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Early Cretaceous fish trails from La Rioja, Spain
Cretaceous Research 30 (2009) 1027–1030
Contents lists available at ScienceDirect
Cretaceous Research journal homepage: www.elsevier.com/locate/CretRes
Early Cretaceous fish trails from La Rioja, Spain Loı¨c Costeur a, *, Rube´n Ezquerra b, c a
Naturhistorisches Museum Basel, Augustinergasse 2, CH 4001 Basel, Switzerland ˜o, La Rioja, Spain Instituto de Estudios Riojanos, Portillo 2, 26001 Logron c ´ n Patrimonio Paleontolo ´gico de La Rioja, 26586 Enciso, La Rioja, Spain Fundacio b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 17 November 2008 Accepted in revised form 26 March 2009 Available online 5 April 2009
We report a new find of the fish trail Undichna unisulca in fluvial sediments of the Lower Cretaceous Oliva´n Group of the Cameros Basin (Aptian-Albian; Valtrujal, La Rioja, northern Spain). It is the second Mesozoic record of Undichna unisulca after the first discovery in the Lower Cretaceous of central Spain (Las Hoyas, Cuenca, central Spain). The trails are characterized by single sinusoidal waves which in this case are often surrounded by well-preserved lateral levees that rule out an undertrail hypothesis. The continental depositional setting, a floodplain fluvial system, reflects the known broad palaeoenvironmental distribution of Undichna and confirms its occurrences in freshwater settings. Ó 2009 Elsevier Ltd. All rights reserved.
Keywords: Fish trails Lower Cretaceous Oliva´n Group Cameros Basin La Rioja Spain
1. Introduction Fish trails related to locomotion are rather scarce in the fossil record and most of these belong to genus Undichna. About 60 records of the genus throughout the world have been identified from the Devonian to the Pleistocene (Minter and Braddy, 2006). They represent simple to quite complex structures showing grooves made by the fish fins while swimming close to the sediment-water interface (Minter and Braddy, 2006). After the first description of U. unisulca in 1999 (de Gibert et al., 1999), the species was found in fluvial, lacustrine or marginal marine settings in Europe and North-America again from the Devonian to the Pleistocene. Nine records including two in Spain (Carboniferous and Cretaceous) were then known (Minter and Braddy, 2006), with thus only one in the Mesozoic, in the Lower Cretaceous locality of Las Hoyas in the central Iberian range (de Gibert et al., 1999). In this context a new discovery of this trace fossil is interesting because it brings new information on the presence of fish were no body fossils are preserved as well as on the palaeoenvironmental conditions that prevailed and on the palaeobiology of extinct fishes. The province of La Rioja is well known for its abundant ichnological record, dominated mainly by dinosaur footprints. More than
* Corresponding author. E-mail address: [email protected] (L. Costeur). 0195-6671/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.cretres.2009.03.004
10 000 fossil footprints of walking dinosaurs have been discovered in the area and most are from the Enciso Group of Early Cretaceous age, just predating the Oliva´n Group where the present discovery was made. Under-water footprints are especially rare, because of difficulties of preservation. However, Ezquerra et al. (2007) recently described a well-preserved example of a theropod dinosaur swimming trackway on a sandstone slab formed in a lacustrine deltaic palaeoenvironment. The discovery reported here is thus the second under-water track discovery in La Rioja and the first one concerning fish. It thus enlarges the ichnological spectrum of the region making it one of the richest places in the world for fossil tracks and traces. Besides its poor ichnological record, very few vertebrate remains are known from the Oliva´n Group as no fish body-fossils have been found and just one record of vertebrate remains, a partial skeleton of an ornithischian dinosaur, has been to ˜ aca, 2001). date published (Torres and Viera, 1994; Ruiz-Omen 2. Settings The locality of Valtrujal is situated in northern central Spain, in the province of La Rioja, northwest of the town of Arnedillo (Fig. 1). It occurs within the Cameros Basin, a high-subsidence basin of the Iberian Rift System that underwent several tectonic phases in the late Paleozoic and Mesozoic. During the Late Jurassic and Early Cretaceous, the basin was a large perennial lake in which siliciclastic and carbonate sediments were deposited (Doublet et al., 2003; Doublet and Garcia, 2004). The locality is Early Cretaceous in
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L. Costeur, R. Ezquerra / Cretaceous Research 30 (2009) 1027–1030
Fig. 1. Geographical, geological, and stratigraphical settings of the Valtrujal locality and eastern Cameros Basin (Geology and Geography modified after Doublet and Garcia, 2004). The synthetic stratigraphical scale is based on Guiraud and Seguret, 1986; Diaz, 1988; Martı´n-Closas and Alonso-Milla´n, 1998 and Pe´rez-Lorente, 2002. Numerical ages are taken from Gradstein and Ogg, 2004. Zigzag lines indicate lateral variations within the eastern Cameros Basin. The gap for the Hauterivian indicates a lag in the sedimentary record. Trace symbols apply to the whole formation or group in front of which they are positioned.
age and is found in the Oliva´n Group of Aptian-Albian age (Guiraud and Seguret, 1986; Martı´n-Closas and Alonso-Milla´n, 1998; Arribas et al., 2007, Fig. 1). The latter is essentially composed of siliciclastic sediments. Previous studies in the Oliva´n Group have indicated that this formation was the result of a tectonic reactivation of the Cameros Basin that generated deposition in a meandering to floodplain fluvial system (Guiraud and Seguret, 1986). The outcrop is a small slab (Fig. 1) of fine-grained sandstones. A complete sedimentological study of the area around the Valtrujal site will be lead to better characterize the relative position of the site within the group and with regards to the other ichnofossil sites numerous in the Enciso Group.
3. Systematic palaeontology Ichnogenus Undichna Anderson, 1976 Ichnospecies Undichna unisulca de Gibert et al., 1999 Material. The slab where the trails are recorded is preserved on the outcrop. Tracings numbered 1VLT1 and 1VLT2 (see also Fig. 3) are deposited at the Centro Paleontolo´gico de Enciso, La Rioja, Spain. The slab contains more than 30 traces (Fig. 2), but only two record long trails continuous enough to yield identifiable undulation traces left by the animal (Fig. 3a–b, respective close-ups). The others are partial traces that only record one short-term contact of the animal with the sediment.
The trails are presented as epichnial grooves that are sharply and rather deeply incised in the sediment (about 1 cm depth) producing a flat V-shaped valley before the actual final incision. The valleys rims show large lateral levees (Fig. 3a–b) that rule out a possible undertrail hypothesis. The trails consist of an unpaired sinusoidal wave with amplitudes of 26 to 65 mm and wavelengths of 99 to 250 mm. The sinuosity is not fully regular in concordance with the original diagnosis (de Gibert et al., 1999) of Unichna unisulca. The grooves are about 0.5 mm to 1 cm wide. Such grooves are classically ascribed to a contact of a fish fin with the sediment. 4. Discussion Since 1999 and its first description by de Gibert et al., the recognition of U. unisulca is much better constrained. Minter and Braddy (2006) recall that previous identifications of simple sinusoidal waves were first attributed to particular conditions of preservation of U. simplicitas which is usually much more complicated with several sets of paired waves (see also Anderson, 1976; Higgs, 1988; Buatois et al., 1998). Later finds in the Cretaceous and Carboniferous of Spain, respectively (de Gibert et al., 1999; SolerGijo´n and Moratalla, 2001) better constrained the morphology and validity of U. unisulca. In general the ichnospecies should show wavelengths in the range of 100 mm to 300 mm, the trails presented here fall just in this range. In particular cases smaller wavelengths can also be attributed to U. unisulca depending on other characteristics such as the sharpness of the incision or the angle of sinuosity, (Morissey et al., 2004). Besides the two long
L. Costeur, R. Ezquerra / Cretaceous Research 30 (2009) 1027–1030
Fig. 2. Picture and drawing of the slab showing the Undichna unisulca fish trails. a (1VLT1) and b (1VLT2) (thick black lines) are the two long traces discussed here and figured as close-ups in Fig. 3. Thin black lines represent lateral levees. Thick grey lines are other fish traces attributable to U. cf. unisulca.
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trails we attribute to U. unisulca, the other much less long imprints found on the same slab can be referred to U. cf. unisulca because they always appear as single unpaired grooves (Fig. 2). Undertrails resulting from the impression of trails on successive laminae can also yield uncertainty in attributing a fish trail to an ichnospecies through the loss of diagnostic characterisctics only preserved on the genuine bedding plane. In many cases it is difficult to judge whether a trace-fossil is an undertrail or not. de Gibert et al. (1999) showed that undisturbed laminae above positive hyporeliefs could indicate the genuine nature of a fossil trace. Another possibility that rules out a derived nature of a trace-fossil is the presence of sedimentological features that would otherwise not be preserved because of their mode of formation. A classic example of such features is present on the traces described here. Lateral levees formed by the expulsion of the sediment from the groove dug by the animal at the moment of trace formation are good indicators of the genuine nature of the trace (Fig. 3a–b; Gierlinski et al., 2004; Ezquerra et al., 2007). When in association with traces as negative epireliefs, they can only be preserved on the bedding plane on which the animal touched the sediment and thus testify to bedding plane origin. Trails attributed to U. unisulca are always made under water, probably as the result of the contact of the anal or caudal fin of a fish with the sediment (de Gibert et al., 1999). A variety of fishes can produce U. unisulca, from pycnodonts (de Gibert et al., 1999) to sharks (Soler-Gijo´n and Moratalla, 2001). It seems that fishes with strong anal or caudal fins can produce a single and continuous sinusoidal groove without additional traces, the other fins being too small or having an orientation preventing contact with the sediment. Also, if a fish moves farther from the bottom, only a single fin can produce a trace. This drawback yields uncertainty in identifying a taxon as tracemaker. Nonetheless, pycnodontiform fish remains have been found associated with the previous Spanish discoveries of Undichna (e.g. Stenamara mia or Turbomesodon praeclarus; de Gibert et al., 1999; Poyato-Ariza and Wenz, 2000; Poyato-Ariza and Wenz, 2004) and were present in other Lower Cretaceous localities of the Iberian Peninsula (Anomoeodus and Coelodus from Spain and Portugal (Kriwet et al., 1999; Cavin et al., 2007) and Bermu´dezRochas and Poyato-Ariza, 2007).
Fig. 3. Close-ups of tracks a (1VLT1) and b (1VLT2) (see also Fig. 2). Note the non-regular sinuosity pattern and the presence of well-preserved lateral levees.
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Pycnodontiform fishes occur throughout the world in the Lower Cretaceous with more than 30 localities, of which about 20 are concentrated in Europe. The majority of the fossil records are found in marine to marginal settings, but 25 percent of the European occurrences are recorded in freshwater settings (Cavin et al., 2007). Such oval deep-bodied fishes are good candidates for trailmakers as they possess a large anal fin that is likely to leave a single groove on the sediment surface. In that case, a subcarangiform to carangiform mode of locomotion can be inferred where only parts of the body are used in the sinusoidal movements involved in swimming (see de Gibert, 2001); here mostly the rear part of the body is used and fish that today employ such locomotion are rather stiff and show less skill at manoeuvrability than speed. Based on estimates made from living fish trails (Wardle et al., 1995), a medium body length of about 25 to 40 cm is proposed for the type of swimming locomotion inferred. It is obviously always difficult to ascribe a fossil trace to a specific species, thus the identification of the possible tracemaker for the trails described here is only a working hypothesis that cannot be firmly tested in the absence of body fossils. Previous records of Undichna unisulca occur exclusively in freshwater to slightly brackish palaeoenvironmental settings (from lacustrine to fluvial, estuarine or at most marginal marine settings, de Gibert et al., 1999; Minter and Braddy, 2006). This new discovery confirms previous findings as the fine-grained sandstone on which the trails are preserved were deposited in a floodplain fluvial setting (Guiraud and Seguret, 1986).
5. Conclusions Fish trails are rare because they are trace fossils made under water by an animal that usually has little contact with the sediment substrate. They are thus an interesting window into fish palaeobiology (swimming mode) but also provide information about faunas that are not preserved as body fossils. The Oliva´n Group in the Lower Cretaceous of La Rioja, Spain never yielded any fish body fossil although it was formed in a fluvial environment. Thus the traces reported here are the first records of the presence of fish in the Aptian-Albian of La Rioja. The trails are ascribed to the single-wave ichnospecies Undichna unisulca and represent the second global record of the ichnospecies in the Mesozoic, testifying to its scarcity. Potential tracemakers can be found in pycnodontiform fishes which have prominent anal fins able to mark the sediment while swimming close to the bottom. Once again, U. unisulca is found in a freshwater setting (a floodplain fluvial environment) where pycnodonts are also known to have lived during the Early Cretaceous.
Acknowledgements We thank the Instituto de Estudios Riojanos for financial support. We are grateful to Fe´lix Pe´rez-Lorente and to the Fundacio´n Patrimonio Paleontolo´gico de La Rioja for supporting this project. We are also very grateful to J. De Gibert, N. Minter, S.J. Braddy, and M. Wisshak for giving their opinions on the trails and for providing important references. We also thank L. Cavin and J.I. ˜ aca for providing references. Finally, we thank Martin Ruiz-Omen Lockley and an anonymous reviewer for their helpful comments.
References Anderson, A.M., 1976. Fish trails from the Early Permian of South Africa. Palaeontology 19 (2), 397–409. Arribas, J., Ochoa, M., Mas, R., Arribas, M.E., Gonza´lez-Acebro´n, L., 2007. Sandstone petrofacies in the northwestern sector of the Iberian Basin. Journal of Iberian Geology 33 (2), 191–206. Bermu´dez-Rochas, D., Poyato-Ariza, F., 2007. New fossiliferous sites with fish faunas from the Basque-cantabrian and Cameros basins. Journal of Vertebrate Paleoontology 27 (Suppl. 3), 47A–48A. Buatois, L.A., Mangano, M.G., Maples, C.G., Lanier, W.P., 1998. Ichnology of an Upper Carboniferous fluvio-estuarine paleovalley: the Tonganoxie sandstone, Buildex quarry, Eastern Kansas, USA. Journal of Paleontology 72 (1), 152–180. Cavin, L., Forey, P.L., Le´cuyer, C., 2007. Correlation between environment and Late Mesozoic ray-finned fish evolution. Palaeogeography, Palaeoclimatology, Palaeoecology 245, 353–367. Diaz, E. 1988. El creta´cico inferior del sector de Jubera (Norte de la Sierra de los Cameros. La Rioja) - Relacio´n entre tecto´nica y sedimentacio´n. Proceedings of the II Congreso Geologico Espana. Doublet, S., Garcia, J.P., Guiraud, M., Me´nard, A., 2003. Wave-dominated siliciclastic and carbonate sedimentation in a Lower Cretaceous lake (Cameros basin, northern Spain). Journal of Iberian Geology 29, 11–30. Doublet, S., Garcia, J.P., 2004. The significance of dropstones in a tropical lacustrine setting, eastern Cameros Basin (Late Jurassic–Early Cretaceous, Spain). Sedimentary Geology 163, 293–309. Ezquerra, R., Doublet, S., Costeur, L., Galton, P.M., Pe´rez-Lorente, F., 2007. Were nonavian theropod dinosaurs able to swim? supportive evidence from an Early Cretaceous trackway, Cameros Basin (La Rioja, Spain). Geology 35 (6), 507–510. de Gibert, J.M., Buatois, L.A., Fregenal-Martinez, Mangano, M.G., Ortega, F., PoyatoAriza, F., Wenz, S., 1999. The fish trace fossil Undichna from the Cretaceous of Spain. Palaeontology 42 (3), 409–427. de Gibert, J.M., 2001. Undichna gosiutensis, isp. nov.: a new fish trace fossils from the Jurassic of Utah. Ichnos 8 (1), 15–22. Gierlinski, G., Pienkowski, G., Niedzwiedzki, G., 2004. Tetrapod track assemblages in the Hettangian of Soltykow, Poland, and its paleoenvironmental background. Ichnos 11, 195–213. Gradstein, F.M., Ogg, J., 2004. Geologic Time Scale 2004 – Why, how, and where next!. Lethaia 37 (2), 175–181. Guiraud, M., Seguret, M., 1986. A releasing solitary overstep model for the late Jurassic - early Cretaceous (Wealdian) Soria strike-slip basin (Northern Spain). In: Biddle, K.T., Christie-Blick, N. (Eds.), Strike Slip Deformation, Basin Formation and Sedimentation. SEPM special publication, 37, pp. 159–175. Higgs, R., 1988. Fish trails in the Upper Carboniferous of south-west England. Palaeontology 31, 255–272. Kriwet, J., Poyato-Ariza, F.J., Wenz, S., 1999. A revision of the pycnodontid fish Coelodus subdiscus Wenz 1989, from the Early Cretaceous of Montsec (Lleida, Spain). Treballs del Museu de Geologia de Barcelona 8, 33–65. Martı´n-Closas, C., Alonso-Milla´n, A., 1998. Estratigrafı´a y bioestratigrafı´a (Charophyta) del Creta´cico inferior en el sector occidental de la Cuenca de Cameros ˜ a 11, 253–269. (Cordillera Ibe´rica). Revista de la Sociedad Geolo´gica de Espan Minter, N.J., Braddy, S.J., 2006. The fish and amphibian swimming traces Undichna and Lunichnium, with examples from the Lower Permian of New Mexico, USA. Palaeontology 49 (5), 1123–1142. Morissey, L.B., Braddy, S.J., Bennett, J.P., Marriott, S.B., Tarrant, P.R., 2004. Fish trails from the Lower Old Red Sandstone of Tredomen Quarry, Powys, southeast Wales. Geological Journal 39, 337–358. Pe´rez-Lorente, F., 2002. La distribucio´n de yacimientos de tipos de huellas de ˜ a). Zubı´a dinosaurio en la Cuenca de Cameros (La Rioja, Burgos, Soria, Espan Monogra´fico 14, 191–210. Poyato-Ariza, F.J., Wenz, S., 2000. A new pycnodontiform fish from the Early Cretaceous of Las Hoyas (Cuenca, Spain). Bulletin de la Socie´te´ Ge´ologique de France 171 (2), 251–258. Poyato-Ariza, F.J., Wenz, S., 2004. The new pycnodontid fish genus Turbomesodon, and a revision of Macromesodon based on new material from the Lower Cretaceous of Las Hoyas, Cuenca, Spain. In: Arratia, G., Tintori, A. (Eds.), Mesozoic Fishes 3 – Systematics, Paleoenvironments and Biodiversity. Pfeil, pp. 341–378. ˜ aca, J.I., 2001. Dinosaurios hipsilofodo´ntidos (Ornithischia: Ornithopoda) Ruiz-Omen en la Penı´nsula Ibe´rica. In: Colectivo Arqueolo´gico-Paleontolo´gico de Salas (eds.), Actas de las I Jornadas internacionales sobre paleontologı´a de dinosaurios y su entorno. Salas de los Infantes, pp. 175–266. Soler-Gijo´n, R., Moratalla, J.J., 2001. Fish and tetrapod trace fossils from the Upper Carboniferous of Puertollano, Spain. Palaeogeography, Palaeoclimatology, Palaeoecology 171, 1–28. Torres, J.A., Viera, L.I., 1994. Hypsilophodon foxii (Reptilia, Ornitischia) en el Cre˜ a). Munibe 46, 3–41. ta´cico inferior de Igea (La Rioja, Espan Wardle, C.S., Videler, J.J., Altringham, J.D., 1995. Tuning in to fish swimming waves: body form, swimming mode and muscle function. Journal of Experimental Biology 198, 1629–1636.
Contents lists available at ScienceDirect
Cretaceous Research journal homepage: www.elsevier.com/locate/CretRes
Early Cretaceous fish trails from La Rioja, Spain Loı¨c Costeur a, *, Rube´n Ezquerra b, c a
Naturhistorisches Museum Basel, Augustinergasse 2, CH 4001 Basel, Switzerland ˜o, La Rioja, Spain Instituto de Estudios Riojanos, Portillo 2, 26001 Logron c ´ n Patrimonio Paleontolo ´gico de La Rioja, 26586 Enciso, La Rioja, Spain Fundacio b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 17 November 2008 Accepted in revised form 26 March 2009 Available online 5 April 2009
We report a new find of the fish trail Undichna unisulca in fluvial sediments of the Lower Cretaceous Oliva´n Group of the Cameros Basin (Aptian-Albian; Valtrujal, La Rioja, northern Spain). It is the second Mesozoic record of Undichna unisulca after the first discovery in the Lower Cretaceous of central Spain (Las Hoyas, Cuenca, central Spain). The trails are characterized by single sinusoidal waves which in this case are often surrounded by well-preserved lateral levees that rule out an undertrail hypothesis. The continental depositional setting, a floodplain fluvial system, reflects the known broad palaeoenvironmental distribution of Undichna and confirms its occurrences in freshwater settings. Ó 2009 Elsevier Ltd. All rights reserved.
Keywords: Fish trails Lower Cretaceous Oliva´n Group Cameros Basin La Rioja Spain
1. Introduction Fish trails related to locomotion are rather scarce in the fossil record and most of these belong to genus Undichna. About 60 records of the genus throughout the world have been identified from the Devonian to the Pleistocene (Minter and Braddy, 2006). They represent simple to quite complex structures showing grooves made by the fish fins while swimming close to the sediment-water interface (Minter and Braddy, 2006). After the first description of U. unisulca in 1999 (de Gibert et al., 1999), the species was found in fluvial, lacustrine or marginal marine settings in Europe and North-America again from the Devonian to the Pleistocene. Nine records including two in Spain (Carboniferous and Cretaceous) were then known (Minter and Braddy, 2006), with thus only one in the Mesozoic, in the Lower Cretaceous locality of Las Hoyas in the central Iberian range (de Gibert et al., 1999). In this context a new discovery of this trace fossil is interesting because it brings new information on the presence of fish were no body fossils are preserved as well as on the palaeoenvironmental conditions that prevailed and on the palaeobiology of extinct fishes. The province of La Rioja is well known for its abundant ichnological record, dominated mainly by dinosaur footprints. More than
* Corresponding author. E-mail address: [email protected] (L. Costeur). 0195-6671/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.cretres.2009.03.004
10 000 fossil footprints of walking dinosaurs have been discovered in the area and most are from the Enciso Group of Early Cretaceous age, just predating the Oliva´n Group where the present discovery was made. Under-water footprints are especially rare, because of difficulties of preservation. However, Ezquerra et al. (2007) recently described a well-preserved example of a theropod dinosaur swimming trackway on a sandstone slab formed in a lacustrine deltaic palaeoenvironment. The discovery reported here is thus the second under-water track discovery in La Rioja and the first one concerning fish. It thus enlarges the ichnological spectrum of the region making it one of the richest places in the world for fossil tracks and traces. Besides its poor ichnological record, very few vertebrate remains are known from the Oliva´n Group as no fish body-fossils have been found and just one record of vertebrate remains, a partial skeleton of an ornithischian dinosaur, has been to ˜ aca, 2001). date published (Torres and Viera, 1994; Ruiz-Omen 2. Settings The locality of Valtrujal is situated in northern central Spain, in the province of La Rioja, northwest of the town of Arnedillo (Fig. 1). It occurs within the Cameros Basin, a high-subsidence basin of the Iberian Rift System that underwent several tectonic phases in the late Paleozoic and Mesozoic. During the Late Jurassic and Early Cretaceous, the basin was a large perennial lake in which siliciclastic and carbonate sediments were deposited (Doublet et al., 2003; Doublet and Garcia, 2004). The locality is Early Cretaceous in
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L. Costeur, R. Ezquerra / Cretaceous Research 30 (2009) 1027–1030
Fig. 1. Geographical, geological, and stratigraphical settings of the Valtrujal locality and eastern Cameros Basin (Geology and Geography modified after Doublet and Garcia, 2004). The synthetic stratigraphical scale is based on Guiraud and Seguret, 1986; Diaz, 1988; Martı´n-Closas and Alonso-Milla´n, 1998 and Pe´rez-Lorente, 2002. Numerical ages are taken from Gradstein and Ogg, 2004. Zigzag lines indicate lateral variations within the eastern Cameros Basin. The gap for the Hauterivian indicates a lag in the sedimentary record. Trace symbols apply to the whole formation or group in front of which they are positioned.
age and is found in the Oliva´n Group of Aptian-Albian age (Guiraud and Seguret, 1986; Martı´n-Closas and Alonso-Milla´n, 1998; Arribas et al., 2007, Fig. 1). The latter is essentially composed of siliciclastic sediments. Previous studies in the Oliva´n Group have indicated that this formation was the result of a tectonic reactivation of the Cameros Basin that generated deposition in a meandering to floodplain fluvial system (Guiraud and Seguret, 1986). The outcrop is a small slab (Fig. 1) of fine-grained sandstones. A complete sedimentological study of the area around the Valtrujal site will be lead to better characterize the relative position of the site within the group and with regards to the other ichnofossil sites numerous in the Enciso Group.
3. Systematic palaeontology Ichnogenus Undichna Anderson, 1976 Ichnospecies Undichna unisulca de Gibert et al., 1999 Material. The slab where the trails are recorded is preserved on the outcrop. Tracings numbered 1VLT1 and 1VLT2 (see also Fig. 3) are deposited at the Centro Paleontolo´gico de Enciso, La Rioja, Spain. The slab contains more than 30 traces (Fig. 2), but only two record long trails continuous enough to yield identifiable undulation traces left by the animal (Fig. 3a–b, respective close-ups). The others are partial traces that only record one short-term contact of the animal with the sediment.
The trails are presented as epichnial grooves that are sharply and rather deeply incised in the sediment (about 1 cm depth) producing a flat V-shaped valley before the actual final incision. The valleys rims show large lateral levees (Fig. 3a–b) that rule out a possible undertrail hypothesis. The trails consist of an unpaired sinusoidal wave with amplitudes of 26 to 65 mm and wavelengths of 99 to 250 mm. The sinuosity is not fully regular in concordance with the original diagnosis (de Gibert et al., 1999) of Unichna unisulca. The grooves are about 0.5 mm to 1 cm wide. Such grooves are classically ascribed to a contact of a fish fin with the sediment. 4. Discussion Since 1999 and its first description by de Gibert et al., the recognition of U. unisulca is much better constrained. Minter and Braddy (2006) recall that previous identifications of simple sinusoidal waves were first attributed to particular conditions of preservation of U. simplicitas which is usually much more complicated with several sets of paired waves (see also Anderson, 1976; Higgs, 1988; Buatois et al., 1998). Later finds in the Cretaceous and Carboniferous of Spain, respectively (de Gibert et al., 1999; SolerGijo´n and Moratalla, 2001) better constrained the morphology and validity of U. unisulca. In general the ichnospecies should show wavelengths in the range of 100 mm to 300 mm, the trails presented here fall just in this range. In particular cases smaller wavelengths can also be attributed to U. unisulca depending on other characteristics such as the sharpness of the incision or the angle of sinuosity, (Morissey et al., 2004). Besides the two long
L. Costeur, R. Ezquerra / Cretaceous Research 30 (2009) 1027–1030
Fig. 2. Picture and drawing of the slab showing the Undichna unisulca fish trails. a (1VLT1) and b (1VLT2) (thick black lines) are the two long traces discussed here and figured as close-ups in Fig. 3. Thin black lines represent lateral levees. Thick grey lines are other fish traces attributable to U. cf. unisulca.
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trails we attribute to U. unisulca, the other much less long imprints found on the same slab can be referred to U. cf. unisulca because they always appear as single unpaired grooves (Fig. 2). Undertrails resulting from the impression of trails on successive laminae can also yield uncertainty in attributing a fish trail to an ichnospecies through the loss of diagnostic characterisctics only preserved on the genuine bedding plane. In many cases it is difficult to judge whether a trace-fossil is an undertrail or not. de Gibert et al. (1999) showed that undisturbed laminae above positive hyporeliefs could indicate the genuine nature of a fossil trace. Another possibility that rules out a derived nature of a trace-fossil is the presence of sedimentological features that would otherwise not be preserved because of their mode of formation. A classic example of such features is present on the traces described here. Lateral levees formed by the expulsion of the sediment from the groove dug by the animal at the moment of trace formation are good indicators of the genuine nature of the trace (Fig. 3a–b; Gierlinski et al., 2004; Ezquerra et al., 2007). When in association with traces as negative epireliefs, they can only be preserved on the bedding plane on which the animal touched the sediment and thus testify to bedding plane origin. Trails attributed to U. unisulca are always made under water, probably as the result of the contact of the anal or caudal fin of a fish with the sediment (de Gibert et al., 1999). A variety of fishes can produce U. unisulca, from pycnodonts (de Gibert et al., 1999) to sharks (Soler-Gijo´n and Moratalla, 2001). It seems that fishes with strong anal or caudal fins can produce a single and continuous sinusoidal groove without additional traces, the other fins being too small or having an orientation preventing contact with the sediment. Also, if a fish moves farther from the bottom, only a single fin can produce a trace. This drawback yields uncertainty in identifying a taxon as tracemaker. Nonetheless, pycnodontiform fish remains have been found associated with the previous Spanish discoveries of Undichna (e.g. Stenamara mia or Turbomesodon praeclarus; de Gibert et al., 1999; Poyato-Ariza and Wenz, 2000; Poyato-Ariza and Wenz, 2004) and were present in other Lower Cretaceous localities of the Iberian Peninsula (Anomoeodus and Coelodus from Spain and Portugal (Kriwet et al., 1999; Cavin et al., 2007) and Bermu´dezRochas and Poyato-Ariza, 2007).
Fig. 3. Close-ups of tracks a (1VLT1) and b (1VLT2) (see also Fig. 2). Note the non-regular sinuosity pattern and the presence of well-preserved lateral levees.
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Pycnodontiform fishes occur throughout the world in the Lower Cretaceous with more than 30 localities, of which about 20 are concentrated in Europe. The majority of the fossil records are found in marine to marginal settings, but 25 percent of the European occurrences are recorded in freshwater settings (Cavin et al., 2007). Such oval deep-bodied fishes are good candidates for trailmakers as they possess a large anal fin that is likely to leave a single groove on the sediment surface. In that case, a subcarangiform to carangiform mode of locomotion can be inferred where only parts of the body are used in the sinusoidal movements involved in swimming (see de Gibert, 2001); here mostly the rear part of the body is used and fish that today employ such locomotion are rather stiff and show less skill at manoeuvrability than speed. Based on estimates made from living fish trails (Wardle et al., 1995), a medium body length of about 25 to 40 cm is proposed for the type of swimming locomotion inferred. It is obviously always difficult to ascribe a fossil trace to a specific species, thus the identification of the possible tracemaker for the trails described here is only a working hypothesis that cannot be firmly tested in the absence of body fossils. Previous records of Undichna unisulca occur exclusively in freshwater to slightly brackish palaeoenvironmental settings (from lacustrine to fluvial, estuarine or at most marginal marine settings, de Gibert et al., 1999; Minter and Braddy, 2006). This new discovery confirms previous findings as the fine-grained sandstone on which the trails are preserved were deposited in a floodplain fluvial setting (Guiraud and Seguret, 1986).
5. Conclusions Fish trails are rare because they are trace fossils made under water by an animal that usually has little contact with the sediment substrate. They are thus an interesting window into fish palaeobiology (swimming mode) but also provide information about faunas that are not preserved as body fossils. The Oliva´n Group in the Lower Cretaceous of La Rioja, Spain never yielded any fish body fossil although it was formed in a fluvial environment. Thus the traces reported here are the first records of the presence of fish in the Aptian-Albian of La Rioja. The trails are ascribed to the single-wave ichnospecies Undichna unisulca and represent the second global record of the ichnospecies in the Mesozoic, testifying to its scarcity. Potential tracemakers can be found in pycnodontiform fishes which have prominent anal fins able to mark the sediment while swimming close to the bottom. Once again, U. unisulca is found in a freshwater setting (a floodplain fluvial environment) where pycnodonts are also known to have lived during the Early Cretaceous.
Acknowledgements We thank the Instituto de Estudios Riojanos for financial support. We are grateful to Fe´lix Pe´rez-Lorente and to the Fundacio´n Patrimonio Paleontolo´gico de La Rioja for supporting this project. We are also very grateful to J. De Gibert, N. Minter, S.J. Braddy, and M. Wisshak for giving their opinions on the trails and for providing important references. We also thank L. Cavin and J.I. ˜ aca for providing references. Finally, we thank Martin Ruiz-Omen Lockley and an anonymous reviewer for their helpful comments.
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