Zoophycos-Like Structures Associated with Dinosaur Tracks in a Tidal-Flat Environment: Lower Jurassic (Southern Alps, Italy)
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Studi Trent. Sci. Nat., Acta Geol., 83 (2008): 123-131 ISSN 0392-0534 © Museo Tridentino di Scienze Naturali, Trento 2008 Zoophycos-like structures associated with dinosaur tracks in a tidal-flat environment: Lower Jurassic (Southern Alps, Italy) Andrea BAUCON1 & Marco AVANZINI2* 1DISGAM – Dipartimento di Scienze Geologiche Ambientali e Marine, Università di Trieste, Via E. Weiss, Trieste, Italy 2 Museo Tridentino di Scienze Naturali, Via Calepina, 14, 38100 Trento, Italy *Corresponding author e-mail: [email protected] SUMMARY - Zoophycos-like structures associated with dinosaur tracks in a tidal-flat environment: Lower Jurassic (Southern Alps, Italy) - This study reports, for the first time, the occurrence ofZoophycos -like structures in the Lower Jurassic of the Italian Alps (Calcari Grigi Group) through the description of the specimens found in the dinosaur tracksite of Lavini di Marco (Trentino, Southern Alps). The sedimentological features of the layers bearing the Zoophycos-like forms can be related to a coastal tidal-flat environment; data in our possession highlight that dinosaurs and the tracemaker of the Zoophycos-like structures frequented adjacent environments and possibly the same ones. The occurrence of Zoophycos-like structures in peritidal environments possibly represents an important contribution to the evolutionary history of Zoophycos in the Eastern Southern Alps. RIASSUNTO - Strutture Zoophycos-like associate a orme di dinosauri in ambiente di piana tidale: Giurassico inferiore (Alpi Meridionali, Italia) - Questo studio documenta per la prima volta la presenza di strutture simili a Zoophycos nel Giurassico inferiore delle Alpi italiane (Gruppo dei Calcari Grigi) attraverso la descrizione di esemplari rinvenuti nel sito ad orme di dinosauro dei Lavini di Marco (Trentino, Alpi meridionali). Le caratteristiche sedimentologiche del livello che conserva gli icnofossili Zoophycos-like corrispondono a quelle di un ambiente di piana tidale; i dati in nostro possesso mettono in evidenza che i dinosauri e i tracemaker di Zoophycos-like popolavano ambienti adiacenti se non il medesimo. La presenza di strutture Zoophycos-like in ambiente di tipo peritidale rappresenta un importante contributo alla storia evolutiva di Zoophycos nel Sudalpino orientale. Key words: Zoophycos, Lavini di Marco, dinosaurs, ichnology, Eastern Southern Alps Parole chiave: Zoophycos, Lavini di Marco, dinosauri, icnologia, Sudalpino orientale 1. Introduction 2002) and Monaco & Garassino (2001) paid attention exclu- sively to the upper part of the Group (Rotzo Formation). 1.1. Geological and geographical setting 1.2. Sedimentological setting The dinosaur tracksite at Lavini di Marco (Southern Alps) is well known for the abundant dinosaurian ichnofau- The stratigraphical succession of Lavini di Marco is or- na (Leonardi & Mietto 2000) (Fig. 1). The dinosaur track be- ganized in peritidal sequences characterized by the alternation aring layers are attributed to the lower part of the Calcari Gri- of mainly subtidal beds (bioclastic and peloidal wackestone; gi Group (Lower Jurassic) (Avanzini et al. 1997). More pre- oolitic packstone) with layers attributed to an inter-supratidal cisely, the trampled layers are included in the “Peritidal unit” environment (wackestone and stromatolitic bindstones with of the Monte Zugna Formation, late Hettangian to Sinemu- desiccation structures and fenestrae) (Fig. 2). rian in age (Masetti et al. 1998). This stratigraphical unit cor- In this study, a basal cycle, corresponding to the la- responds to a coastal tidal-flat environment and is characte- yers named in the Lavini di Marco literature (i.e. Avanzini rized by peritidal shallowing-up metrical cycles: each cycle et al. 1997) 104, 105 and 106, is described (Fig. 2B). Abun- presents an inferior subtidal bed on which an inter-supratidal dant iron oxides concentrated into veins, laminae or globu- layer is superimposed (Masetti et al. 1998). lar bodies, fragments of red carbonated mud, small holes fil- While the dinosaur traces have been the subject of wi- led with silt and sparry calcite, and chemical transformations de study (Lanzinger & Leonardi 1991; Leonardi & Avanzini are indicative of subaerial exposure of these levels (Avanzi- 1994; Dalla Vecchia 1994; Avanzini et al. 1997; Leonardi & ni et al. 1997). Mietto 2000) on the other hand non-dinosaurian ichnology has Layer 104 is constituted of mudstones coarsening up- not been discussed, either for Lavini di Marco or for the basal wards to packstone/grainstone, with coated grains and skele- part of the Calcari Grigi Group. Monaco & Giannetti (2001, tal remains. The depressions present in the upper part of the 124 Baucon & Avanzini Zoophycos-like structures associated with dinosaur tracks Fig. 1 - Geographic location of the Lavini di Marco ichnosite with an indication of the main Lower Jurassic palaeogeographic domains of the Southern Alps. Fig. 1 - Localizzazione dell’icnosito dei Lavini di Marco nell’ambito dei principali domini plaeogeografici delle Alpi Meridionali nel Giurassico inferiore. layer were filled by laminated mud covered by a crust com- posed of dolomite, calcite, clays and iron oxides. The topo- graphic characteristics of level 104 are reminiscent of the la- teral microkarstification of present day tropical islands. This layer is interpreted (Avanzini et al. 1997) as a representative of a subtidal environment, which came rapidly to subaerial exposure when karstification of the tidal flat occurred. Layers 105 and 106 constitute a decimetre-scale set (Fig. 2). Level 105 is an alternance of stromatolitic bands, pe- loidal grey wackestone and reddish mudstone with a thick- ness that ranges from 15 mm to 100 mm. Its upper part pre- Fig. 2 - Stratigraphical setting. A. Stratigraphy of the Calcari Grigi sents mud-cracks with a diameter of 10-30 up to 50 mm. This Group (from Avanzini et al. 2006). B. Detail of the facies cycle level shows a vertical alternance of several emersions and considered in the text; Zoophycos-like structures have been found drowning with strong environmental changes in a few cen- in the inter-supratidal dolomitized facies corresponding to layers timeters of section. The influence of both marine and fresh 105 and 106 . water is testified respectively by the presence of the marine Fig. 2 - Assetto stratigrafico. A. Stratigrafia del Gruppo dei Calcari alga Thaumatoporella and the values of the isotopic ratios grigi (da Avanzini et al. 2006). B. Dettaglio della sequenza di base (δ18O from -0.5 to 10/00) and by the absence of typical mari- considerata nel testo; le strutture Zoophycos-like sono state iden- ne organisms (i.e. foraminifers, brachiopods) and microtex- tificate nelle facies dolomitizzate inter-sopratidali corrispondenti ai livelli 105 e 106. tures. Diagenetic and sedimentologic features seem to indi- cate a prevalence of emersion and a possibly distant coastli- ne. Comparison with present carbonate platforms in a tropi- The whole set consisting of layers 105 and 106 is in- cal humid climate shows that the lenses of fresh water were terpreted (Avanzini et al. 1997) as a representative of a com- ephemeral when level 105 formed (Avanzini et al. 1997). plex inter-supratidal environment. Level 106 is 100 to 150 mm thick and presents a hi- gh lateral variability. Stromatolites and flat pebbles breccias 1.3. The trace fossils assemblage are common. Level 106 was less exposed to subaerial altera- tion than level 105. It represents sedimentation in brackish Although the assemblage of trace fossils from Lavini pond (δ18O from + 2 to +40/00), emerged bars and shallow di Marco is in a study stage, we can anticipate that the pre- pools intercalated. valently subtidal bed 104 bears convex structures ascribed Studi Trent. Sci. Nat., Acta Geol., 83 (2008): 123-131 125 to Thalassinoides at the basis, while the stratum body ap- of the lamellae, corresponding to a single whorl of the tra- pears totally bioturbated. On the upper surface, layer 104 ce, ranges between 1.4 and 4 mm. When the structures are shows several dinosaur footprints (mainly theropods) and examined in polished slabs, the dark lamellae usually have various locomotion traces constituted by narrow crests or a homogeneous appearance, even if a structure made up of parallel furrows (they are interpreted as produced by ga- peloids can sometimes be observed (Fig. 3C). When the tra- stropods and, doubtfully, by arthropods – cf. Pterichnus in ces are observed in thin sections, the dark lamellae show a Gaillard et al. 2005). structure supported by calcite peloids (sometimes of skeletal The inter- supratidal set (layers 105 and 106) main- origin) floating in a matrix that contrasts with the host sedi- tains a moderately diversified ichnoassociation. On the lo- ment. On the contrary, light lamellae have textural features wer bed junction of each layer, convex branched structures that reflect those of the host sediment (Fig. 4B). are commonly found. They are comparable with Thalassi- The above described structures are reminiscent of the noides; on the upper layers junctions very similar traces are ichnogenus Zoophycos. found preserved as concave burrows. Abundant footprints of According to Rodríguez-Tovar & Uchman (2004) (af- small (i.e. Grallator, Kayentapus, Anomoepus) and large di- ter Frey 1970; Häntzschel 1975; Wetzel & Werner 1981; Fil- nosaurs (i.e. Parabrontopodus, Lavinipes) are common. Se- lion & Pickerill 1984) Zoophycos is identified as a spreite veral invertebrate trace fossils of the bedset corresponding structure