Ichnotaxobases for bird-like footprints: towards a uniform approach

RICARDO NESTOR MELCHOR; SILVINA DE VALAIS,

Praga

Workshop; Third Workshop on Ichnotaxonomy; 2006

Institute of Geology, AS CR, Prague

The conceptual and working approach to the ichnotaxonomy of vertebrate footprints, including that of bird-like footprints, is far from uniform. Ichnologists have recognised ichnofamilies, ichnogenera and ichnospecies of bird-like footprints on different grounds. As examples of the current inconsistency in criteria, some ichnofamilies are defined with greater detail than some ichnogenera, and commonly the geologic age of the track-bearing lithostratigraphic unit and the supposed trackmaker are considered as a first-order criteria for ichnotaxonomy. The aim of this paper is to revise and discuss the ichnotaxobases used for the classification of avian footprints and suggests those considered as most appropriate for this kind of footprints. The proposed procedures are applied to the ichnotaxonomic analysis of the Santo Domingo Formation (Late Triassic, NW Argentina) avian-like footprints.             There are ichnogeneric taxobases that are used for most authors to classify tracks with avian affinities, including the number, morphology, relative length, orientation of the impressions of the digits or the total divarication, and trackway parameters. Certain features are regarded as ichnogeneric taxobases by some authors, such as the footprint length/width ratio, divarication between the impressions of the digits II-III and III-IV, presence or absence of the sole impression, heel, and webbing, and the correspondence of the axis of the impression the digit I with that of the III. Other characters were used indistinctly to name ichnogenera or ichnospecies by different authors, including: presence of claw marks, number of phalangeal pads, and size of the footprint. Most authors consider the details of the morphology, outline, and range and absolute length of the digit impressions, especially of the hallux, as an ichnospecific taxobases. Other features often regarded in an secondary level are: the morphology of the impressions of claws, phalangeal pads, sole, and heels; and the range of the divarication between the impressions of the digits II-III, III-IV, and II-IV; and the distance between the tips of the middle and the inner and outer digit imprints and projection of the impression of the III beyond the external digit imprints. Ichnofamilies should be erected to include ichnogenera that share key morphological features. In this respect, it is suggested that the number, relative position and proportionate length of digits; and the presence or absence of webbing impressions are useful taxobases to define ichnofamilies of bird-like footprints. Similarly, the proposed ichnogeneric taxobases to be used with bird-like footprints are: the number, morphology, relative length, and orientation of the digit imprints (specially that of digit I), length/width ratio of footprints, number of phalangeal pad marks, divarication between the impressions of digits, and presence or absence of webbing. The ichnospecific taxobases used in this study are: range of length/width ratio of footprints, proportion of digit length I:II:III:IV, range of the angles among the impressions of the digits, morphology of the sole, footprint size, pace angulation, stride length, and departure of tracks from the midline. Regarding size of footprints, an arbitrary position is considered necessary. It is proposed that a representative (modal or average) size value for the holotype series be estimated (using a number of readings no lower than 30) and a relative or absolute range of variability be defined for each ichnospecies. The listed criteria are potentially useful for distinction of new ichnotaxa, although the importance given to each one will vary depending on the features of the track population under study. Erection of new ichnotaxa should always be based on a large sample size and considering the full variability of the footprint population. The material selected as type should be preferentially a trackway, instead of single footprints. Diagnosis of ichnotaxa must be as concise a possible, highlighting the key features of the ichnotaxa, and avoiding interpretative terms like “shorebird footprint”, “charadriform footprint”, etc. If the only material available are a few specimens of footprints of potentially distinctive morphology that cannot be compared with a known ichnotaxa, it is considered desirable to leave it in open nomenclature, instead of creating a new ichnotaxa.