2007: Between a rock and a hard place: arthropod trackways and ichnotaxonomy. Lethaia , Vol. 40, Several challenges exist in ichnotaxonomy: overcoming the perceived distinction between invertebrate and vertebrate ichnotaxonomy, standardizing terminology, rationalizing the plethora of ichnotaxa already in existence, and developing principles for diagnosing new ichnotaxa. Ichnotaxa should be based on morphology, and this morphology incorporates three key components; the behaviour expressed, the producer, and the substrate. Invertebrate and vertebrate ichnotaxa can both be accommodated within this framework, but they differ in the relative contributions of these components. The key to justifying the synonymy of existing ichnotaxa is the recognition of intergrading specimens. However, this is only the case for minor morphological variants (i.e. those representing minor differences in behaviour, such as gait parameters or stance; or minor differences in preservation, such as undertrack fallout or slight differences in substrate conditions). Intergrading specimens should not be used to justify synonymy between major morphological variants (i.e. those representing major behavioural differences, defined herein as ethological categories; or major differences in preservation, such as formation in soup, soft and firmgrounds), and such specimens should be denoted as hybrids (e.g. Cruziana × Rusophycus ). New ichnotaxa should ideally be based on observations of large samples of material, so that recurrence is demonstrable, and morphological continuums, or subset relationships, representing minor morphological variation, are identified. Ichnotaxa may only be erected on the basis of limited material if they truly represent a unique morphology. These principles have been developed with arthropod trackways in mind, but it is hoped that they will be of more general utility. ᮀ Computer simulation , ichnology , ichnotaxobases , neoichnology , nomenclature , trace fossils .Nicholas J. Minter [N.J.Minter@bris.ac.uk], Simon J. Braddy [S.J.Braddy@bris.ac.uk], and Robert B. Davis [rbd501@york.ac.uk]

The colonization of land was one of the major events in Earth history, leading to the expansion of life and laying the foundations for the modern biosphere. We examined trace fossils, the record of the activities of past life, to understand how animals diversify both behaviourally and ecologically when colonizing new habitats. The faunal invasion of land was preceded by excursions of benthic animals into very shallow, marginal-marine environments during the latest Ediacaran Period and culminated in widespread colonization of non-marine niches by the end of the Carboniferous Period. Trace-fossil evidence for the colonization of new environments shows repeated early-burst patterns of maximal ichnodisparity (the degree of difference among basic trace-fossil architectural designs), ecospace occupation, and level of ecosystem engineering prior to maximal ichnodiversity. Similarities across different environments in the types of behavioural programmes employed (as represented by different trace fossils), modes of life present, and the ways in which animals impacted their environments, suggest constraints on behavioural and ecological diversification. The early-burst patterns have the hallmark of novelty events. The underlying drivers of these events likely were the extrinsic limitation of available ecospace and intrinsic controls of genomic and developmental plasticity that enabled trace-maker morphological and behavioural novelty.

The collection and dissemination of vertebrate ichnological data is struggling to keep up with techniques that are becoming commonplace in the wider palaeontological field. A standard protocol is required to ensure that data is recorded, presented and archived in a manner that will be useful both to contemporary researchers, and to future generations. Primarily, our aim is to make the 3D capture of ichnological data standard practice, and to provide guidance on how such 3D data can be communicated effectively (both via the literature and other means) and archived openly and in perpetuity. We recommend capture of 3D data, and the presentation of said data in the form of photographs, false‐colour images, and interpretive drawings. Raw data (3D models of traces) should always be provided in a form usable by other researchers (i.e. in an open format). If adopted by the field as a whole, the result will be a more robust and uniform literature, supplemented by unparalleled availability of datasets for future workers.

The ichnotaxonomy and stratigraphic, geographic and environmental distribution of fish (Undichna) and amphibian (Lunichnium) swimming traces are reviewed. The ichnospecies of Undichna consist of various combinations of sinusoidal waves of differing complexity. Some of the more complex ichnospecies are made up of elements of the simpler forms, and morphological subset relationships between them are presented. Such subset series represent potential taphoseries relationships (i.e. preservational variants that reflect, for example, undertrails), or series of minor behavioural variations. Such a system can be used to highlight that different ichnospecies occurring at a locality may be taphonomic or minor behavioural variants of each other. Caution should, therefore, be exercised before erecting new ichnospecies on the basis of limited material if its morphology is a subset of an existing ichnospecies. However, the naming of such simpler ichnospecies is valid if they represent a recurrent morphology, and it is valid to erect new ichnospecies whose morphology is not a subset of an existing ichnospecies. Specimens that demonstrate intergradation between ichnotaxa can be used to justify their synonomy. Ichnotaxonomic revisions reduce the number of ichnospecies in Undichna from 14 to nine. U. radnicensis, a highly variable ichnospecies, is synonymized with U. britannica on the basis of material from China that demonstrates they can intergrade. U. prava is a partial U. tricosta, which falls within the minimum diagnosis of U. simplicitas. U. gosiutensis is regarded as a subjective junior synonym of U. quina. U. westerbergensis, originally attributed to a 'crossopterygian' fish performing a tetrapodlike gait, is reassigned as a distinct ichnospecies within Lunichnium because it demonstrates the same morphology, representing similar behaviour, albeit by a different producer. L. anceps and L. gracile are synonymized with L. rotterodium. New specimens of U. bina and L. rotterodium are also described from the Lower Permian Robledo Mountains Formation of southern New Mexico, USA.

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