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Gerromorphan bugs in Early Cretaceous French amber (Insecta: Heteroptera): first representatives of Gerridae and their phylogenetic and palaeoecological implications
Cretaceous Research 26 (2005) 793e800 www.elsevier.com/locate/CretRes
Gerromorphan bugs in Early Cretaceous French amber (Insecta: Heteroptera): first representatives of Gerridae and their phylogenetic and palaeoecological implications Vincent Perrichot a, Andre´ Nel b,*, Didier Neraudeau a a
Ge´osciences Rennes and CNRS UMR 6118, Campus de Beaulieu, baˆtiment 15, 263 Avenue du Ge´ne´ral Leclerc, 35042 Rennes Cedex, France b CNRS UMR 5143, Muse´um National d’Histoire Naturelle, Entomologie, 45 Rue Buffon, F-75005 Paris, France Received 9 September 2004; accepted in revised form 5 May 2005 Available online 2 November 2005
Abstract Three specimens of gerromorphan bugs in Late Albian amber from south-west France are described. One is regarded as an incertae sedis within the Gerromorpha, the other two are assigned to Cretogerris albianus gen. et sp. nov., the oldest representative of the aquatic bug family Gerridae. The discovery confirms the great antiquity of the Gerridae, until now only inferred from an Early Cretaceous representative of the sister family Veliidae. The phylogenetic affinities of Cretogerris within the Gerridae are still rather uncertain, but this fossil taxon shows highly specialized body and leg structures that are very similar to those of the marine Halobatinae, suggesting that it was possibly a marine surface skater. The Gerridae and the Chresmodidae, another extinct group of Mesozoic surface skaters, were contemporaneous during at least the early Cenomanian. The discovery of these gerromorphan bugs in the Albian amber supports the hypothesis of a selective trap of a litter fauna, originating from a beach environment, for this resin. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: Aquatic insects; Heteroptera; Gerromorpha; Gerridae; Albian amber; South-west France
1. Introduction Gerromorpha or water striders live mainly in, or on the surface film of, fresh or marine water. These aquatic insects are thus not prone to be caught in resin exuded by trees on land. Indeed, most insects found as inclusions in amber belong to terrestrial forms whereas aquatic forms are quite rare. Among gerromorphan bugs, only a few fossil species of Gerridae have been described from the Eocene in north-east Italy (Andersen et al., 1994), Late Eocene Baltic amber (Andersen, 2000a; Weitschat et al., 2002), the Eocene and OligoceneeMiocene of North America and PaleoceneeEocene of Denmark (Andersen, 1982), and Miocene Dominican amber (Andersen and Poinar, 1992; Andersen, 2000b; Wappler and Andersen,
* Corresponding author. E-mail address: [email protected] (A. Nel). 0195-6671/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.cretres.2005.05.003
2004). Water striders are therefore only known from the Cenozoic, even if the presence of their sister family Veliidae in Australian Early Cretaceous deposits suggests that both families appeared at least during this period (Andersen et al., 1994; Andersen, 2000b). Thus, the present discovery of two representatives of the Gerridae in French Late Albian amber is of great interest in connection with the early history of this family of aquatic bugs. 2. Systematic palaeontology Order: Heteroptera Latreille, 1810 Infraorder: Gerromorpha Popov, 1971 Family: incertae sedis Genus and species undetermined Figs. 1e2
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Fig. 1. Specimen MNHN ARC 226.13, photograph of general habitus, dorsal view; scale bar represents 1 mm.
Material. Specimen MNHN ARC 226.13, fossilized with 83 other arthropods in the same piece of amber (cf. list in Perrichot, 2004, table 2, sample Arc 226), deposited in the Laboratoire de Pale´ontologie, Muse´um national d’Histoire naturelle, Paris. Locality and stratigraphic horizon. Archingeay-Les Nouillers, Charente-Maritime, south-west France, Lower Cretaceous, uppermost Albian (Ne´raudeau et al., 2002). Description. A nearly complete specimen but with the head and thorax partly dislocated; body 1.12 mm long, covered by small setae 0.05 mm long; head longer than wide, 0.29 mm long, about 0.2 mm wide; a median impressed line on dorsal surface of head; anterior part of head protruding; no visible ocelli; eyes globular but not posteriorly produced, 0.12 mm diameter, 0.12 mm apart; ocular setae not visible; three pairs of long cephalic trichobothria on dorsal surface of head, 0.05 mm long, disposed in two lines along the inner margins of the two eyes; apex of rostrum surpassing for 0.29 mm the prosternal hind margin; first labial segment 0.06 mm long, second 0.05 mm long, third very long at 0.41 mm, and fourth 0.14 mm long; no intercalary sclerites between labial segments 3 and 4; antennal tubercles situated before eyes and visible from above, directed forward; four antennal segments covered with setae but without trichobothria-like setae, second segment 0.18 mm long, third 0.23 mm long, first and fourth segments not completely preserved; all segments straight and cylindrical; ventral lobes of head not protruding in front and not hiding the rostral base. Thorax elongate, 0.37 mm long, 0.44 mm wide; separation between pronotum and mesonotum not well preserved, no pronotal lobe extending medially; no trace of wing structure,
Fig. 2. Specimen MNHN ARC 226.13, reconstruction of general habitus, dorsal view; scale bar represents 1 mm.
probably apterous; sternal structures poorly visible; tarsal claws at apex of tarsi; dorsal and ventral arolia visible but not differentiated, at least in one prothoracic leg, as curved and strong ‘‘trichobothria’’; all femora bearing a long seta in their apical outer quarter; all tarsi composed of one-segment; femora, tibia and tarsi of all legs subcylindrical, elongate and slender; prothoracic femur straight, 0.44 mm long and 0.06 mm wide, tibia straight, 0.36 mm long and 0.04 mm wide, tarsus 0.14 mm long; mesothoracic femur 0.49 mm long and 0.06 mm wide, tibia 0.5 mm long and 0.04 mm wide, tarsus 0.23 mm long; metathoracic femur 0.72 mm long and 0.09 mm wide, tibia 0.92 mm long and 0.04 mm wide, bearing six strong setae, tarsus 0.31 mm long; legs pilose but without rows of spinous hairs. Abdomen elongate, 0.55 mm long and 0.34 mm wide; genital structure not visible, if present. Discussion. The small size, absence of visible genital structures, and absence of wing structure suggest that this specimen is a larva. After Andersen (1982) and Schuh and Slater (1995), this fossil shows two of the main synapomorphies of the Gerromorpha, i.e., the presence of three pairs of cephalic trichobothria inserted in deep cuticular pits along the inner margins of the eyes, and a pretarsus with undifferentiated dorsal and ventral arolia. Its position within this suborder is more difficult to establish. The characters that are currently used to separate the families are not available in this fossil (Andersen, 1982, pp. 255e257). Therefore, we prefer to maintain it as an incertae sedis within the Gerromorpha.
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Family: Gerridae Leach, 1815 Genus Cretogerris gen. nov. Type species. Cretogerris albianus sp. nov. Derivation of name. Combination of the Cretaceous Period and the modern genus Gerris. Diagnosis. Body entirely globose, ‘‘drop-like’’ shaped; ventral lobes of head reduced; no median impressed line on dorsal surface; ocular setae longer than four facets; three pairs of long cephalic trichobothria on dorsal surface of head; no subapical spinous hairs on first antennal segment; antennal tubercle not situated in front; absence of trichobothria-like setae on second antennal segment; rostrum surpassing the prosternal hind margin; tarsi consist of two-segments on all legs in the adult specimen; first segment of prothoracic tarsus less than one-third the length of second segment; mesothoracic femur longer than tibia; meso- and metathoracic legs long and slender; meso- and metathoracic coxae inserted laterally on thorax; pronotum small and narrow; mesonotum very large; abdomen very reduced. Cretogerris albianus sp. nov. Figs. 3e5 Derivation of name. After the Albian age of these fossils.
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Material. Holotype specimen MNHN ARC 262.1 (larger specimen), paratype specimen MNHN ARC 262.2 (smaller specimen); both specimens fossilized in the same piece of amber with two adult Diptera: Dolichopodidae; deposited in the Laboratoire de Pale´ontologie, Muse´um national d’Histoire naturelle, Paris. Type locality and stratigraphic horizon. Archingeay-Les Nouillers, Charente-Maritime, south-west France, Lower Cretaceous, uppermost Albian (Ne´raudeau et al., 2002). Diagnosis. As for genus. Description. The smaller specimen is probably a last instar larva, 1.52 mm long and with one-segmented metathoracic tarsi; the larger is an adult, 1.84 mm long, with two-segmented metathoracic tarsi, and the genital appendages present. Unless otherwise indicated, the following description is based on the adult specimen. Head 0.2 mm long and 0.6 mm wide; no median impressed line on dorsal surface; anterior part of head regularly curved but not strongly protruding; no ocelli; eyes globular but not posteriorly produced, 0.15 mm long and 0.13 mm wide, 0.36 mm apart; ocular setae 0.05 mm long, longer than four facets; three pairs of long cephalic trichobothria on dorsal surface of head, disposed in two lines along the inner margins of the two eyes; apex of rostrum surpassing for
Fig. 3. Cretogerris albianus gen. et sp. nov., holotype, MNHN ARC 262.1 (larger specimen) and paratype MNHN ARC 262.2 (smaller specimen); photograph of general habitus, ventral view; scale bar represents 1 mm.
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Fig. 4. Cretogerris albianus gen. et sp. nov., holotype and paratype as in Fig. 3; reconstruction of general habitus, ventral view; scale bar represents 2 mm.
0.06 mm the prosternal hind margin; length of first labial segment 0.12 mm, second 0.07 mm, third 0.18 mm, and fourth 0.13 mm; antennal tubercles situated beneath the eyes and not visible from above, but directed forward; four antennal segments, length of first basal segment 0.16 mm, second 0.22 mm, third 0.24 mm, and fourth 0.21 mm; second and third segments slightly curved, without trichobothria-like setae; fourth segment spindle-shaped but not twisted nor bent; all segments cylindrical; ventral lobes of head not produced in front of the rostral base. Thorax 0.84 mm long, 1.0 mm wide, not very elongate but very broad, with its broadest part opposite the mesothoracic coxae; separation between pronotum and mesonotum not preserved although there is apparently no pronotal lobe extending medially, but a structure with two posterior lateral extensions probably corresponding to a very large mesonotum; no trace of wing structure; sternal structures poorly visible in both specimens but there is apparently a very large triangular structure that occupies nearly all of the ventral surface of thorax, probably corresponding to the mesothoracic sternite; all tarsi twosegmented (larger specimen); prothoracic femur straight, 0.68 mm long and 0.07 mm wide, tibia curved, 0.52 mm long and 0.04 mm wide, first tarsal segment 0.04 mm long, shorter than one-third of the length of second, second tarsal segment 0.15 mm long and 0.06 mm wide; prothoracic claws strong but rather short, slightly shorter than the width of segment, arolia not visible; mesothoracic and metathoracic femora, tibia and tarsi subcylindrical, elongate and slender; meso- and metathoracic coxae inserted laterally on thorax, with coxal axes almost horizontal; mesothoracic femur 1.44 mm long and 0.09 mm wide, longer than tibia; tibia
0.92 mm long and 0.03 mm wide, suture between first and second tarsal segments clearly visible; first tarsal segment 0.25 mm long, second segment more than 0.54 mm long; metathoracic femur 1.52 mm long and 0.09 mm wide, tibia 1.20 mm long and 0.04 mm wide, suture between first and second tarsal segment clearly visible; first tarsal segment 0.75 mm long, second tarsal segment 0.34 mm long, with apical claw visible; meso- and metathoracic femora, tibia and tarsus pilose but without rows of spinous hairs. Abdomen very short, 0.45 mm long and 0.82 mm wide, extended for only 0.22 mm beyond the level of the metathoracic coxae, broadly triangular, with reduced pregenital segments, especially in its ventral part; it is not possible to determine whether the first abdominal laterotergites are fused with the sternum; genital structure poorly visible, but present at least in the large specimen. The smaller specimen differs from the larger one in having meso- and metathoracic tarsi consisting of only one-segment. The suture between the first and second mesothoracic tarsal segments is not visible on the smaller specimen, but the preserved part of the tarsus is longer than the first tarsal segment of the larger specimen; metathoracic tarsus of smaller specimen 1.02 mm long, with apical claw visible; prothoracic tarsus of small specimen not preserved. Discussion. The very distinctive ‘‘drop-shape’’ of the body, with the long and slender legs, large mesothorax and reduced abdomen is characteristic of several taxa of three insect families: Hermatobatidae Coutie`re and Martin, 1901, Veliidae Amyot and Serville, 1843 and Gerridae Leach, 1815. Affinities with the Hermatobatidae, the sister group of
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Fig. 5. Cretogerris albianus gen. et sp. nov. A, B, holotype, MNHN ARC 262.1. A, photograph of prothoracic tarsus, arrow indicates separation between tarsal segments; B, photograph of mesothoracic tarsus, arrow indicates separation between tarsal segments; scale bars represent 0.25 mm. C, paratype MNHN ARC 262.2, photograph of metathoracic tarsus, arrow indicates tarsal claws; scale bar represents 1 mm.
(Veliidae C Gerridae), can be excluded because the adult specimen of Cretogerris has two-segmented tarsi and a rather elongate head (Andersen, 1982, p. 249). According to the keys of the gerromorphan families and the cladogram of the gerrid subfamilies proposed by Andersen (1982, p. 416, 1998, pp. 110, 111, 2000a, pp. 258, 259), Cretogerris differs from the Veliidae and falls within the Gerridae because of the absence of an impressed median line on the dorsal surface of head, the meso- and metathoracic coxae inserted laterally on the thorax, the prolonged mesothorax, which is much longer than metathorax, the long, slender metathoracic femur, and the twosegmented tarsi. It also shares with the Gerridae the absence of ocelli (Andersen, 1982, p. 233), but it has three pairs of cephalic trichobothria disposed in a manner similar to those of the modern Eurymetra Esaki, 1926, unlike many other Gerridae. It is much more difficult to establish its precise relationships within the Gerridae because of the lack of information on numerous thoracic and abdominal structures. Nevertheless,
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it can be attributed to the clade Gerridae non Rhagadotarsinae Lundblad, 1933 [Z (Trepobatinae Matsuda, 1960 C (Electrobatinae Andersen and Poinar, 1992 C (Charmatometrinae Andersen, 1975 C ((Gerrinae Leach, 1815 C Eotrechinae Matsuda, 1960) C (Cylindrostethinae Matsuda, 1960 C (Ptilomerinae Bianchi, 1896 C Halobatinae Bianchi, 1896))))))], after the apomorphic character state ‘‘ventral lobes of head reduced’’ (Andersen, 1982, pp. 234, 235, 1998; Andersen and Poinar, 1992, pp. 262, 263). Ocular setae longer than four facets, the absence of trichobothria-like setae on second antennal segment, and the rostrum surpassing the prosternal hind margin, are plesiomorphic character states that exclude affinities with the clade [Charmatometrinae C ((Gerrinae C Eotrechinae) C (Cylindrostethinae C (Ptilomerinae C Halobatinae)))]. The first segment of the prothoracic tarsus being shorter than one-third of the length of second segment is also a plesiomorphic character state of Cretogerris and present in Rhagadotarsinae and Trepobatinae, which excludes it from the clade [Electrobatinae C (Charmatometrinae C ((Gerrinae C Eotrechinae) C (Cylindrostethinae C (Ptilomerinae C Halobatinae))))]. Unlike Cretogerris, Electrobates Andersen and Poinar, 1992 has long subapical spinous hairs on the first antennal segment, an antennal tubercle situated in front, and a large pronotum. Nevertheless, the head with three pairs of trichobothria is an apomorphy of the (Ptilomerinae C Halobatinae), and present in both Cretogerris and Electrobates (Andersen, 1982, 2000a). The Trepobatinae can be excluded because the mesothoracic femur of Cretogerris is longer than the tibia (plesiomorphic state; Andersen, 1982, p. 241, 2000b, pp. 412, 413; Andersen and Poinar, 1992). Lastly, unlike Cretogerris, all subfamilies, except the fossil subfamily Electrobatinae and the fossil and modern Halobatinae, usually have elongate abdomens. In conclusion, the phylogenetic relationships of Cretogerris within the Gerridae remain partly uncertain. Cretogerris could either be in a position basal to the clade Gerridae non Rhagadotarsinae (Fig. 6A) or basal to the clade Gerridae non {Rhagadotarsinae and Trepobatinae} (Fig. 6B). It is unlikely to be closely related to the Halobatinae, although its general habitus is very similar to that of some extant species of this subfamily, e.g., Halobates and Eurymetra. All of these taxa are flightless and have a short, broad body, with a reduced abdomen and a large and broad thorax. 3. Conclusions 3.1. Palaeoecology The presence of gerrid bugs in amber is somewhat surprising, considering the hydrophobic (in the chemical sense) character of the resin, and the water-surface skating behaviour of the bugs. Andersen (2000b, p. 429) proposed two possible explanations for such an occurrence. In the first, specimens were trapped in a flow of resin produced by plants growing in a near-shore location. Alternatively, bugs living in a pond or stream that dried up temporarily may have wandered around and encountered the resin (see also Weitschat et al., 2002).
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Fig. 6. Two putative locations (A and B) of Cretogerris gen. nov. in the phylogenetic tree for the families of Gerromorpha and subfamilies of Gerridae (modified from Andersen, 2000b). The temporal distribution of fossils is indicated by heavy branches; unbroken branches denote range extensions inferred from fossils; broken branches denote range extensions inferred from sister group relationships, the question marks correspond to the possible presence of Mesoveliidae in the Early Jurassic (Shcherbakov and Popov, 2002, p. 151).
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In the present case, the occurrence of two specimens in the same small lump of amber rather supports the first hypothesis. Some modern Gerridae live in marine environments. They are distributed among the three subfamilies Rhagadotarsinae, Trepobatinae and Halobatinae (Andersen, 1995). It is not possible to infer any conclusion on the freshwater versus marine habitat of Cretogerris because of the relative uncertainty of its phylogenetic position. Nevertheless, its body shape is very similar to that of fossil and modern marine Halobatinae (Andersen, 1982; Andersen et al., 1994), although representatives of this subfamily also live in fresh water. A possible adaptation to marine environments in the Halobatinae is the loss of wings (Andersen, 1999, p. 239). Since this is a characteristic of Cretogerris, a marine habitat for this fossil bug is possible. On the other hand, the prolonged mesothorax, long and slender legs, and short abdomen of Cretogerris are also features of freshwater Halobatinae. The reconstruction of the general palaeoenvironment supports the hypothesis of resin production in areas very close to the sea (Perrichot, 2003), but under freshwater influence. The third gerromorphan bug from Archingeay (specimen MNHN ARC 226.13) has unspecialized legs (apical claws, no specialized setae), suggesting that it was better adapted for ‘‘walking’’ on land and water surfaces near shore (Andersen, 1995). Thus the present discoveries support the hypothesis of Perrichot (2004) of a selective trap on, or close to, the ground litter for most of the arthropod fauna in this French Cretaceous amber. Both specimens of Cretogerris were trapped together with two Diptera: Dolichopodidae, whose larvae are potential inhabitants of sandy ground in moist forest environments (Evenhuis, 1994; Borkent, 1995; Nel et al., 2004a). The specimen MNHN ARC 226.13 was also trapped in a piece of amber together with several other arthropods potentially living on the litter, i.e., some representatives of the Acari, Blattodea, Orthoptera: Grylloidea, Psocoptera, and Diptera: Dolichopodidae and Ceratopogonidae (Perrichot, 2004, table 2). 3.2. Antiquity of Gerridae Cretogerris has acquired the main specializations for surface skating of the more advanced modern Gerridae. This suggests that the Gerridae is a very old group, having appeared early in the Early Cretaceous or even the Late Jurassic. The present discovery also undoubtedly confirms the presence of the clade [Veliidae C Gerridae] during the Early Cretaceous, the only Cretaceous bug previously attributed to the Veliidae being based on two rather poorly preserved impressions from the Aptian Koonwarra fossil bed in Australia (Jell and Duncan, 1986). The apparent absence of both these families in other Early Cretaceous deposits is surprising. The Polyneoptera: Chresmodidae were very different Late Jurassic and Early Cretaceous surface skaters. Some probably lived in marine environments but others inhabited lakes. They were probably also carnivorous, eating small nektonic animals, in common with the Gerridae. The most recent representatives of the Chresmodidae have recently been discovered in
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middle Cenomanian deposits in Lebanon (Nel et al., 2004b). Thus the Gerridae and Chresmodidae were contemporaneous for at least two million years; hence, the hypothesis that the replacement of the Chresmodidae by the Gerridae as a potential cause of the extinction of the former is poorly supported by our study.
Acknowledgements We are greatly indebted to M. Paul Marchand, owner of the quarry in which the Albian amber was discovered, for his helpful contribution to the excavation of the material. We acknowledge M. Thierry Guyot, collector of the undetermined specimen reported here, who generously gave us his collection of amber from Archingeay, which contains numerous fossil inclusions. We also thank Gilbert Hodebert (MNHN) for drawing and mounting of figures. This paper is a contribution to the ‘‘ECLIPSE’’ CNRS program ‘‘Interactions Climat/Ecosyste`mes entre l’Aptien et le Pale´oce`ne’’.
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Gerromorphan bugs in Early Cretaceous French amber (Insecta: Heteroptera): first representatives of Gerridae and their phylogenetic and palaeoecological implications Vincent Perrichot a, Andre´ Nel b,*, Didier Neraudeau a a
Ge´osciences Rennes and CNRS UMR 6118, Campus de Beaulieu, baˆtiment 15, 263 Avenue du Ge´ne´ral Leclerc, 35042 Rennes Cedex, France b CNRS UMR 5143, Muse´um National d’Histoire Naturelle, Entomologie, 45 Rue Buffon, F-75005 Paris, France Received 9 September 2004; accepted in revised form 5 May 2005 Available online 2 November 2005
Abstract Three specimens of gerromorphan bugs in Late Albian amber from south-west France are described. One is regarded as an incertae sedis within the Gerromorpha, the other two are assigned to Cretogerris albianus gen. et sp. nov., the oldest representative of the aquatic bug family Gerridae. The discovery confirms the great antiquity of the Gerridae, until now only inferred from an Early Cretaceous representative of the sister family Veliidae. The phylogenetic affinities of Cretogerris within the Gerridae are still rather uncertain, but this fossil taxon shows highly specialized body and leg structures that are very similar to those of the marine Halobatinae, suggesting that it was possibly a marine surface skater. The Gerridae and the Chresmodidae, another extinct group of Mesozoic surface skaters, were contemporaneous during at least the early Cenomanian. The discovery of these gerromorphan bugs in the Albian amber supports the hypothesis of a selective trap of a litter fauna, originating from a beach environment, for this resin. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: Aquatic insects; Heteroptera; Gerromorpha; Gerridae; Albian amber; South-west France
1. Introduction Gerromorpha or water striders live mainly in, or on the surface film of, fresh or marine water. These aquatic insects are thus not prone to be caught in resin exuded by trees on land. Indeed, most insects found as inclusions in amber belong to terrestrial forms whereas aquatic forms are quite rare. Among gerromorphan bugs, only a few fossil species of Gerridae have been described from the Eocene in north-east Italy (Andersen et al., 1994), Late Eocene Baltic amber (Andersen, 2000a; Weitschat et al., 2002), the Eocene and OligoceneeMiocene of North America and PaleoceneeEocene of Denmark (Andersen, 1982), and Miocene Dominican amber (Andersen and Poinar, 1992; Andersen, 2000b; Wappler and Andersen,
* Corresponding author. E-mail address: [email protected] (A. Nel). 0195-6671/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.cretres.2005.05.003
2004). Water striders are therefore only known from the Cenozoic, even if the presence of their sister family Veliidae in Australian Early Cretaceous deposits suggests that both families appeared at least during this period (Andersen et al., 1994; Andersen, 2000b). Thus, the present discovery of two representatives of the Gerridae in French Late Albian amber is of great interest in connection with the early history of this family of aquatic bugs. 2. Systematic palaeontology Order: Heteroptera Latreille, 1810 Infraorder: Gerromorpha Popov, 1971 Family: incertae sedis Genus and species undetermined Figs. 1e2
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Fig. 1. Specimen MNHN ARC 226.13, photograph of general habitus, dorsal view; scale bar represents 1 mm.
Material. Specimen MNHN ARC 226.13, fossilized with 83 other arthropods in the same piece of amber (cf. list in Perrichot, 2004, table 2, sample Arc 226), deposited in the Laboratoire de Pale´ontologie, Muse´um national d’Histoire naturelle, Paris. Locality and stratigraphic horizon. Archingeay-Les Nouillers, Charente-Maritime, south-west France, Lower Cretaceous, uppermost Albian (Ne´raudeau et al., 2002). Description. A nearly complete specimen but with the head and thorax partly dislocated; body 1.12 mm long, covered by small setae 0.05 mm long; head longer than wide, 0.29 mm long, about 0.2 mm wide; a median impressed line on dorsal surface of head; anterior part of head protruding; no visible ocelli; eyes globular but not posteriorly produced, 0.12 mm diameter, 0.12 mm apart; ocular setae not visible; three pairs of long cephalic trichobothria on dorsal surface of head, 0.05 mm long, disposed in two lines along the inner margins of the two eyes; apex of rostrum surpassing for 0.29 mm the prosternal hind margin; first labial segment 0.06 mm long, second 0.05 mm long, third very long at 0.41 mm, and fourth 0.14 mm long; no intercalary sclerites between labial segments 3 and 4; antennal tubercles situated before eyes and visible from above, directed forward; four antennal segments covered with setae but without trichobothria-like setae, second segment 0.18 mm long, third 0.23 mm long, first and fourth segments not completely preserved; all segments straight and cylindrical; ventral lobes of head not protruding in front and not hiding the rostral base. Thorax elongate, 0.37 mm long, 0.44 mm wide; separation between pronotum and mesonotum not well preserved, no pronotal lobe extending medially; no trace of wing structure,
Fig. 2. Specimen MNHN ARC 226.13, reconstruction of general habitus, dorsal view; scale bar represents 1 mm.
probably apterous; sternal structures poorly visible; tarsal claws at apex of tarsi; dorsal and ventral arolia visible but not differentiated, at least in one prothoracic leg, as curved and strong ‘‘trichobothria’’; all femora bearing a long seta in their apical outer quarter; all tarsi composed of one-segment; femora, tibia and tarsi of all legs subcylindrical, elongate and slender; prothoracic femur straight, 0.44 mm long and 0.06 mm wide, tibia straight, 0.36 mm long and 0.04 mm wide, tarsus 0.14 mm long; mesothoracic femur 0.49 mm long and 0.06 mm wide, tibia 0.5 mm long and 0.04 mm wide, tarsus 0.23 mm long; metathoracic femur 0.72 mm long and 0.09 mm wide, tibia 0.92 mm long and 0.04 mm wide, bearing six strong setae, tarsus 0.31 mm long; legs pilose but without rows of spinous hairs. Abdomen elongate, 0.55 mm long and 0.34 mm wide; genital structure not visible, if present. Discussion. The small size, absence of visible genital structures, and absence of wing structure suggest that this specimen is a larva. After Andersen (1982) and Schuh and Slater (1995), this fossil shows two of the main synapomorphies of the Gerromorpha, i.e., the presence of three pairs of cephalic trichobothria inserted in deep cuticular pits along the inner margins of the eyes, and a pretarsus with undifferentiated dorsal and ventral arolia. Its position within this suborder is more difficult to establish. The characters that are currently used to separate the families are not available in this fossil (Andersen, 1982, pp. 255e257). Therefore, we prefer to maintain it as an incertae sedis within the Gerromorpha.
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Family: Gerridae Leach, 1815 Genus Cretogerris gen. nov. Type species. Cretogerris albianus sp. nov. Derivation of name. Combination of the Cretaceous Period and the modern genus Gerris. Diagnosis. Body entirely globose, ‘‘drop-like’’ shaped; ventral lobes of head reduced; no median impressed line on dorsal surface; ocular setae longer than four facets; three pairs of long cephalic trichobothria on dorsal surface of head; no subapical spinous hairs on first antennal segment; antennal tubercle not situated in front; absence of trichobothria-like setae on second antennal segment; rostrum surpassing the prosternal hind margin; tarsi consist of two-segments on all legs in the adult specimen; first segment of prothoracic tarsus less than one-third the length of second segment; mesothoracic femur longer than tibia; meso- and metathoracic legs long and slender; meso- and metathoracic coxae inserted laterally on thorax; pronotum small and narrow; mesonotum very large; abdomen very reduced. Cretogerris albianus sp. nov. Figs. 3e5 Derivation of name. After the Albian age of these fossils.
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Material. Holotype specimen MNHN ARC 262.1 (larger specimen), paratype specimen MNHN ARC 262.2 (smaller specimen); both specimens fossilized in the same piece of amber with two adult Diptera: Dolichopodidae; deposited in the Laboratoire de Pale´ontologie, Muse´um national d’Histoire naturelle, Paris. Type locality and stratigraphic horizon. Archingeay-Les Nouillers, Charente-Maritime, south-west France, Lower Cretaceous, uppermost Albian (Ne´raudeau et al., 2002). Diagnosis. As for genus. Description. The smaller specimen is probably a last instar larva, 1.52 mm long and with one-segmented metathoracic tarsi; the larger is an adult, 1.84 mm long, with two-segmented metathoracic tarsi, and the genital appendages present. Unless otherwise indicated, the following description is based on the adult specimen. Head 0.2 mm long and 0.6 mm wide; no median impressed line on dorsal surface; anterior part of head regularly curved but not strongly protruding; no ocelli; eyes globular but not posteriorly produced, 0.15 mm long and 0.13 mm wide, 0.36 mm apart; ocular setae 0.05 mm long, longer than four facets; three pairs of long cephalic trichobothria on dorsal surface of head, disposed in two lines along the inner margins of the two eyes; apex of rostrum surpassing for
Fig. 3. Cretogerris albianus gen. et sp. nov., holotype, MNHN ARC 262.1 (larger specimen) and paratype MNHN ARC 262.2 (smaller specimen); photograph of general habitus, ventral view; scale bar represents 1 mm.
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Fig. 4. Cretogerris albianus gen. et sp. nov., holotype and paratype as in Fig. 3; reconstruction of general habitus, ventral view; scale bar represents 2 mm.
0.06 mm the prosternal hind margin; length of first labial segment 0.12 mm, second 0.07 mm, third 0.18 mm, and fourth 0.13 mm; antennal tubercles situated beneath the eyes and not visible from above, but directed forward; four antennal segments, length of first basal segment 0.16 mm, second 0.22 mm, third 0.24 mm, and fourth 0.21 mm; second and third segments slightly curved, without trichobothria-like setae; fourth segment spindle-shaped but not twisted nor bent; all segments cylindrical; ventral lobes of head not produced in front of the rostral base. Thorax 0.84 mm long, 1.0 mm wide, not very elongate but very broad, with its broadest part opposite the mesothoracic coxae; separation between pronotum and mesonotum not preserved although there is apparently no pronotal lobe extending medially, but a structure with two posterior lateral extensions probably corresponding to a very large mesonotum; no trace of wing structure; sternal structures poorly visible in both specimens but there is apparently a very large triangular structure that occupies nearly all of the ventral surface of thorax, probably corresponding to the mesothoracic sternite; all tarsi twosegmented (larger specimen); prothoracic femur straight, 0.68 mm long and 0.07 mm wide, tibia curved, 0.52 mm long and 0.04 mm wide, first tarsal segment 0.04 mm long, shorter than one-third of the length of second, second tarsal segment 0.15 mm long and 0.06 mm wide; prothoracic claws strong but rather short, slightly shorter than the width of segment, arolia not visible; mesothoracic and metathoracic femora, tibia and tarsi subcylindrical, elongate and slender; meso- and metathoracic coxae inserted laterally on thorax, with coxal axes almost horizontal; mesothoracic femur 1.44 mm long and 0.09 mm wide, longer than tibia; tibia
0.92 mm long and 0.03 mm wide, suture between first and second tarsal segments clearly visible; first tarsal segment 0.25 mm long, second segment more than 0.54 mm long; metathoracic femur 1.52 mm long and 0.09 mm wide, tibia 1.20 mm long and 0.04 mm wide, suture between first and second tarsal segment clearly visible; first tarsal segment 0.75 mm long, second tarsal segment 0.34 mm long, with apical claw visible; meso- and metathoracic femora, tibia and tarsus pilose but without rows of spinous hairs. Abdomen very short, 0.45 mm long and 0.82 mm wide, extended for only 0.22 mm beyond the level of the metathoracic coxae, broadly triangular, with reduced pregenital segments, especially in its ventral part; it is not possible to determine whether the first abdominal laterotergites are fused with the sternum; genital structure poorly visible, but present at least in the large specimen. The smaller specimen differs from the larger one in having meso- and metathoracic tarsi consisting of only one-segment. The suture between the first and second mesothoracic tarsal segments is not visible on the smaller specimen, but the preserved part of the tarsus is longer than the first tarsal segment of the larger specimen; metathoracic tarsus of smaller specimen 1.02 mm long, with apical claw visible; prothoracic tarsus of small specimen not preserved. Discussion. The very distinctive ‘‘drop-shape’’ of the body, with the long and slender legs, large mesothorax and reduced abdomen is characteristic of several taxa of three insect families: Hermatobatidae Coutie`re and Martin, 1901, Veliidae Amyot and Serville, 1843 and Gerridae Leach, 1815. Affinities with the Hermatobatidae, the sister group of
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Fig. 5. Cretogerris albianus gen. et sp. nov. A, B, holotype, MNHN ARC 262.1. A, photograph of prothoracic tarsus, arrow indicates separation between tarsal segments; B, photograph of mesothoracic tarsus, arrow indicates separation between tarsal segments; scale bars represent 0.25 mm. C, paratype MNHN ARC 262.2, photograph of metathoracic tarsus, arrow indicates tarsal claws; scale bar represents 1 mm.
(Veliidae C Gerridae), can be excluded because the adult specimen of Cretogerris has two-segmented tarsi and a rather elongate head (Andersen, 1982, p. 249). According to the keys of the gerromorphan families and the cladogram of the gerrid subfamilies proposed by Andersen (1982, p. 416, 1998, pp. 110, 111, 2000a, pp. 258, 259), Cretogerris differs from the Veliidae and falls within the Gerridae because of the absence of an impressed median line on the dorsal surface of head, the meso- and metathoracic coxae inserted laterally on the thorax, the prolonged mesothorax, which is much longer than metathorax, the long, slender metathoracic femur, and the twosegmented tarsi. It also shares with the Gerridae the absence of ocelli (Andersen, 1982, p. 233), but it has three pairs of cephalic trichobothria disposed in a manner similar to those of the modern Eurymetra Esaki, 1926, unlike many other Gerridae. It is much more difficult to establish its precise relationships within the Gerridae because of the lack of information on numerous thoracic and abdominal structures. Nevertheless,
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it can be attributed to the clade Gerridae non Rhagadotarsinae Lundblad, 1933 [Z (Trepobatinae Matsuda, 1960 C (Electrobatinae Andersen and Poinar, 1992 C (Charmatometrinae Andersen, 1975 C ((Gerrinae Leach, 1815 C Eotrechinae Matsuda, 1960) C (Cylindrostethinae Matsuda, 1960 C (Ptilomerinae Bianchi, 1896 C Halobatinae Bianchi, 1896))))))], after the apomorphic character state ‘‘ventral lobes of head reduced’’ (Andersen, 1982, pp. 234, 235, 1998; Andersen and Poinar, 1992, pp. 262, 263). Ocular setae longer than four facets, the absence of trichobothria-like setae on second antennal segment, and the rostrum surpassing the prosternal hind margin, are plesiomorphic character states that exclude affinities with the clade [Charmatometrinae C ((Gerrinae C Eotrechinae) C (Cylindrostethinae C (Ptilomerinae C Halobatinae)))]. The first segment of the prothoracic tarsus being shorter than one-third of the length of second segment is also a plesiomorphic character state of Cretogerris and present in Rhagadotarsinae and Trepobatinae, which excludes it from the clade [Electrobatinae C (Charmatometrinae C ((Gerrinae C Eotrechinae) C (Cylindrostethinae C (Ptilomerinae C Halobatinae))))]. Unlike Cretogerris, Electrobates Andersen and Poinar, 1992 has long subapical spinous hairs on the first antennal segment, an antennal tubercle situated in front, and a large pronotum. Nevertheless, the head with three pairs of trichobothria is an apomorphy of the (Ptilomerinae C Halobatinae), and present in both Cretogerris and Electrobates (Andersen, 1982, 2000a). The Trepobatinae can be excluded because the mesothoracic femur of Cretogerris is longer than the tibia (plesiomorphic state; Andersen, 1982, p. 241, 2000b, pp. 412, 413; Andersen and Poinar, 1992). Lastly, unlike Cretogerris, all subfamilies, except the fossil subfamily Electrobatinae and the fossil and modern Halobatinae, usually have elongate abdomens. In conclusion, the phylogenetic relationships of Cretogerris within the Gerridae remain partly uncertain. Cretogerris could either be in a position basal to the clade Gerridae non Rhagadotarsinae (Fig. 6A) or basal to the clade Gerridae non {Rhagadotarsinae and Trepobatinae} (Fig. 6B). It is unlikely to be closely related to the Halobatinae, although its general habitus is very similar to that of some extant species of this subfamily, e.g., Halobates and Eurymetra. All of these taxa are flightless and have a short, broad body, with a reduced abdomen and a large and broad thorax. 3. Conclusions 3.1. Palaeoecology The presence of gerrid bugs in amber is somewhat surprising, considering the hydrophobic (in the chemical sense) character of the resin, and the water-surface skating behaviour of the bugs. Andersen (2000b, p. 429) proposed two possible explanations for such an occurrence. In the first, specimens were trapped in a flow of resin produced by plants growing in a near-shore location. Alternatively, bugs living in a pond or stream that dried up temporarily may have wandered around and encountered the resin (see also Weitschat et al., 2002).
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Fig. 6. Two putative locations (A and B) of Cretogerris gen. nov. in the phylogenetic tree for the families of Gerromorpha and subfamilies of Gerridae (modified from Andersen, 2000b). The temporal distribution of fossils is indicated by heavy branches; unbroken branches denote range extensions inferred from fossils; broken branches denote range extensions inferred from sister group relationships, the question marks correspond to the possible presence of Mesoveliidae in the Early Jurassic (Shcherbakov and Popov, 2002, p. 151).
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In the present case, the occurrence of two specimens in the same small lump of amber rather supports the first hypothesis. Some modern Gerridae live in marine environments. They are distributed among the three subfamilies Rhagadotarsinae, Trepobatinae and Halobatinae (Andersen, 1995). It is not possible to infer any conclusion on the freshwater versus marine habitat of Cretogerris because of the relative uncertainty of its phylogenetic position. Nevertheless, its body shape is very similar to that of fossil and modern marine Halobatinae (Andersen, 1982; Andersen et al., 1994), although representatives of this subfamily also live in fresh water. A possible adaptation to marine environments in the Halobatinae is the loss of wings (Andersen, 1999, p. 239). Since this is a characteristic of Cretogerris, a marine habitat for this fossil bug is possible. On the other hand, the prolonged mesothorax, long and slender legs, and short abdomen of Cretogerris are also features of freshwater Halobatinae. The reconstruction of the general palaeoenvironment supports the hypothesis of resin production in areas very close to the sea (Perrichot, 2003), but under freshwater influence. The third gerromorphan bug from Archingeay (specimen MNHN ARC 226.13) has unspecialized legs (apical claws, no specialized setae), suggesting that it was better adapted for ‘‘walking’’ on land and water surfaces near shore (Andersen, 1995). Thus the present discoveries support the hypothesis of Perrichot (2004) of a selective trap on, or close to, the ground litter for most of the arthropod fauna in this French Cretaceous amber. Both specimens of Cretogerris were trapped together with two Diptera: Dolichopodidae, whose larvae are potential inhabitants of sandy ground in moist forest environments (Evenhuis, 1994; Borkent, 1995; Nel et al., 2004a). The specimen MNHN ARC 226.13 was also trapped in a piece of amber together with several other arthropods potentially living on the litter, i.e., some representatives of the Acari, Blattodea, Orthoptera: Grylloidea, Psocoptera, and Diptera: Dolichopodidae and Ceratopogonidae (Perrichot, 2004, table 2). 3.2. Antiquity of Gerridae Cretogerris has acquired the main specializations for surface skating of the more advanced modern Gerridae. This suggests that the Gerridae is a very old group, having appeared early in the Early Cretaceous or even the Late Jurassic. The present discovery also undoubtedly confirms the presence of the clade [Veliidae C Gerridae] during the Early Cretaceous, the only Cretaceous bug previously attributed to the Veliidae being based on two rather poorly preserved impressions from the Aptian Koonwarra fossil bed in Australia (Jell and Duncan, 1986). The apparent absence of both these families in other Early Cretaceous deposits is surprising. The Polyneoptera: Chresmodidae were very different Late Jurassic and Early Cretaceous surface skaters. Some probably lived in marine environments but others inhabited lakes. They were probably also carnivorous, eating small nektonic animals, in common with the Gerridae. The most recent representatives of the Chresmodidae have recently been discovered in
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middle Cenomanian deposits in Lebanon (Nel et al., 2004b). Thus the Gerridae and Chresmodidae were contemporaneous for at least two million years; hence, the hypothesis that the replacement of the Chresmodidae by the Gerridae as a potential cause of the extinction of the former is poorly supported by our study.
Acknowledgements We are greatly indebted to M. Paul Marchand, owner of the quarry in which the Albian amber was discovered, for his helpful contribution to the excavation of the material. We acknowledge M. Thierry Guyot, collector of the undetermined specimen reported here, who generously gave us his collection of amber from Archingeay, which contains numerous fossil inclusions. We also thank Gilbert Hodebert (MNHN) for drawing and mounting of figures. This paper is a contribution to the ‘‘ECLIPSE’’ CNRS program ‘‘Interactions Climat/Ecosyste`mes entre l’Aptien et le Pale´oce`ne’’.
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