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Chironomidae (Diptera) in Dominican amber as indicators for ecosystem stability in the Caribbean
Palaeogeography, Palaeoclimatology, Palaeoecology 241 (2006) 410 – 416 www.elsevier.com/locate/palaeo
Chironomidae (Diptera) in Dominican amber as indicators for ecosystem stability in the Caribbean Martin Grund ⁎ Institut für Paläontologie, Universität Bonn, Nussallee 8, 53115 Bonn, Germany Received 23 December 2005; received in revised form 31 March 2006; accepted 24 April 2006
Abstract A first overview on fossil chironomids in Dominican amber is given. This fossil assemblage seems to represent an insular fauna, very similar to its living relatives. Stenochironomus sp. and especially the true xylophagous, neotropical/southern North American Xestochironomus spp. prove the persistence of submerged dead wood in nutrient poor mountain streams in the Greater Antilles from the Miocene until today. Their abundance indicates that the special ecological conditions in extant Caribbean tropical mountain streams already ruled the ancient ecosystem. The results arising from the fossils of these freshwater organisms do not coincide with the faunal changes shown by other groups of insects. General systematic descriptions of new fossil representatives of Xestochironomus and Stenochironomus are given. © 2006 Elsevier B.V. All rights reserved. Keywords: Diptera; Chironomidae; Dominican amber; Paleoecology; Climate change
1. Introduction Inclusions of fossil plants and animals in amber enable us to reconstruct the lifestyle and habitat of former organisms and give insight in ancient ecosystems. Of course, different organisms are of different utility in this attempt. It is therefore reasonable to study fossil taxa with high explanatory power for ecological analyses. Aquatic insects are particularly suitable in this regard (Wichard and Weitschat, 1996), especially Chironomids (non-biting midges), which are frequently used as bioindicators in classifying and monitoring different freshwater environments (Armitage et al., 1995). Chironomid larvae occur in virtually all aquatic habitats, ⁎ Tel.: +49 228734843; fax: +49 228733509. E-mail address: [email protected]. 0031-0182/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.palaeo.2006.04.005
except the open sea. Remains from chironomid larvae in aquatic sediments are already used for paleoecological reconstructions, mainly of Quarternary limnic environments (Walker, 1993; Armitage et al., 1995). Based on biostratigraphic and paleogeographic data Iturralde-Vinent and MacPhee (1996) dated the exploitable fossil bearing Dominican amber deposits as of late Early Miocene through early Middle Miocene age (15– 20 Ma). The resin has formed on paleo-Hispaniola and was transported to a sedimentary basin by slope-wash and through river channels dissecting the landscape. The material found today in the La Toca formation of the Northern mining district originated in the forested mountain ranges of the ancestral Cordillera Central. The amber of the Yanigua formation in the eastern mining district is from elsewhere around the basin (IturraldeVinent and MacPhee, 1996; Iturralde-Vinent, 2001).
M. Grund / Palaeogeography, Palaeoclimatology, Palaeoecology 241 (2006) 410–416
Although winged adult Chironomids are very common in fossil resins, the group is rather poorly studied. This is very probably due to taxonomic difficulties, because the systematic determination of chironomids can be a challenging task. Often only the males are described in taxonomic literature and identification needs high power magnification of many structures that are easily obscured in amber-preserved specimens. Despite a single species, Ablabesmyia electrohispaniolana (Grund, 2005), Dominican amber Chironomidae are completely unstudied. The present paper proves that in many cases the fossils can be identified to extant genera and valuable information about the ancient ecosystem, diversity, ecology and biogeography can be ascertained. The only modern synopsis of an amber chironomid fauna is that of Seredszus (2003) on Baltic amber Chironomids, which are of Eocene age. In a catalogue on the chironomids of the Neotropical Region, Spies and Reiss (1996) listed 29 genera with 41 species of chironomids recorded from the Greater Antilles. However, our knowledge on the chironomid fauna of the region is far from complete. Because of the intensive ecological studies carried out on Puerto Rican ecosystems (Reagan and Waide, 1996), this island is predestined to serve as a representative model for the Greater Antilles, which are large and mountainous islands, clearly isolated from the mainland. Ferrington et al. (1993) studied faunal composition, temporal abundance and ecological aspects of the chironomids of a small mountain stream in Puerto Rico. They found abundant adults of the genera Xestochironomus and Stenochironomus whose larvae are xylophagous. Xestochironomus and Stenochironomus belong to the tribe Chironomi in the subfamily Chironominae. The larvae of Xestochironomus are highly specialized wood miners in lotic freshwaters in the Neotropics and in the southern USA. Stenochironomus is a cosmopolitan genus, with representatives living below tree line and mining in wood or leaves in different freshwater habitats. The woodmining larvae are found in dead submerged angiosperm wood. The wood as a substrate lies immobile in the water and is of firm constitution and free from surface contamination (Borkent, 1984). The distribution and abundance of today's flora and fauna in the Circum-Caribbean region is influenced by climatic history: for example, by cool and dry periods from the late Pleistocene to present. In the climate scenario of Curtis et al. (2001), several lake beds in the region were devoid of water during dry periods in the late Pleistocene, temperature was 6.5 to 8.0 °C lower than today and eventually drought was responsible even for the collapse of the Maya in the 9th century A.D.
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Probably climate changes varied through the region and their impacts on the environment were different in different types of ecosystems or habitats. Poinar and Poinar (1999) outlined a reconstruction of the Miocene Dominican amber forest, based upon a huge database of fossil plants and animals in Dominican amber, the world's best source for terrestrial invertebrate fossils derived from a tropical ecosystem. Their analysis led to the conclusion that the ecosystem on Hispaniola changed since amber times, because many taxa are now extinct from the island and there is no remaining forest on Earth today with such a community of plants and animals. Mainly taxa adapted to a narrow range of environmental conditions went extinct during Pliocene– Pleistocene climatic changes and were unable to recolonize the island. The stingless bees for example, present in Dominican amber, are extinct on the island today (Michener and Poinar, 1996). The ecologically dominant ants have undergone significant changes via extinction and invasion, with higher extinction rates in the ecologically more specialized taxa (Wilson, 1988). However, the degree of faunal turnover since amber times is still not known. The reconstruction of the Dominican amber-producing forest by Grimaldi (1996) differed little from recent Neotropical rainforests. Penney (1999) compared fossil and recent spider families of the region and concluded that the taxa found in amber but not today on Hispaniola still exist on the island but are yet to be found. 2. Materials and methods The chironomid fossils in the Dominican amber collections of SMNS (Staatliches Museum für Naturkunde Stuttgart, Germany) and AMNH (American Museum of Natural History, New York, USA) were examined. The pieces were cut and ground for observation with classical microscope techniques (stereomicroscopy and light microscopy). It is often not possible to dissect and finish amber stones with many inclusions without damaging fossils. Thus assemblages of more than five chironomids in one amber piece were left untreated and therefore not included in this study. For taxonomic reasons female specimens were also not considered. A total of 163 specimens, of variable conservation condition, were available for identification. The morphological terminology follow Sæther (1980) and Borkent (1984), for diagnostic adult characters of the genera see Cranston et al. (1989). The two fossil specimens exemplarily described here are Do-4250-B for Xestochironomus and Do-4215-M for Stenochironomus, both housed in the SMNS.
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2.1. Taxonomic limitation The fossils described in the present study can be assigned to extant genera with certainty. They are not described and named as new fossil species because in the authors opinion it is not possible to safely distinguish fossil species of the same genus, since important characters like subtle genital features and many setae are either not visible or lacking. 3. Results Forty individual specimens could be identified to genus level. This is 25% of the stock. Seven different genera, all extant, were identified (Table 1).
3.2.3. Wing Membrane without setae. Costa not extended. R2 + 3 missing. FCu distal to RM. R, R1 and R4 + 5 with setae. Halteres brown. Wing length 1.8 mm (Fig. 1C). 3.2.4. Legs Fore tibia with distinct apical inner scale, without apical spine. Mid and hind tibiae with two fused combs and each with a distinct, slightly sinuous spur. Pulvilli present. Femur of foreleg with one distal brown band and one at mid-length. Fore tibia proximal and distal brown. Femora of mid and hindlegs proximal and distal dark, tibiae proximal dark. Tarsi of mid and hindlegs pale, tarsi of forelegs missing. 3.2.5. Abdomen Tergites 2–4 with posterior brown band.
3.1. Stenochironomus sp. One fossil of Stenochironomus has been found in the SMNS collection. 3.2. Description of the fossil Do-4215-M Habitus: The wings are spread and the abdomen is contorted in the anterior–posterior-axis and around the right hind leg. It shows brown arrays and bands on legs, abdomen and thorax (Fig. 1A). 3.2.1. Head Eyes with strong parallel-sided dorsomedial extension. Palpus 5-segmented. AR > 1. 3.2.2. Thorax Antepronotal lobes strongly reduced, scutum distinctly overreaching antepronotum (Fig. 1B). Acrostichals starting at anterior margin of scutum. Lateral vittae and posterior part of median vittae brown. Postnotum brown. Table 1 Chironomid genera found in Dominican amber and number of fossil specimens Genus
Number of specimens
Ablabesmyia Monopelopia Tanypus Dicrotendipes Polypedilum Stenochironomus Xestochironomus Total
14 1 1 2 6 1 15 40
3.2.6. Hypopygium Median anal tergital setae in circular patch. Anal point prominent, t-shaped in cross-section at base, flattened apically and curved ventrally. Tufts of setae lateral and ventral of anal point. Inferior volsella long, narrow and bowed dorsoventrally, with apical setae. Gonostylus rather long, narrow and curved medially (Fig. 1D). 3.2.7. Diagnosis As in Xestochironomus, the antepronotal lobes in Stenochironomus are strongly reduced and the scutum distinctly overreaches the antepronotum. In contrast to Xestochironomus, the tibial combs in Stenochironomus are fused. Additionally the inferior volsella of Stenochironomus is narrower, mostly longer and bowed dorsoventrally. Due to these character states the fossil can be identified confidently as Stenochironomus. 3.3. Xestochironomus spp. Thirteen fossil inclusions of Xestochironomus have been found in the SMNS collection and two in the AMNH collection. The different shapes of the gonostylus (forked or unforked, and if forked, the form of inner appendage) and variations in abdominal colour pattern suggest that there are at least four different species among these fossils. 3.4. Description of the fossil Do-4250-B 3.4.1. Habitus The distal third of the left wing is bent down slightly (Fig. 1G). The fossil shows no colour pattern (Fig. 1E).
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Fig. 1. Chironomids in Dominican amber. Stenochironomus sp.: A: Habitus, B: Thorax, C: Wing, D: Hypopygium; Xestochironomus sp.: E: Habitus, F: Thorax, G: Wing, H: Hypopygium.
3.4.2. Head Antenna with thirteen flagellomeres. AR < 1. Eyes with strong parallel-sided dorsomedial extension. Palpus 5-segmented.
3.4.3. Thorax Antepronotal lobes strongly reduced, scutum distinctly overreaching antepronotum. Scutum uniformly light brown. Acrostichals starting at anterior
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margin of scutum. Acrostichals and dorsocentrals uniserial (Fig. 1F). 3.4.4. Wing Membrane without setae. Costa not extended. R2 + 3 weak, curving and ending near apex of R1. R4 + 5 ending at wing apex. FCu distal to RM. Wing length 1.4 mm (Fig. 1G; distal third of the wing is bent down slightly). 3.4.5. Legs Uniformly brown. Mid and hind tibiae with two closely approximated combs, each bearing distinct, slightly sinuous spur. 3.4.6. Abdomen Uniformly brown. Setation evenly distributed. 3.4.7. Hypopygium Median anal tergital setae in circular patch. Anal point towering, hook-shaped in lateral view. Superior volsella slenderly digitiform. Inferior volsella not distinctly swollen, bearing apical/subapical setae and one apical, posteriorly directed, elongate seta. Gonostylus forked with inner appendage short with rather blunt tip, outer appendage longer with pointed tip bearing one long seta (Fig. 1H). 3.4.8. Diagnosis As in Stenochironomus the antepronotal lobes in Xestochironomus are strongly reduced and the scutum distinctly overreaches the antepronotum. In contrast to Stenochironomus, the tibial combs in Xestochironomus are separated. Additionally the inferior volsella of Xestochironomus is shorter, thicker and not noticeably curved. The gonostylus of this fossil is forked, as characteristic for several Xestochironomus species known from the Caribbean region (Sublette and Wirth, 1972) and elsewhere in the Neotropics. Due to these character states the fossil can be identified confidently as Xestochironomus. 4. Discussion The survey of Miocene Dominican amber chironomids presented here yields 40 specimens unevenly distributed among seven extant genera. In comparison, Eocene Baltic amber Chironomids were assigned to 18 extant genera with no genus represented by more than six individuals (Seredszus, 2003). It therefore appears as if the Dominican amber fauna is less diverse, containing fewer genera and possibly represents a depleted island fauna.
All of the seven genera found in the amber are recorded today from at least one island in the Caribbean Sea. The distribution of Xestochironomus is limited to Central and South America and the southern USA, the other genera are widely distributed, most of them on all continents, excepting Antarctica. It is possible that there are chironomid genera in Dominican amber not yet detected, which have become extinct in the Caribbean, Neotropics or worldwide. Although not all genera reported today from the Greater Antilles have been found in studied amber, all genera identified from amber remain extant in the Caribbean. The fossil fauna is dominated by Xestochironomus and Ablabesmyia, with 3/4 of those identified to genus belonging to these taxa. The high number of Ablabesmyia is probably an artefact of perception, because of their conspicuous colour pattern (Grund, 2005). The currently known Ablabesmyia fossils all look quite similar and it is not known how many species they comprise. The high number of Xestochironomus cannot be explained by especially conspicuous individuals. We must assume that this genus is common in the fossil record because it was common in the ecosystem. It is supposed that the fifteen fossils found, comprise four different species. From phylogenetic studies Cranston and Mckie (2006) concluded that the habit of wood-mining in Chironomidae had probably already been adopted sometime in the Mesozoic. This and the well-established principle of behavioural fixity lead to the conclusion that both taxa showed the same ecological specialization in the Miocene. In the food webs of the streams in Luquillo Experimental Forest on Puerto Rico, detritus is an important energy source and aquatic insects are important detritivores (Covich and McDowell, 1996). Ferrington et al. (1993) found that Xestochironomus and Stenochironomus formed 45% (specimens) of chironomid emergence from a Puerto Rican mountain stream in an emergence trap over a one-year period. Sublette and Wirth (1972) reported that Xestochironomus is one of the most characteristic genera on the islands Dominica and Jamaica, caught near small rocky streams in the mountains, and that Stenochironomus leptopus Kieffer is one of the most abundant Chironomini on Dominica at lowland sites and in the mountains. With seven species recorded, Xestochironomus is one of the most species rich chironomid genera of the Caribbean islands. The biotic and abiotic factors in small Neotropical streams are often similar (Covich and McDowell, 1996). As found in Puerto Rico, low nutrient concentrations,
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low insolation (canopy-shaded) and a relatively species poor island fauna should occur in many flowing freshwaters in the Caribbean Islands. The author assumes that small mountain streams throughout the Greater Antilles are detritus based and constitute habitats for a chironomid fauna with high proportion of the genera Xestochironomus and Stenochironomus. Larvae of the vast majority of Chironomids live in freshwater. There are only a few genera that include drought resistant species (Armitage et al., 1995). An adaptation to survive drought is known for neither Xestochironomus or Stenochironomus. This means that the members of both taxa need continued presence of freshwater and a population of trees in the close vicinity. Serious drought or deforestation due to climate change would result in extirpation. While Stenochironomus occurs in standing and flowing waters, Xestochironomus is reliant on lotic habitats. Chironomids, like all animals capable of flight, can be dispersed by wind. However, it is highly unlikely that the same genera that lived in lotic freshwaters in Miocene Hispaniola became extinct due to climate change and reached the Caribbean islands and recolonized the same habitats during the Holocene, what is only a short time span in the islands' history. Consequently the fossils presented here show that a noteworthy portion of the Miocene chironomid fauna of Hispaniola (on genus level) persisted in the Caribbean without interruption for at least 15 Ma. Especially Xestochironomus proves that climate changes were not severe enough to prohibit the existence of sufficient lotic freshwater habitats and tree populations on at least one of the larger Antillean islands. Like today the fauna in the streams of Miocene Hispaniola was at least partly based on allochthonous plant material. Probably the same factors like nutrient poor, canopy-shaded, small mountain streams produced a situation comparable to today's ecosystem. Following this interpretation the fossil chironomid fauna indicates stability of lotic freshwater habitats in the Greater Antilles. A contrasting picture is demonstrated by the ants, which act as indicators for changes in the terrestrial ecosystem. Studying the ants living in the West Indies and the fossil ant fauna in Dominican amber, Wilson (1988) found: “Of 38 genera and well-defined subgenera identified in the fossil deposits, 34 have survived somewhere in the New World tropics to the present, although the species studied thus far are extinct. Of the surviving genera and subgenera, 22 persist on Hispaniola.” And the comparison of the fossil and recent faunas led him to the conclusion that: “The Hispaniolan fauna has undergone considerable turnover since Dominican amber
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times”. The results gained from the ants, like the reconstruction of Poinar and Poinar (1999), show that there have been obvious changes in the terrestrial ecosystem of Hispaniola between amber times and today. Climate change obviously occurred in the CircumCaribbean region (Curtis et al., 2001), but on what scale and with what impact on different ecosystems remains a challenging question. Seemingly different environments were affected in different ways with Chironomids of lotic freshwater habitats in the Greater Antilles showing stability, while the surrounding terrestrial biota changed. Stable flowing freshwater habitats on islands are hardly imaginable within periods of serious drought. But a climate drier than today, with some sort of refuge areas like wetter windward sites on the larger islands is imaginable. Acknowledgements I thank J. Rust and P. S. Cranston for their comments on the manuscript and G. Bechly (SMNS) and D. Grimaldi (AMNH) for the loans. I also thank B. Sinclair and M. Spies.
References Armitage, P.D., Cranston, P.S., Pinder, L.C.V., 1995. The Chironomidae: The Biology and Ecology of Non-biting Midges. Chapman and Hall, London. Borkent, A., 1984. The systematics and phylogeny of the Stenochironomus complex (Xestochironomus, Harrisius, and Stenochironomus) (Diptera: Chironomidae). Memoirs of the Entomological Society of Canada 128. Covich, A.P., McDowell, W.H., 1996. The stream community. In: Reagan, D.P., Waide, R.B. (Eds.), The Food Web of a Tropical Rainforest. University of Chicago Press, pp. 433–459. Cranston, P.S., Mckie, B., 2006. Aquatic wood — an insect perspective. In: Grove, S.J., Hanula, J.L. (Eds.), Insect Biodiversity and Dead Wood: Proceedings of a Symposium for the 22nd International Congress of Entomology. General Technical Report. U.S. Department of Agriculture, Forest Service, Southern Research Station, Asheville, NC. Cranston, P.S., Dillon, M.E., Pinder, L.C.V., Reiss, F., 1989. The adult males of Chironominae (Diptera: Chironomidae) of the Holarctic region — keys and diagnoses. Entomologica Scandinavica Supplement 34, 353–502. Curtis, J.H., Brenner, M., Hodell, D.A., 2001. Climate change in the Circum-Caribbean (Late Pleistocene to present) and implications for regional biogeography. In: Woods, C. (Ed.), Biogeography of the West Indies: Past. Present, and Future. Sandhill Crane Press, Gainesville, Florida, pp. 35–54. Ferrington Jr., L.C., Buzby, K.M., Masteller, E.C., 1993. Composition and temporal abundance of chironomidae emergence from a tropical rainforest stream at El Verde, Puerto Rico. Journal of the Kansas Entomological Society 66 (2), 167–180.
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Grimaldi, D.A., 1996. Amber. Window to the past. American Museum of Natural History. Harry N. Abrams, Incorporated, New York. Grund, M., 2005. Chironomids (Diptera: Chironomidae) of Dominican amber. Ablabesmyia electrohispaniolana, sp.n. and paleoecological indications due to subfamily proportions. Insect Systematics and Evolution 36, 29–34. Iturralde-Vinent, M.A., 2001. Geology of the amber-bearing deposits of the Greater Antilles. Caribbean Journal of Science 37 (3–4), 141–167. Iturralde-Vinent, M.A., MacPhee, R.D.E., 1996. Age and paleogeographical origin of Dominican amber. Science 273, 1850–1852. Michener, C.D., Poinar Jr., G., 1996. The known bee fauna of the Dominican amber. Journal of the Kansas Entomological Society 69 (4), 353–361. Penney, D., 1999. Hypotheses for the recent Hispaniolan spider fauna based on the Dominican Republic amber fauna. Journal of Arachnology 27, 64–70. Poinar Jr., G.O., Poinar, R., 1999. The Amber Forest: A Reconstruction of a Vanished World. Princeton University Press, Princeton, New Jersey. Reagan, D.P., Waide, R.B., 1996. The Food Web of a Tropical Rain Forest. University of Chicago Press, Chicago. Sæther, O.A., 1980. Glossary of chironomid morphology terminology (Diptera: Chironomidae). Entomologica Scandinavica Supplement 14, 1–51.
Seredszus, F. 2003. Wasserinsekten des Baltischen Bernsteins unter besonderer Berücksichtigung der Chironomiden. Grundlagen zum Verständnis von aquatischen Lebensräumen und Lebensgemeinschaften im eozänen Bernsteinwald. - PhD Thesis, Erziehungswissenschaftliche Fakultät at University of Cologne, Germany, available at: http://kups.ub.uni-koeln.de/volltexte/2004/ 1126/. Spies, M., Reiss, F., 1996. Catalog and bibliography of Neotropical and Mexican Chironomidae (Insecta, Diptera). Spixiana Supplement 22, 61–119. Sublette, J.E., Wirth, W.W., 1972. New genera and species of West Indian Chironomidae (Diptera). The Florida Entomologist 55, 1–17. Walker, I.R., 1993. Paleolimnological biomonitoring using freshwater benthic macroinvertebrates. In: Rosenberg, D.M., Resh, V.H. (Eds.), Freshwater Biomonitoring and Benthic Macroinvertebrates. Chapman & Hall, New York, London, pp. 306–343. Wichard, W., Weitschat, W., 1996. Wasserinsekten im Bernstein-Eine paläobiologische Studie. Entomologische Mitteilungen aus dem Löbbecke-Museum und Aquazoo 4, 1–122. Wilson, E.O., 1988. The biogeography of the West Indian ants (Hymenoptera: Formicidae). In: Liebherr, J.K. (Ed.), Zoogeography of Caribbean Insects. Cornell University Press, New York, pp. 214–230.
Chironomidae (Diptera) in Dominican amber as indicators for ecosystem stability in the Caribbean Martin Grund ⁎ Institut für Paläontologie, Universität Bonn, Nussallee 8, 53115 Bonn, Germany Received 23 December 2005; received in revised form 31 March 2006; accepted 24 April 2006
Abstract A first overview on fossil chironomids in Dominican amber is given. This fossil assemblage seems to represent an insular fauna, very similar to its living relatives. Stenochironomus sp. and especially the true xylophagous, neotropical/southern North American Xestochironomus spp. prove the persistence of submerged dead wood in nutrient poor mountain streams in the Greater Antilles from the Miocene until today. Their abundance indicates that the special ecological conditions in extant Caribbean tropical mountain streams already ruled the ancient ecosystem. The results arising from the fossils of these freshwater organisms do not coincide with the faunal changes shown by other groups of insects. General systematic descriptions of new fossil representatives of Xestochironomus and Stenochironomus are given. © 2006 Elsevier B.V. All rights reserved. Keywords: Diptera; Chironomidae; Dominican amber; Paleoecology; Climate change
1. Introduction Inclusions of fossil plants and animals in amber enable us to reconstruct the lifestyle and habitat of former organisms and give insight in ancient ecosystems. Of course, different organisms are of different utility in this attempt. It is therefore reasonable to study fossil taxa with high explanatory power for ecological analyses. Aquatic insects are particularly suitable in this regard (Wichard and Weitschat, 1996), especially Chironomids (non-biting midges), which are frequently used as bioindicators in classifying and monitoring different freshwater environments (Armitage et al., 1995). Chironomid larvae occur in virtually all aquatic habitats, ⁎ Tel.: +49 228734843; fax: +49 228733509. E-mail address: [email protected]. 0031-0182/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.palaeo.2006.04.005
except the open sea. Remains from chironomid larvae in aquatic sediments are already used for paleoecological reconstructions, mainly of Quarternary limnic environments (Walker, 1993; Armitage et al., 1995). Based on biostratigraphic and paleogeographic data Iturralde-Vinent and MacPhee (1996) dated the exploitable fossil bearing Dominican amber deposits as of late Early Miocene through early Middle Miocene age (15– 20 Ma). The resin has formed on paleo-Hispaniola and was transported to a sedimentary basin by slope-wash and through river channels dissecting the landscape. The material found today in the La Toca formation of the Northern mining district originated in the forested mountain ranges of the ancestral Cordillera Central. The amber of the Yanigua formation in the eastern mining district is from elsewhere around the basin (IturraldeVinent and MacPhee, 1996; Iturralde-Vinent, 2001).
M. Grund / Palaeogeography, Palaeoclimatology, Palaeoecology 241 (2006) 410–416
Although winged adult Chironomids are very common in fossil resins, the group is rather poorly studied. This is very probably due to taxonomic difficulties, because the systematic determination of chironomids can be a challenging task. Often only the males are described in taxonomic literature and identification needs high power magnification of many structures that are easily obscured in amber-preserved specimens. Despite a single species, Ablabesmyia electrohispaniolana (Grund, 2005), Dominican amber Chironomidae are completely unstudied. The present paper proves that in many cases the fossils can be identified to extant genera and valuable information about the ancient ecosystem, diversity, ecology and biogeography can be ascertained. The only modern synopsis of an amber chironomid fauna is that of Seredszus (2003) on Baltic amber Chironomids, which are of Eocene age. In a catalogue on the chironomids of the Neotropical Region, Spies and Reiss (1996) listed 29 genera with 41 species of chironomids recorded from the Greater Antilles. However, our knowledge on the chironomid fauna of the region is far from complete. Because of the intensive ecological studies carried out on Puerto Rican ecosystems (Reagan and Waide, 1996), this island is predestined to serve as a representative model for the Greater Antilles, which are large and mountainous islands, clearly isolated from the mainland. Ferrington et al. (1993) studied faunal composition, temporal abundance and ecological aspects of the chironomids of a small mountain stream in Puerto Rico. They found abundant adults of the genera Xestochironomus and Stenochironomus whose larvae are xylophagous. Xestochironomus and Stenochironomus belong to the tribe Chironomi in the subfamily Chironominae. The larvae of Xestochironomus are highly specialized wood miners in lotic freshwaters in the Neotropics and in the southern USA. Stenochironomus is a cosmopolitan genus, with representatives living below tree line and mining in wood or leaves in different freshwater habitats. The woodmining larvae are found in dead submerged angiosperm wood. The wood as a substrate lies immobile in the water and is of firm constitution and free from surface contamination (Borkent, 1984). The distribution and abundance of today's flora and fauna in the Circum-Caribbean region is influenced by climatic history: for example, by cool and dry periods from the late Pleistocene to present. In the climate scenario of Curtis et al. (2001), several lake beds in the region were devoid of water during dry periods in the late Pleistocene, temperature was 6.5 to 8.0 °C lower than today and eventually drought was responsible even for the collapse of the Maya in the 9th century A.D.
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Probably climate changes varied through the region and their impacts on the environment were different in different types of ecosystems or habitats. Poinar and Poinar (1999) outlined a reconstruction of the Miocene Dominican amber forest, based upon a huge database of fossil plants and animals in Dominican amber, the world's best source for terrestrial invertebrate fossils derived from a tropical ecosystem. Their analysis led to the conclusion that the ecosystem on Hispaniola changed since amber times, because many taxa are now extinct from the island and there is no remaining forest on Earth today with such a community of plants and animals. Mainly taxa adapted to a narrow range of environmental conditions went extinct during Pliocene– Pleistocene climatic changes and were unable to recolonize the island. The stingless bees for example, present in Dominican amber, are extinct on the island today (Michener and Poinar, 1996). The ecologically dominant ants have undergone significant changes via extinction and invasion, with higher extinction rates in the ecologically more specialized taxa (Wilson, 1988). However, the degree of faunal turnover since amber times is still not known. The reconstruction of the Dominican amber-producing forest by Grimaldi (1996) differed little from recent Neotropical rainforests. Penney (1999) compared fossil and recent spider families of the region and concluded that the taxa found in amber but not today on Hispaniola still exist on the island but are yet to be found. 2. Materials and methods The chironomid fossils in the Dominican amber collections of SMNS (Staatliches Museum für Naturkunde Stuttgart, Germany) and AMNH (American Museum of Natural History, New York, USA) were examined. The pieces were cut and ground for observation with classical microscope techniques (stereomicroscopy and light microscopy). It is often not possible to dissect and finish amber stones with many inclusions without damaging fossils. Thus assemblages of more than five chironomids in one amber piece were left untreated and therefore not included in this study. For taxonomic reasons female specimens were also not considered. A total of 163 specimens, of variable conservation condition, were available for identification. The morphological terminology follow Sæther (1980) and Borkent (1984), for diagnostic adult characters of the genera see Cranston et al. (1989). The two fossil specimens exemplarily described here are Do-4250-B for Xestochironomus and Do-4215-M for Stenochironomus, both housed in the SMNS.
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2.1. Taxonomic limitation The fossils described in the present study can be assigned to extant genera with certainty. They are not described and named as new fossil species because in the authors opinion it is not possible to safely distinguish fossil species of the same genus, since important characters like subtle genital features and many setae are either not visible or lacking. 3. Results Forty individual specimens could be identified to genus level. This is 25% of the stock. Seven different genera, all extant, were identified (Table 1).
3.2.3. Wing Membrane without setae. Costa not extended. R2 + 3 missing. FCu distal to RM. R, R1 and R4 + 5 with setae. Halteres brown. Wing length 1.8 mm (Fig. 1C). 3.2.4. Legs Fore tibia with distinct apical inner scale, without apical spine. Mid and hind tibiae with two fused combs and each with a distinct, slightly sinuous spur. Pulvilli present. Femur of foreleg with one distal brown band and one at mid-length. Fore tibia proximal and distal brown. Femora of mid and hindlegs proximal and distal dark, tibiae proximal dark. Tarsi of mid and hindlegs pale, tarsi of forelegs missing. 3.2.5. Abdomen Tergites 2–4 with posterior brown band.
3.1. Stenochironomus sp. One fossil of Stenochironomus has been found in the SMNS collection. 3.2. Description of the fossil Do-4215-M Habitus: The wings are spread and the abdomen is contorted in the anterior–posterior-axis and around the right hind leg. It shows brown arrays and bands on legs, abdomen and thorax (Fig. 1A). 3.2.1. Head Eyes with strong parallel-sided dorsomedial extension. Palpus 5-segmented. AR > 1. 3.2.2. Thorax Antepronotal lobes strongly reduced, scutum distinctly overreaching antepronotum (Fig. 1B). Acrostichals starting at anterior margin of scutum. Lateral vittae and posterior part of median vittae brown. Postnotum brown. Table 1 Chironomid genera found in Dominican amber and number of fossil specimens Genus
Number of specimens
Ablabesmyia Monopelopia Tanypus Dicrotendipes Polypedilum Stenochironomus Xestochironomus Total
14 1 1 2 6 1 15 40
3.2.6. Hypopygium Median anal tergital setae in circular patch. Anal point prominent, t-shaped in cross-section at base, flattened apically and curved ventrally. Tufts of setae lateral and ventral of anal point. Inferior volsella long, narrow and bowed dorsoventrally, with apical setae. Gonostylus rather long, narrow and curved medially (Fig. 1D). 3.2.7. Diagnosis As in Xestochironomus, the antepronotal lobes in Stenochironomus are strongly reduced and the scutum distinctly overreaches the antepronotum. In contrast to Xestochironomus, the tibial combs in Stenochironomus are fused. Additionally the inferior volsella of Stenochironomus is narrower, mostly longer and bowed dorsoventrally. Due to these character states the fossil can be identified confidently as Stenochironomus. 3.3. Xestochironomus spp. Thirteen fossil inclusions of Xestochironomus have been found in the SMNS collection and two in the AMNH collection. The different shapes of the gonostylus (forked or unforked, and if forked, the form of inner appendage) and variations in abdominal colour pattern suggest that there are at least four different species among these fossils. 3.4. Description of the fossil Do-4250-B 3.4.1. Habitus The distal third of the left wing is bent down slightly (Fig. 1G). The fossil shows no colour pattern (Fig. 1E).
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Fig. 1. Chironomids in Dominican amber. Stenochironomus sp.: A: Habitus, B: Thorax, C: Wing, D: Hypopygium; Xestochironomus sp.: E: Habitus, F: Thorax, G: Wing, H: Hypopygium.
3.4.2. Head Antenna with thirteen flagellomeres. AR < 1. Eyes with strong parallel-sided dorsomedial extension. Palpus 5-segmented.
3.4.3. Thorax Antepronotal lobes strongly reduced, scutum distinctly overreaching antepronotum. Scutum uniformly light brown. Acrostichals starting at anterior
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margin of scutum. Acrostichals and dorsocentrals uniserial (Fig. 1F). 3.4.4. Wing Membrane without setae. Costa not extended. R2 + 3 weak, curving and ending near apex of R1. R4 + 5 ending at wing apex. FCu distal to RM. Wing length 1.4 mm (Fig. 1G; distal third of the wing is bent down slightly). 3.4.5. Legs Uniformly brown. Mid and hind tibiae with two closely approximated combs, each bearing distinct, slightly sinuous spur. 3.4.6. Abdomen Uniformly brown. Setation evenly distributed. 3.4.7. Hypopygium Median anal tergital setae in circular patch. Anal point towering, hook-shaped in lateral view. Superior volsella slenderly digitiform. Inferior volsella not distinctly swollen, bearing apical/subapical setae and one apical, posteriorly directed, elongate seta. Gonostylus forked with inner appendage short with rather blunt tip, outer appendage longer with pointed tip bearing one long seta (Fig. 1H). 3.4.8. Diagnosis As in Stenochironomus the antepronotal lobes in Xestochironomus are strongly reduced and the scutum distinctly overreaches the antepronotum. In contrast to Stenochironomus, the tibial combs in Xestochironomus are separated. Additionally the inferior volsella of Xestochironomus is shorter, thicker and not noticeably curved. The gonostylus of this fossil is forked, as characteristic for several Xestochironomus species known from the Caribbean region (Sublette and Wirth, 1972) and elsewhere in the Neotropics. Due to these character states the fossil can be identified confidently as Xestochironomus. 4. Discussion The survey of Miocene Dominican amber chironomids presented here yields 40 specimens unevenly distributed among seven extant genera. In comparison, Eocene Baltic amber Chironomids were assigned to 18 extant genera with no genus represented by more than six individuals (Seredszus, 2003). It therefore appears as if the Dominican amber fauna is less diverse, containing fewer genera and possibly represents a depleted island fauna.
All of the seven genera found in the amber are recorded today from at least one island in the Caribbean Sea. The distribution of Xestochironomus is limited to Central and South America and the southern USA, the other genera are widely distributed, most of them on all continents, excepting Antarctica. It is possible that there are chironomid genera in Dominican amber not yet detected, which have become extinct in the Caribbean, Neotropics or worldwide. Although not all genera reported today from the Greater Antilles have been found in studied amber, all genera identified from amber remain extant in the Caribbean. The fossil fauna is dominated by Xestochironomus and Ablabesmyia, with 3/4 of those identified to genus belonging to these taxa. The high number of Ablabesmyia is probably an artefact of perception, because of their conspicuous colour pattern (Grund, 2005). The currently known Ablabesmyia fossils all look quite similar and it is not known how many species they comprise. The high number of Xestochironomus cannot be explained by especially conspicuous individuals. We must assume that this genus is common in the fossil record because it was common in the ecosystem. It is supposed that the fifteen fossils found, comprise four different species. From phylogenetic studies Cranston and Mckie (2006) concluded that the habit of wood-mining in Chironomidae had probably already been adopted sometime in the Mesozoic. This and the well-established principle of behavioural fixity lead to the conclusion that both taxa showed the same ecological specialization in the Miocene. In the food webs of the streams in Luquillo Experimental Forest on Puerto Rico, detritus is an important energy source and aquatic insects are important detritivores (Covich and McDowell, 1996). Ferrington et al. (1993) found that Xestochironomus and Stenochironomus formed 45% (specimens) of chironomid emergence from a Puerto Rican mountain stream in an emergence trap over a one-year period. Sublette and Wirth (1972) reported that Xestochironomus is one of the most characteristic genera on the islands Dominica and Jamaica, caught near small rocky streams in the mountains, and that Stenochironomus leptopus Kieffer is one of the most abundant Chironomini on Dominica at lowland sites and in the mountains. With seven species recorded, Xestochironomus is one of the most species rich chironomid genera of the Caribbean islands. The biotic and abiotic factors in small Neotropical streams are often similar (Covich and McDowell, 1996). As found in Puerto Rico, low nutrient concentrations,
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low insolation (canopy-shaded) and a relatively species poor island fauna should occur in many flowing freshwaters in the Caribbean Islands. The author assumes that small mountain streams throughout the Greater Antilles are detritus based and constitute habitats for a chironomid fauna with high proportion of the genera Xestochironomus and Stenochironomus. Larvae of the vast majority of Chironomids live in freshwater. There are only a few genera that include drought resistant species (Armitage et al., 1995). An adaptation to survive drought is known for neither Xestochironomus or Stenochironomus. This means that the members of both taxa need continued presence of freshwater and a population of trees in the close vicinity. Serious drought or deforestation due to climate change would result in extirpation. While Stenochironomus occurs in standing and flowing waters, Xestochironomus is reliant on lotic habitats. Chironomids, like all animals capable of flight, can be dispersed by wind. However, it is highly unlikely that the same genera that lived in lotic freshwaters in Miocene Hispaniola became extinct due to climate change and reached the Caribbean islands and recolonized the same habitats during the Holocene, what is only a short time span in the islands' history. Consequently the fossils presented here show that a noteworthy portion of the Miocene chironomid fauna of Hispaniola (on genus level) persisted in the Caribbean without interruption for at least 15 Ma. Especially Xestochironomus proves that climate changes were not severe enough to prohibit the existence of sufficient lotic freshwater habitats and tree populations on at least one of the larger Antillean islands. Like today the fauna in the streams of Miocene Hispaniola was at least partly based on allochthonous plant material. Probably the same factors like nutrient poor, canopy-shaded, small mountain streams produced a situation comparable to today's ecosystem. Following this interpretation the fossil chironomid fauna indicates stability of lotic freshwater habitats in the Greater Antilles. A contrasting picture is demonstrated by the ants, which act as indicators for changes in the terrestrial ecosystem. Studying the ants living in the West Indies and the fossil ant fauna in Dominican amber, Wilson (1988) found: “Of 38 genera and well-defined subgenera identified in the fossil deposits, 34 have survived somewhere in the New World tropics to the present, although the species studied thus far are extinct. Of the surviving genera and subgenera, 22 persist on Hispaniola.” And the comparison of the fossil and recent faunas led him to the conclusion that: “The Hispaniolan fauna has undergone considerable turnover since Dominican amber
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times”. The results gained from the ants, like the reconstruction of Poinar and Poinar (1999), show that there have been obvious changes in the terrestrial ecosystem of Hispaniola between amber times and today. Climate change obviously occurred in the CircumCaribbean region (Curtis et al., 2001), but on what scale and with what impact on different ecosystems remains a challenging question. Seemingly different environments were affected in different ways with Chironomids of lotic freshwater habitats in the Greater Antilles showing stability, while the surrounding terrestrial biota changed. Stable flowing freshwater habitats on islands are hardly imaginable within periods of serious drought. But a climate drier than today, with some sort of refuge areas like wetter windward sites on the larger islands is imaginable. Acknowledgements I thank J. Rust and P. S. Cranston for their comments on the manuscript and G. Bechly (SMNS) and D. Grimaldi (AMNH) for the loans. I also thank B. Sinclair and M. Spies.
References Armitage, P.D., Cranston, P.S., Pinder, L.C.V., 1995. The Chironomidae: The Biology and Ecology of Non-biting Midges. Chapman and Hall, London. Borkent, A., 1984. The systematics and phylogeny of the Stenochironomus complex (Xestochironomus, Harrisius, and Stenochironomus) (Diptera: Chironomidae). Memoirs of the Entomological Society of Canada 128. Covich, A.P., McDowell, W.H., 1996. The stream community. In: Reagan, D.P., Waide, R.B. (Eds.), The Food Web of a Tropical Rainforest. University of Chicago Press, pp. 433–459. Cranston, P.S., Mckie, B., 2006. Aquatic wood — an insect perspective. In: Grove, S.J., Hanula, J.L. (Eds.), Insect Biodiversity and Dead Wood: Proceedings of a Symposium for the 22nd International Congress of Entomology. General Technical Report. U.S. Department of Agriculture, Forest Service, Southern Research Station, Asheville, NC. Cranston, P.S., Dillon, M.E., Pinder, L.C.V., Reiss, F., 1989. The adult males of Chironominae (Diptera: Chironomidae) of the Holarctic region — keys and diagnoses. Entomologica Scandinavica Supplement 34, 353–502. Curtis, J.H., Brenner, M., Hodell, D.A., 2001. Climate change in the Circum-Caribbean (Late Pleistocene to present) and implications for regional biogeography. In: Woods, C. (Ed.), Biogeography of the West Indies: Past. Present, and Future. Sandhill Crane Press, Gainesville, Florida, pp. 35–54. Ferrington Jr., L.C., Buzby, K.M., Masteller, E.C., 1993. Composition and temporal abundance of chironomidae emergence from a tropical rainforest stream at El Verde, Puerto Rico. Journal of the Kansas Entomological Society 66 (2), 167–180.
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Grimaldi, D.A., 1996. Amber. Window to the past. American Museum of Natural History. Harry N. Abrams, Incorporated, New York. Grund, M., 2005. Chironomids (Diptera: Chironomidae) of Dominican amber. Ablabesmyia electrohispaniolana, sp.n. and paleoecological indications due to subfamily proportions. Insect Systematics and Evolution 36, 29–34. Iturralde-Vinent, M.A., 2001. Geology of the amber-bearing deposits of the Greater Antilles. Caribbean Journal of Science 37 (3–4), 141–167. Iturralde-Vinent, M.A., MacPhee, R.D.E., 1996. Age and paleogeographical origin of Dominican amber. Science 273, 1850–1852. Michener, C.D., Poinar Jr., G., 1996. The known bee fauna of the Dominican amber. Journal of the Kansas Entomological Society 69 (4), 353–361. Penney, D., 1999. Hypotheses for the recent Hispaniolan spider fauna based on the Dominican Republic amber fauna. Journal of Arachnology 27, 64–70. Poinar Jr., G.O., Poinar, R., 1999. The Amber Forest: A Reconstruction of a Vanished World. Princeton University Press, Princeton, New Jersey. Reagan, D.P., Waide, R.B., 1996. The Food Web of a Tropical Rain Forest. University of Chicago Press, Chicago. Sæther, O.A., 1980. Glossary of chironomid morphology terminology (Diptera: Chironomidae). Entomologica Scandinavica Supplement 14, 1–51.
Seredszus, F. 2003. Wasserinsekten des Baltischen Bernsteins unter besonderer Berücksichtigung der Chironomiden. Grundlagen zum Verständnis von aquatischen Lebensräumen und Lebensgemeinschaften im eozänen Bernsteinwald. - PhD Thesis, Erziehungswissenschaftliche Fakultät at University of Cologne, Germany, available at: http://kups.ub.uni-koeln.de/volltexte/2004/ 1126/. Spies, M., Reiss, F., 1996. Catalog and bibliography of Neotropical and Mexican Chironomidae (Insecta, Diptera). Spixiana Supplement 22, 61–119. Sublette, J.E., Wirth, W.W., 1972. New genera and species of West Indian Chironomidae (Diptera). The Florida Entomologist 55, 1–17. Walker, I.R., 1993. Paleolimnological biomonitoring using freshwater benthic macroinvertebrates. In: Rosenberg, D.M., Resh, V.H. (Eds.), Freshwater Biomonitoring and Benthic Macroinvertebrates. Chapman & Hall, New York, London, pp. 306–343. Wichard, W., Weitschat, W., 1996. Wasserinsekten im Bernstein-Eine paläobiologische Studie. Entomologische Mitteilungen aus dem Löbbecke-Museum und Aquazoo 4, 1–122. Wilson, E.O., 1988. The biogeography of the West Indian ants (Hymenoptera: Formicidae). In: Liebherr, J.K. (Ed.), Zoogeography of Caribbean Insects. Cornell University Press, New York, pp. 214–230.