type age (Eocene) are very probably incorrect and linked to inversion of labels with another sample (Annelise Folie, personal communication 2010). For these reasons, we consider Penaeus smyrnacus to be a subjective junior synonym of Palaeobenthesicymus libanensis ( Brocchi, 1875) . In conclusion, based upon all the previous remarks, we consider it important to design a neotype for Palaeobenthesicymus libanensis because the holotype and sole specimen of Brocchi (1875) was not found either in the old Collections de La Sorbonne now housed at the Université Pierre & Marie Curie (Paris 6) or in the collections of the Muséum national d’Histoire naturelle, Paris. The neotype for Penaeus libanensis Brocchi, 1875 is the specimen MNHN.F. A30593 (Arambourg collection) that shows almost all the diagnostic characters of the species and also of the genus ( Fig. 2B 1 ). The type locality is the Late Cretaceous Sahel Alma Lagerstätte ( Lebanon, Middle East).

Stratigraphic and geographic range.—As for genus.

Reconstruction.—Our new observations provide a more accurate description and allow a detailed iconographic reconstruction of Palaeobenthesicymus libanensis ( Fig. 9 ). The general view is different from that proposed by Glaessner (1945). It is more precise and reveals new anatomical features that provide several lines of evidence to demonstrate the relationship of the fossil crest−bearing shrimps to the extant Benthesicymidae .

the pleonal region bearing numerous infra−millimetric fragments corresponding to unknown coprolithic material ( Figs. 4A 2 , 6B ). Some specimens (MNHN.F.A30584, A30587, A30599) show calcite spheres on the surface of their carapace (Fig. 8). Based on their morphology (spheres 1 to 1.5 mm in diameter) and position in the cephalothorax, Roger (1946) interpreted them as “eggs”. Examination of similar structures in those specimens and other decapods from Lebanon (e.g., Palinurus , Linuparus , Acanthochirana , Homarus ) revealed that they are calcite crystal bundles ( Briggs and Wilby 1996). The bundles formed almost exclusively within the body of the crustaceans, suggesting that the cuticle envelope played a role in maintaining ion concentrations and pH levels generated by decay bacteria. The precipitation of these very early authigenic calcite bundles occurs in decaying carcasses and is influenced by microbial activity. Their presence in close association with phosphatized soft−tissues indicates that phosphate forms initially due to a decay induced drop in pH, calcite may precipitate subsequently once the pH has risen sufficiently ( Briggs and Wilby 1996). The rarity of crystal bundles reveals very probable rapid burial and relatively closed conditions during the early stages of fossilization of the Sahel Alma fauna ( Briggs and Wilby 1996).

Palaeopathology.—Epibionts have been recognized on several specimens of Palaeobenthesicymus libanensis . They are particularly visible after ultraviolet illumination. For instance, they occupy a large part of the scaphocerite of specimen MNHN.F.A30595 and form infra−millimetric isolated ovoid structures ( Fig. 4A 1 ). Similar subspheric structures have been recognized on the rostrum of other Sahel Alma crustaceans by Petit and Charbonnier (2012). They interpreted these epibionts to be marine sponge gemmules. Other specimens (MNHN.F.A30589, A30593, A30596) show indentations cutting the thick margin of the rostral crest (e.g., Fig. 3B ). Their positions are not regular and probably correspond to small healed wounds.

Taphonomy, paleobiology, cymus Palaeobiology libanensis .— remains The exact an mode open of question life of. Palaeobenthesi− Most of the au− and palaeoecology thors ( Brocchi 1875; Glaessner 1945; Roger 1946) did not discuss the palaeoecology of this species. Some important of crest−bearing shrimps features of P. libanensis would suggest that crest−bearing shrimps were necto−benthic animals with well−developed ca− Taphonomy.—The exquisite preservation of the studied pacities for swimming: (1) the whole exoskeleton is very thin specimens allows detailed anatomical observations. Soft−tis− and smooth, without ornamentation, as observed in numersues such as muscles and gills are preserved in grey−brown ous modern nektonic shrimps (Pérez Farfante and Kensley siliceous and probably phosphatized mineral displaying a 1997); (2) the antennae are very long and therefore may act weak fluorescence when exposed to ultraviolet light. Mus− as a parachute in the water column ( Bauer 2004); (3) the long cles are mainly located under desquamated cuticle in the and well−developed pleopods suggest active swimming; (4) pleonal region and in the pereiopods while remains of gills the strong tail fan composed of uropods which exceed the are located in the branchiostegal region ( Fig. 4A ). The intes− length of the telson by three−quarters may indicate very actine is also preserved and forms a small cylindrical canal in tive propulsion; and (5) the well−developed rostral crest may +

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have played an important role in the stability and steering that these animals were visual predators and possibly were during the swimming. The relatively flattened pereiopods re− able to detect prey in dim light conditions (relatively deep eninforced by longitudinal ridges also suggest good capacities vironment and/or turbid waters). for benthic locomotion. The very slender chelae of the first

Palaeoenvironmental significance.— Palaeobenthesicymus three pereiopods may indicate that P. libanensis was an ac−

libanensis is relatively similar in size and general morphology tive predator and principally fed on small organisms living in

to some extant Benthesicymidae such as Benthesicymus crethe soft muddy bottoms of the Sahel Alma Lagerstätte. The

natus Bate, 1881. This species is a deep−water shrimp and only possible predatory habits of the crest−bearing shrimps are few biological and ecological data are available because it also confirmed by the presence of their bilobed eyes. Bilobed lives at depths where in situ observations are rare and dependeyes are very rare among recent crustaceans. They are known ent upon submersibles or camera sledges (Pérez Farfante and in predator euphausiacids that live in very dim−light condi− Kensley 1997). However, Jamieson et al. (2009) reported that tions, below the euphotic zone ( Land 1980). Their eyes are B. crenatus lives at depths exceeding 7000 m and is an active characterized by two lobes: (1) the lower lobe with relatively predator that was observed preying upon smaller scavenging large ommatidia to detect the bioluminescence emitted by amphipods. There are also morphological resemblances (e.g., potential preys and/or predators in the darkness of the water elongate chelipeds, small rostrum, long multiarticulated pleocolumn and (2) the upper lobe with small ommatidia shows pods, body shape) between P. libanensis and some caridean relatively limited capacities to detect faint lights but offers a shrimps of Pasipheidae that are known to make daily migrahigher image resolution to discern the outline of preys (often tions moving to the water surface at night and returning to using counter−illumination or transparency) in the residual deep water during the day. These species are predator and daylight coming from the water surface ( Land 1980). Thus, principally feed on mysids, chaetognaths and small fishes bilobed eyes in P. libanensis may be interpreted as an adap− ( Cartes 1993). The comparisons with Recent Benthesicymitation to vision at low light intensities. They strongly suggest dae and Pasipheidae seem to indicate that P. libanensis might + Fig. 8. The crest−bearing shrimp Palaeobenthesicymus libanensis ( Brocchi, 1875) from the Sahel Alma Lagerstätte (late Santonian, Lebanon). A. Complete specimen (MNHN.F.A30587), left lateral view, note cephalothorax with white traces of decay (A 1); detailed view of the cuticle with a concentration of calcite bundles probably influenced by microbial activity (A 2); detailed view of the eye showing a network of quadrate ommatidia (A 3). B. Sub−complete specimen (MNHN.F.A30584), right lateral (B 1) and detailed (B 2) views, calcite bundles are concentrated to the ventral margin of the cephalothorax and to the branchial region, note the branchiocardiac groove limited by two parallel ridges.

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have been an active predatory and have played a key role between the benthic and pelagic palaeoecosytems. More precisely, P. libanensis was probably nectobenthic and might have preyed both on the small−sized crustaceans living in the muddy substrate typical of the chalky sediments of Sahel Alma and on those living in the water column. In summary, P. libanensis is an important component of the Sahel Alma Lagerstätte that has morphological and possible ecological analogues among the extant deep−water Benthesicymidae . That the crest−bearing shrimps lived in relatively deep water conditions is plausible and confirmed by other fossils such as coleoid cephalopods ( Lukeneder and Harzhauser 2004), actinopterygians, chondrichtyians ( Gayet et al. 2003) and crinoids ( Roger 1946). All these organisms support the notion that the Sahel Alma area was situated in the lower circalittoral zone with a water depth most probably exceeding 150 m. This interpretation is compatible with the geological data suggesting that the chalky limestones from Sahel Alma were probably deposited in a small depression around the plateform−basin transition in the lower circalittoral zone or in the upper bathyal zone (depth ca. 150–200 m).

Figures

Fig. 2. Type material of crest−bearing shrimp Palaeobenthesicymus libanensis (Brocchi, 1875) from the Sahel Alma Lagerstätte (upper Santonian, Lebanon). A. Reproduction of Brocchi’s (1875) lithography, left lateral view, specimen considered to be definitively lost. B. Complete specimen (MNHN.F.A30593, Arambourg collection), left lateral view, specimen chosen as neotype for P. libanensis (Brocchi, 1875). Photograph (B1) and camera lucida line drawing (B2), note the well−developed rostral crest and the bilobed eyes.

Fig. 3. Anatomical details of the neotype (MNHN.F.A30593, Arambourg collection) of crest−bearing shrimp Palaeobenthesicymus libanensis (Brocchi, 1875) from the Sahel Alma Lagerstätte (upper Santonian, Lebanon). A. Anterior part showing the pair of bilobed eyes and some fragmentary cephalic appendages, note the short rostrum with blunt anterior end. B. Rostral crest, very thin, with a thickened dorsal ridge, note two indentations (black arrows) probably corresponding to small healed wounds. C. Tail fan with well−developed uropods, note the very elongate uropodal exopod bearing a rounded diaeresis (black arrow).

Fig. 4. The crest−bearing shrimp Palaeobenthesicymus libanensis (Brocchi, 1875) from the Sahel Alma Lagerstätte (late Santonian, Lebanon). A. Sub−complete specimen (MNHN.F.A30595), dorsal view (A1) and camera lucida line drawing (A2), note the cephalothoracic grooves, the scaphocerites with white small spheres on the outer margins (epibionts), and the fragment of intestinal canal; grey areas correspond to mineralized soft−tissues. B. Cephalic region of fragmentary specimen (MNHN.F.A30585), ventral view, note the two pairs of antennulae. C. Fragmentary specimen (MNHN.F.SHA.545), left lateral view, cephalic region showing well−preserved antennulae and antennae, note the very pointed distal extremity of the scaphocerite. D. Sub−complete specimen (MNHN.F.A30599), left lateral view, note the eyes and the multiarticulated antennae.

Fig. 1. Map of Lebanon showing the main fossiliferous localities yielding exceptionally preserved Cenomanian faunas (Hadjoula, Hakel, En Nammoura, Maifouk, Lagerstätten) and Santonian faunas (Sahel Alma Lagerstätte).

Fig. 6. The crest−bearing shrimp Palaeobenthesicymus libanensis (Brocchi, 1875) from the Sahel Alma Lagerstätte (late Santonian, Lebanon). A. Male specimen (MNHN.F.A30709), left lateral view (A1), note the petasma, a modified endopodite of the first pleopod in copulatory appendage (black frame). Detailed view of the same petasma composed of unit 1 (two thin elements joined on their distal extremity) and unit 2 (subtriangular scale) (A2). B. Pleon of fragmentary specimen (MNHN.F.A30708), right lateral view, note the well−developed pleopods connected to cylindrical protopodites and the long intestinal canal parallel to the pleonal axis. C. Pleon of sub−complete specimen (MNHN.F.A30602), left lateral view, note the rounded ventral margin of pleurae and the well−marked dorsal ridge on somites 5 and 6. D. Small specimen (MNHN.F.A30596), right lateral view, note the well−preserved tail fan.

Fig. 9. Reconstruction of crest−bearing shrimp Palaeobenthesicymus libanensis (Brocchi, 1875). Drawing: Charlène Letenneur (MNHN, scientific draughtsman).