Author: Avanzini, M
Date published: July 1, 2010
1. Introduction and geological setting
A grey (creamy on the surface) medium-grained sandstone slab (37x24x10 cm) bearing a lacertoid track - among other invertebrate traces and possible reptile scratch marks - was collected from the Lastres Formation of Villaviciosa (Asturias, Northern Spain) and is now exhibited in the Museo del Jurasico de Asturias (MUJA) (Fig.1).
The Upper Jurassic (Kimmeridgian) Lastres Formation contains many vertebrate tracksites with a large variety of taxa. Most are of dinosaurs, but several sites include pterosaur, turtle and crocodilian tracks (García-Ramos et al., 2002; 2006; Avanzini et al., 2005; 2007).
The sedimentary palaeoenvironment of Lastres Formation was a fluvial-dominated deltaic system at a paleolatitude of about 33° N.
The material described here (reported for the first time by García-Ramos et al., 2002), consists of an isolated footprint preserved as a natural cast (convex hyporelief). The track's features are typical of "lacertoid" footprints: a pentadactyl manual print with digits increasing in length from I to IV.
Paleozoic and Mesozoic lacertoid footprints have been compared to this newly discovered track, but only Rhynchosauroides ichnogenus share most of the features. Although dinosaur tracks are well known in Late Jurassic, tracks of non dinosaur vertebrates are fairly rare, and no lizard-like ones have so far been reported.
2. Ichnologic description
The track consists of an isolated pentadactyl ectaxonic right manus footprint of length 25mm and width 24mm. It is very asymmetric, plantigrade, with digits and palm deeply marked on the substrate (Fig. 2).
The functional prevalence is external thus digits III and IV are the best impressed ones.
Traces of an outward slipping are preserved on the medial side of the print and on digits I-III.
The digits are straight or slightly turned inwards. Digit group I-III is nearly parallel while digit I shows a slight inward rotation. The divergence of digit group I-IV is equal to 50°. The angle between digit I and V is 105°.
Digit IV is only a little longer than digit III while digits II and I are nearly equal in length and only a little shorter than III and IV Digits IV and III show the metacarpalphalangeal articulation almost aligned and more distal with respect to digit I-II group. Digit V is short and stout with the base lined up with the base of digit IV Its claw is imprinted below the first pad of digit I. All the claw marks are sub-triangular in shape with a large base and an arched pointed tip; they turn either inward (I and II) or outward (III and IV).
Digit I is slightly outward arched and shows two phalangeal pads (free digit Length - 10 mm). Digit II is slightly inward arched and shows three possible phalangeal pads (free digit Length - 10 mm). Digit III is stout, without well recognisable phalangeal pads and with a robust and arched claw on the tip (free digit Length - 11mm). Digit IV is the longest one; it is slender with a marked and pointed claw marks and shows four well recognisable and one faint phalangeal pads (free digit Length - 12 mm). Digit V is the shortest, outward rotated, with a short triangular claw on the tip (free digit Length - 7 mm).
The phalangeal formula, calculated by counting the phalangeal pads, seems to be 2, 3, ?, ?5, ?
The overall footprint morphology is typical of a lizard - like or "lacertoid" track.
In the Jurassic the tracks of non - dinosaurian reptiles are scarcely documented and tracks generically attributable to lizard-like reptiles are reported only in the Lower Jurassic of the United States (Irmis, 2005; Lockley, 1991; Olsen and Rainforth, 2003; Rainforth, 1997). They were attributed to Rhynchosauroides and Lacertipus ichnogenera. Lockley and Hunt (1995) doubt the validity of Lacertipus, suggesting that this track can be explained as a cursorial version of Brasilichnium (a mammal-like reptile track). For this reason the only lizard-like ichnogenus actually accepted for the whole Jurassic would be Rhynchosauroides.
The ichnogenus Rhynchosauroides and the type-species Rhynchosauroides rectipes were erected in 1911 on the basis of some Lower Triassic footprints reported from the Keuper Sandstone Fm. (now Helsby Sandstone) of Runcorn, Cheshire, UK (Beasley in Maidwell, 1911).
The original diagnosis was revised by Haubold (1971), who described this ichnite as an ectaxonic track, asymmetric, from semiplantigrade to plantigrade. In Haubold's opinion, the large pace as well as the proportions between the digits (which correspond to a 2:3:4:5:3 phalangeal formula), and the great length of pes digit IV, which is external to the area of maximal load, are characteristic of "lacertoid" trackmakers. In typical Rhynchosauroides manus and pes tracks, the digits increase in length from I to IV, while digit V is much shorter than the others and is turned outwards. The pes, digitigrade, is larger and slenderer than the manus, and generally anterior to it. The digits from I to IV can be inwardly curved and the fourth is the longest. The manus is more internal located than the pes, more plantigrade, wide and has shorter digits.
All the characteristics listed above fit this newly discovered footprint well and we can infer that, the attribution to the morphofamily Rhynchosauroidea is acceptable. In particular the general outline of the footprint, the width to length ratio approximately equal to 1 (1.04), the marked plantigrady and the substantial similarity in length of digits IV-I are coherent with a Rhynchosauroidea manual footprint.
The presence of some burrows produced by bivalves (Lockeid), the absence of emersion signs as mud cracks, the absence of the tail impressions, and mainly there are not tracks in front and behind of this manual print, forming a trackway, would suggest a possibly subaqueously produced track (probably semi-aquatic tracks, sensu Silva et al. 2008).
Footprints of other small reptiles such as crocodiles are excluded due to their distinctive morphology (Lockley and Meyer, 2004). In the typical Lower Jurassic croco-dilomorph taxon Batrachopus Hitchcock, 1845 (Olsen and Padian, 1986, Lockley and Meyer, 2004), the manus has five fingers and is usually rotated so that digit II points forwards, digit IV laterally and digit V backwards. Digit I and V are very divergent with an angle of about 180°. The crocodilian ichnogenus Crocodylopodus from the Early Cretaceous of Spain (Fuentes Vidarte and Meijide Calvo, 2001; Lockley and Meyer, 2004), is also characterized by five manual digit impressions with a much greater interdigital angulation and the marks of digits I and V are almost aligned. A similar feature is common in all the small fossil crocodilian footprints described up to now (Fuentes Vidarte and Meijide Calvo, 2001; Lockley and Meyer, 2004; Avanzini et al., 2007).
4. A possible trackmaker attribution
In 1842 the palaeontologist Richard Owen assigned the name Rhynchosauroides articeps to some tracks occurring in the Grinsill quarries, near Shrewsbury (UK). He based his attribution on the evidence that the tracks dimensionally corresponded to the Rhynchosaurus body fossils and both occurred in the same beds.
However, Rhynchosaurus is characterised by shorter limbs and larger digits compared to what we would expect for a Rhynchosauroides trackmaker (Baird, 1957). Despite of this fact, the correlation between rhynchosaurs and Rhynchosauroides has for a long time been accepted probably due to the abundance of rhynchosaur body fossils pertaining to Middle and Upper Triassic deposits of the southern hemisphere, in which rhynchosaurs were the dominant herbivorous.
Today there is no general agreement about Rhynchosauroides zoological attribution.
The structure of rhynchosaurs pes (e. g. Noteosuchus Watson 1912) is rather primitive and therefore not too different from that of some lepidosauromorphs or of other basal archosauromorphs (Benton 1985; Carroll 1988). Over the years trackmakers have been associated with areoscelids, nothosaurs, prolacertiforms and neodiapsids. The possibility that the Rhynchosauroides track-maker may have been a "lepidosauromorph" reptile has been suggested by Haubold (1971), Lockley and Hunt (1994), Olsen and Rainforth (2003) and Domnanovich et al. (2008). It has also been proposed that the morphology of some Rhynchosauroides ichnospecies closely matches the foot structure of prolacertiform reptiles (Avanzini and Renesto, 2002, Diedrich, 2002).
It has to be pointed out, however, that the ichnogenus Rhyncosauroides shows a wide chronological occurrence which extends for most of the Permian and Mesozoic (Valentini et al., 2007). It is therefore very likely that the several ichnospecies attributed to Rhynchosauroides could really have been made by very different trackmakers.
Studies on Upper Jurassic lizard or lizard - like reptiles are more detailed than those on Triassic and Lower Jurassic microvertebrates (Evans, 2003 and bibliography herein).
The Lepidosauria differentiation into Squamata and Rhynchocephalia extends back into the Triassic. Squamates (lizard, snakes and amphisbaenians), with the exception of fragmentary remains from Africa and India, are only known from localities in northern continents (Laurasia). Among Squamata, only Iguania and Scleroglossa show relatively large bodies and autopodia. In the Scleroglossa group, we can exclude Gekkota and Scincomorpha, which are small lizards with a slender and graceful skeletal structure. Anguimorpha encompassing anguids, xenosaurus and the successful varanids (monitor lizards) have a more robust structure. However, all Jurassic anguimorpha are still too small when compared to this latest manus footprint, which suggests a body length of almost 30 cm.
The Rhynchocephalia had a worldwide distribution in the Late Jurassic (Evans, 1984; Gauthier et al., 1988) and their relatively robust structure could be compared to this new manus footprint.
In Rhynchocephalia manus all the digits are relatively stout. Digit V is the shortest, digits IV and III are nearly equal in length wile digits II and I are shorter. The claw on the tip of the fifth digit is the longest as appears in the here described footprint.
Peabody (1948) attributed Triassic Rhynchosauroides to the Rhynchocephalia. Gaston et al. (2003) attributed Rhynchosauroides from Chinle Group (Upper Triassic) of the Gateway area in western Colorado to a sphenodontid/lizard-like form and Silva et al. (2008) also attributed Rhynchosauroides trackways from the Upper Triassic (Santa Maria Fm.) of Southern Brazil to lacertilian reptiles, possibly sphenodontids.
In conclusion, even though it is not possible to be certain of the Asturian footprint's trackmaker, it seems very likely that it was a rhyncocephalian reptile.
The morphofamily Rhynchosauroidea, represented by two ichnogenus (Rhynchosauroides and Ganasauripus) and nearly thirty ichnospecies, is one of the most common and geographically widespread Late Permian and Triassic ichnotaxa. The Rhynchosauroides ichnogenus has been reported in Europe, North America and South America. The oldest tracks are Late Permian in age and belong to the Val Gardena Sandstone in Northern Italy (Valentini et al., 2007) but in fact Rhynchosauroides is almost exclusively Triassic, where it is abundant and very common. The most recent known occurrence of Rhynchosauroides belongs to basal Jurassic deposits from the North American Newark Supergroup (Olsen and Rainforth, 2003); Szajna and Hartline, 2003; Rainforth, 2002; Hunt and Lucas, 2007).
The Asturian footprint appears to represent the first reported occurrence for the Late Jurassic (Lockley, 2002), and therefore, the most recent example of the ichnogenus discovered so far.
We are indebted to Jean-Michel Mazin (Université Lyon, France), Angela Delgado Buscalioni (Universidad Autónoma de Madrid) and to an anonymous referee for the formal and constructive reviews. Financial support was provided by the Government of Principado de Asturias, the University of Oviedo (Protocolo CN-04-226), the Spanish Government (Ministry of Science and Innovation, Project CGL2007-62469 /BTE) and the Museo Tridentino di Scienze Naturali.
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M. Avanzini1, L. Piñuela2, J. C. García-Ramos2
1 Museo Tridentino di Scienze Naturali, Via Calepina 14,1-38120 Trento, Italy. avanzini@mtsn. tn. it
2 MUJA, Museo del Jurdsico de Asturias, Rasa de San Telmo, 33328, Colunga, Asturias, Espana; Depto. de Geologia. Universidad de Oviedo, C/Jesus Arias de Velasco, s/n, 33005 Oviedo, Asturias, Spain. email@example.com
Received: 07/12/09 / Accepted: 30/06/10