Vertebrate Tracks Research Articles

Syndepositional tectonics recorded by soft-sediment deformation and liquefaction structures (continental Lower Permian sediments, Southern Alps, Northern Italy): Stratigraphic significance

The Lower Permian succession of the Central Southern Alps (Lombardy, Northern Italy) was deposited in fault-controlled continental basins, probably related to transtensional tectonics. We focussed our study on the stratigraphic record of the Lower Permian Orobic Basin, which consists of a 1000 m thick succession of prevailing continental clastics with intercalations of ignimbritic flows and tuffs (Pizzo del Diavolo Formation, PDV) resting on the underlying prevailing pyroclastic flows of the Cabianca Volcanite. The PDV consists of a lower part (composed of conglomerates passing laterally to sandstones and distally to silt and shales), a middle part (pelitic, with carbonates) and an upper part (alternating sandstone, silt and volcanic flows). Syndepositional tectonics during the deposition of the PDV is recorded by facies distribution, thickness changes and by the presence of deformation and liquefaction structures interpreted as seismites. Deformation is recorded by both ductile structures (ball-and-pillow, plastic intrusion, disturbed lamination, convolute stratification and slumps) and brittle structures (sand dykes and autoclastic breccias). Both the sedimentological features and the geodynamic setting of the depositional basin confidently support the interpretation of the described deformation features as related to seismic shocks. The most significant seismically-induced deformation is represented by a slumped horizon (about 4 m thick on average) which can be followed laterally for more than 5 km. The slumped bed consists of playa-lake deposits (alternating pelites and microbial carbonates, associated with mud cracks and vertebrate tracks). The lateral continuity and the evidence of deposition on a very low-angle surface along with the deformation/liquefaction of the sediments suggest that the slump was triggered by a high-magnitude earthquake. The stratigraphic distribution of the seismites allows us to identify time intervals of intense seismic activity, which correspond to rapid and basin-wide changes in the stratigraphical architecture of the depositional basin and/or to the reprise of the volcanic activity. The nature of the structures and their distribution suggest that the magnitude of the earthquakes responsible for the observed structures was likely higher than 5 (in order to produce sediment liquefaction) and probably reached intensity as high as 7 or more. The basin architecture suggests that the foci of these earthquakes were located close to the fault-controlled borders of the basin or within the basin itself.

Taphonomic Controls on Animal Tracks at Saline, Alkaline Lake Bogoria, Kenya Rift Valley: Impact of Salt Efflorescence and Clay Mineralogy

Abstract The morphologies of subaerial vertebrate tracks and invertebrate traces in surficial sediments can be used in paleoenvironmental reconstructions by providing details about the initial substrate conditions (e.g., water content) and any changes experienced during trace preservation. A qualitative field study was undertaken to identify the factors affecting track taphonomy in saline lake-margin environments, supported by experiments to isolate selected factors that modify track morphology and substrate characteristics. Flamingo tracks, ubiquitous around saline, alkaline Lake Bogoria in the Kenya Rift, were the focus of field study, although other vertebrate tracks and surficial invertebrate trails and tunnels were also considered. A complex and interacting set of environmental factors were identified, such as the effect of lake-level fluctuations on substrate grain size and capillary evaporation. Several factors affect the early taphonomic modification, destruction, stabilization, and/or cementation of tracks and their associated substrates. The laboratory experiments tested the effects of pore-water salinity, clay mineralogy (e.g., swelling or non-swelling clays), drying method (e.g., solar radiation or wind), and wetting-and-drying cycles on tracks. These data were supported by statistical analyses of impression depth and planimetric area. The results show that track morphology in smectitic substrates is altered rapidly by wetting and drying, particularly if the substrates contain saline pore waters, but morphology is less affected in non-swelling clays. The degree of morphologic alteration and/or resistance to wetting and drying varies with the rate of substrate drying and the type of the resulting salt efflorescence (e.g., interstitial vs. surficial crust). These short-term, process-related taphonomic effects help to explain the morphologic variants of tracks in lake-marginal settings, and provide insights into the preserved distribution patterns of subaerial ichnofossil assemblages.

FOSSIL VERTEBRATE TRACKS AS PALEOPENETROMETERS: CONFOUNDING EFFECTS OF FOOT MORPHOLOGY

The depth to which a vertebrate track is indented can provide a wealth of information, being a direct result of the weight, duty factor, and limb kinematics of the animal as well as media ( = substrate or sediment) consistency. In order to recreate the formation of the track and elucidate media consistency at the time of track formation, such factors as animal mass, duty factor, and foot morphology must be taken into consideration. This study uses Finite Element Analysis and physical modeling to demonstrate for the first time that the shape of the foot is an important factor that influences the depth to which the sediment is penetrated. In cohesive sediment, less compact morphology allows more sediment to move vertically upwards at the edges of the foot, dissipating force at the surface, and retarding transmission of load vertically down into the sediment. The reverse of this effect is seen in noncohesive sediment. Foot morphology, therefore, has a direct impact on preservation potential, both of surface tracks and undertracks, that is irrespective of the pressure exerted on the sediment surface by the foot and independent of mass and duty factor.

Toward the Origin of Amniotes: Diadectomorph and Synapsid Footprints from the Early Late Carboniferous of Germany

Ichnotaxonomic revision of two extended sequences of large tetrapod footprints from the Westphalian A Bochum Formation of western Germany suggests assignment of the specimens to the well-known Permo-Carboniferous ichnogenera Ichniotherium and Dimetropus. Trackway parameters and imprint morphology strongly support basal diadectomorphs and “pelycosaurian” -grade synapsid reptiles, respectively, as the most likely trackmakers. The ichnofossils thereby extend the first appearance of these two important groups of basal tetrapods by about 5–10 million years, to the early Late Carboniferous, which is in accordance with the minimum age for the evolutionary origin of the clades following widely accepted phylogenetic analyses. These trackways provide not only direct evidence bearing on activity and behaviour of large terrestrial tetrapods close to the origin of amniotes, but also serve as a valuable benchmark for the assessment of controversially interpreted vertebrate tracks from other localities of similar age.

Bird Tracks at the Hot Springs Mammoth Site, South Dakota, USA

The Hot Springs Mammoth Site, South Dakota, USA, has been excavated for over three decades, during which time numerous body fossils have been recorded. The site is particularly well known for the skeletal remains of mammalian megafauna. Bedding plane surfaces were studied that displayed the first record of small vertebrate (avian) and invertebrate traces. While large vertebrate tracks, often observed in cross-section, are well known at the site, the new traces form a hitherto unstudied assemblage. The presence of distinct didactyl and tridactyl avian tracks from the site are described here for the first time. The small (∼20 mm long) tracks and associated invertebrate traces suggest relatively high moisture content in the substrate on surfaces that experienced aerial or subaerial exposure. This is consistent with the interpretation that the upper layers of the site represent the latter stages of a sinkhole setting with a pond undergoing cyclical drying out.

Fluvial and eolian ichnofaunas from the Lower Permian of South America (Patquía Formation, Paganzo Basin)

The Lower Permian Patquia Formation is the youngest unit of the Paganzo Basin, western Argentina. The lower section consists of red mudstones, and fine- and coarse grained sandstones deposited in fluvial systems with extensive and thick floodplain deposits. These rocks contain a low-diversity and relatively abundant association of trace fossils suggesting the activity of a sub-superficial to superficial fauna. The association is characterized by Rusophycus carbonarius (Cubichnia), Cruziana problematica (Repichnia), and Palaeophycus tubularis (Domichnia) of the Scoyenia ichnofacies. Disarticulated fish remains are also present. The upper section is dominated by red, cross-bedded, medium- to fine-grained sandstones deposited in eolian systems that host a lowdiversity and low-abundance association of trace fossils that indicates the activity of a mainly superficial fauna. Tetrapod footprints (Chelichnus duncani, oval digit imprints, short parallel grooves, and sinusoidal grooves), horizontal to vertical burrows (Palaeophycus tubularis and Skolithos isp. respectively), and arthropod trackways are the typical components of these deposits. The association shows elements of the three presently proposed eolian ichnofacies (Chelichnus, Octopodichnus and Entradichnus ichnofacies) suggesting the necessity of revision and the possible integration of these separate ichnofacies into a single model. The record of fossil vertebrate tracks is uncommon in Lower Permian strata of South America. Therefore, the ichnologic record of the Patquia Formation is a significant contribution to the understanding of Lower Permian South American ichnofaunas.

The Sodium Hypochlorite Solution for the Removal of Lichen from Vertebrate Track Surfaces

The removal of extraneous biological materials from vertebrate ichnological specimens may be necessary if they have been exposed to subaerial processes in the field for long periods of time. It is not uncommon for fossil track specimens to be found nearly completely covered with lichen colonies, especially those recovered from alpine areas. This paper describes a technique using a sodium hypochlorite solution (bleach) to remove lichen from a track surface containing numerous avian prints and small theropod prints with skin impressions.

Vertebrate/Invertebrate Trackways

The Ichnology Workshop held in 1998 in Bornholm, DK, evidenced significant differences between approaches used for studying vertebrate and invertebrate ichnites. In particular, trackways are used more in invertebrate ichno-studies than in vertebrate studies. This is due to the intrinsic characteristics of each category, primarily the multiple consequences of the order of magnitude gap in sizes, the opportunity to have more taxonomic information of the trackmaker with vertebrate ichnites, the usefulness of correlations with biotopes and facies with invertebrate ichnites, and practical parameters such as the probability of findings. That vertebrate and invertebrate ichnologists exchange ideas could certainly bring progress, however, significant differences between the two fields certainly remain. Using information on animal behavior to define ichnotaxobases would not be acceptable for most vertebrate tracks.

LABORATORY-CONTROLLED SIMULATIONS OF DINOSAUR FOOTPRINTS IN SAND: A KEY TO UNDERSTANDING VERTEBRATE TRACK FORMATION AND PRESERVATION

Dinosaur tracks and trackways yield invaluable information as to the identity, size, and gait of the trackmaker and the conditions of the media (=substrate) it traversed. Correctly interpreting tracks requires consideration of their three-dimensional morphology. Laboratory-controlled simulations were conducted to investigate the subsurface track morphology formed from differently shaped feet, as the shape of the footprint deteriorates with depth. A circular, triangular, and a tridactyl dinosaur foot-shaped template, or indenter, were indented vertically into two types of sand, with four moisture contents—dry, 10%, 20%, and saturated. The morphology of all three indenters was preserved most accurately in the moist sand. Tracks in dry and saturated sand were distorted by a greater degree of media deformation. Digit imprints of tridactyl tracks were only clearly discernible in near-surface layers and were deformed by shear zones or inward movement of sediment in dry and saturated sand. The long digits of the template produced the greatest degree of outward displacement, and tracks became wider with depth and deepest in the heel region. This was most distinct in dry sand, where extensive shear zones in cross section demonstrated the outward and upward movement of sediment. All tracks in saturated sand were characterized by considerable downward displacement of sediment and features related to the upward pull of sediment as the templates were withdrawn. These diagnostic features allow vertebrate tracks to be differentiated from nonbiogenic, soft-sediment deformation. Fossil tracks studied from the Middle Jurassic succession of the Cleveland Basin, Yorkshire, demonstrate affinities to the experimental tracks formed in saturated sand.

Biogenic structures in exhumed surfaces around saline lakes: An example from Lake Bogoria, Kenya Rift Valley

Five overprinted suites of animal and plant traces around saline, alkaline Lake Bogoria, Kenya, are described from a set of interfingering Late Pleistocene and Holocene exhumed surfaces that reveal a complex story of changes in lake level and environmental controls on trace assemblages through time. These stratigraphic surfaces are amalgamated and form a co-planar surface. The sediments were first deposited during intermediate lake levels, then reworked and cemented during low lake levels, and were subsequently overlain by transgressive lacustrine sediments during higher lake levels. Ongoing exhumation of the surface near the present shoreline has reactivated the sediments as a substrate for the activities of epifaunal and infaunal animals, as well as plant growth. The modern environmental setting at Lake Bogoria is very similar to that of the preserved exhumed surfaces, which allows comparisons between observed sets of environmental factors affecting animal behaviour and their representation in the fossilized examples. Pronounced lateral environmental heterogeneity around saline and hypersaline lakes may lead to concentrations of life around sites with vital resources, such as freshwater. At Lake Bogoria, groups of fault-controlled hot springs represent these “oases”, and have been a relatively consistent feature in and around the lake during the development of the exhumed surfaces. Together, the diverse assemblage of animal trace suites from Lake Bogoria represent the Mermia and Scoyenia ichnofacies, but can be divided into five suites that reveal details of laterally heterogeneous sets of environmental factors, and are useful indicators of lake level, substrate moisture content, substrate cohesion, etc. Suite 1 comprises the traces of chironomid larvae (Diptera: Chironomidae) formed in subaqueous lacustrine settings. Suite 2 includes the traces of flamingos ( Phoeniconaias minor and Phoenicopterus ruber) formed at the shoreline. Suite 3 compares well with the Mermia ichnofacies and comprises trails and burrow systems associated with relatively fresh, saturated to extremely shallow subaqueous substrates. Suite 4 is comparable to both the pre- and post-desiccation suites of the Scoyenia ichnofacies, is the most diverse suite at Lake Bogoria, and mainly comprises the burrows of beetles, earwigs, and their larvae as well as a diverse set of rhizoliths and vertebrate tracks. Suite 5 contains a moderately diverse set of traces attributed to termites and ants.

Aeolian sand sea development along the mid‐Cretaceous western Tethyan margin (Spain): erg sedimentology and palaeoclimate implications

AbstractThe existence of a mid‐Cretaceous erg system along the western Tethyan margin (Iberian Basin, Spain) was recently demonstrated based on the occurrence of wind‐blown desert sands in coeval shallow marine deposits. Here, the first direct evidence of this mid‐Cretaceous erg in Europe is presented and the palaeoclimate and palaeoceanographic implications are discussed. The aeolian sand sea extended over an area of 4600 km2. Compound crescentic dunes, linear draa and complex aeolian dunes, sand sheets, wet, dry and evaporitic interdunes, sabkha deposits and coeval extradune lagoonal deposits form the main architectural elements of this desert system that was located in a sub‐tropical arid belt along the western Tethyan margin. Sub‐critically climbing translatent strata, grain flow and grain fall deposits, pin‐stripe lamination, lee side dune wind ripples, soft‐sediment deformations, vertebrate tracks, biogenic traces, tubes and wood fragments are some of the small‐scale structures and components observed in the aeolian dune sandstones. At the boundary between the aeolian sand sea and the marine realm, intertonguing of aeolian deposits and marine facies occurs. Massive sandstone units were laid down by mass flow events that reworked aeolian dune sands during flooding events. The cyclic occurrence of soft sediment deformation is ascribed to intermittent (marine) flooding of aeolian dunes and associated rise in the water table. The aeolian erg system developed in an active extensional tectonic setting that favoured its preservation. Because of the close proximity of the marine realm, the water table was high and contributed to the preservation of the aeolian facies.A sand‐drift surface marks the onset of aeolian dune construction and accumulation, whereby aeolian deposits cover an earlier succession of coastal coal deposits formed in a more humid period. A prominent aeolian super‐surface forms an angular unconformity that divides the aeolian succession into two erg sequences. This super‐surface formed in response to a major tectonic reactivation in the basin, and also marks the change in style of aeolian sedimentation from compound climbing crescentic dunes to aeolian draas.The location of the mid‐Cretaceous palaeoerg fits well to both the global distribution of other known Cretaceous erg systems and with current palaeoclimate data that suggest a global cooling period and a sea‐level lowstand during early mid‐Cretaceous times. The occurrence of a sub‐tropical coastal erg in the mid‐Cretaceous of Spain correlates with the exposure of carbonate platforms on the Arabian platform during much of the Late Aptian to Middle Albian, and is related to this eustatic sea‐level lowstand.

Millions of reptile tracks—Early to Middle Triassic carbonate tidal flat migration bridges of Central Europe—reptile immigration into the Germanic Basin

Discoveries of vertebrate track sites in Central Europe have occurred in 75 localities in carbonate tidal flats of the Middle Triassic. In the Germanic Basin carbonate tidal flats were wide-span mapped, resulting in the finding of millions of small- to medium-sized reptile tracks. In the west of the basin the sediment of the Lower Muschelkalk to basal Upper Muschelkalk contains at least 21 track horizons, whereas in the eastern part more typically marine conditions were present. Here, tidal flats with additional track beds started earlier during the Upper Bunter and demonstrated marine ingression from the eastern Silesian gate. During low stands these tidal flats comprised inter-peninsula bridges, which allowed migration of reptiles. Only two medium to small prolacertilian reptiles, which were fully adapted to these environments, left any kind of track. A large thecodont reptile such as Euparkeria was the potential predator that may rarely have hunted the main small trackmakers Macrocnemus and Hescherleria.

A reindeer track from a drill core, and lake basin development of the Late Glacial Lille Slotseng kettle-hole basin, South-East Jylland, Denmark

IA subfossil reindeer track is found in lake shore sediments in a drill core through the Late Glacial and Holocene lacustrine succession from the Lille Slotseng kettle-hole basin, located in the southeastern part of Jylland. The track is dated to 11,795 ± 80 14C yr BP or 13,635 cal. yr BP. This is the first vertebrate track recognized from a soft sediment drill core. Hitherto, convincing vertebrate trace fossils have only been described from boreholes through lithified Triassic sediments. During a previous excavation at the site, skeletal remains of at least ten reindeer were recovered from the Bølling succession and a vertebra was dated to approximately 14,100 cal. yr BP. The Lille Slotseng basin is semicircular with a maximum diameter of 23 m and the overall transgressive–regressive succession covers the time period from 16,000 to around 8,000 cal. yr BP. The oldest basin-fill sediments are melt-water deposits. They are overlain by a succession belonging to the Bølling Interstadial (GI 1-e), older Dryas (GI 1-d), Allerød, (GI 1-c, 1-b, 1-a), and Younger Dryas (GS 1), which terminates the Late Glacial succession. Then follows Preboreal algal gyttja and nearshore woody peat from the Boreal and Early Atlantic times, filling the basin.

A Tale of Two Ichnologies: The Different Goals and Potentials of Invertebrate and Vertebrate (Tetrapod) Ichnotaxonomy and How They Relate to Ichnofacies Analysis

Invertebrate and vertebrate (tetrapod) ichnology have evolved as separate disciplines, largely due to differences in the morphology and behavior of the tracemakers and the practical impossibility of treating them as uniform subjects of analysis. Thus, invertebrate ichnotaxa reflect behavior to a much greater degree than they reflect tracemaker morphology. The situation is essentially reversed in the case of vertebrates, where morphology, especially of the foot, plays an important primary role in ichnotaxonomy (and a secondary role in trackmaker identification), whereas behavior is of much lesser importance and in many cases can compromise consistent and effective ichnotaxonomy. Inherent differences in morphological and behavioral complexity also have significant implications for our concept of the different potentials of vertebrate and invertebrate ichnotaxonomy and ichnofacies analysis—as shown in distinctions made between biotaxonichnofacies and ethoichnofacies, respectively. Vertebrate tracks not only inform us about morphology, but in the most general sense also have made huge contributions to our understanding of vertebrate behavior that have only limited supra-morphological relevance to ichnotaxonomy or ichnofacies analysis. These contributions have done much to settle and/or stimulate debate about the posture, speed, abundance, spatio-temporal distributions and social behavior of dinosaurs, pterosaurs, birds and other vertebrates, and so have had a dynamic impact on thinking in the field of vertebrate paleontology and even biostratigraphy. Nonetheless, this influence may affect ichnotaxonomy as in the case of trackway evidence of posture that proved important in differentiating the wide- and narrow-gauge trackways Brontopodus and Parabrontopodus. By contrast, the impact of invertebrate ichnology on our understanding of invertebrate behavior is complexly interwoven with an ichnotaxonomy that has a much less obvious relationship to tracemaker morphology. Thus, the use of invertebrate traces to decipher the evolution of fossil behavior has proved an ambiguous and subtle pursuit that does not readily demonstrate parallels with the evolution of easily identified taxonomic groups. Because vertebrates (especially tetrapods) are inherently complex organisms, in comparisons with invertebrates, their traces play a less ambiguous and more significant role in helping us understand major phases in vertebrate evolution. In many cases they have pointed to interpretations of vertebrate paleobiology that were not confirmed by other lines of evidence until generations later (e.g., polar dinosaur migrations). When vertebrate tracks are used for palichnostratigraphy or as evidence to support faunal turnover and extinction events, ichnotaxonomy assumes a significant role in applied biostratigraphy. Such biostratigraphic utility is far more ambiguous and much less significant in the case of invertebrate ichnology. Despite these inherent differences, in the field of ichnofacies analysis, invertebrate and vertebrate ichnology come together as integrated components of facies analysis, and potentially have great synergistic utility in paleoecological and paleoenvironmental analysis. Nevertheless vertebrate (tetrapod) ichnofacies nomenclature, and terrestrial ichnofacies analysis in general, is not yet well-established and is currently generating disparate schools of thought. It is argued that tetrapod ichnofacies are inherently more diverse and differentiated than invertebrate ichnofacies.

Small Theropod Track Assemblages from Middle Jurassic Eolianites of Eastern Utah: Paleoecological Insights from Dune Ichnofacies in a Transgressive Sequence

New discoveries show that very small theropod tracks (cf. Wildeichnus) are abundant in the upper part of the Moab Member, recently assigned to the Curtis Formation (formerly considered part of the Entrada Formation) in the Mid-?Late Jurassic of eastern Utah. The tracks represent a distinct small-theropod ichnofacies associated with eolian dune deposits that is easily differentiated from the water-lain beds of the overlying Megalosauripus-Therangospodus ichnofacies, which comprises the single-surface Moab megatracksite. Pterosaur track assemblages, representing the Pteriachnus ichnofacies, are found a few meters above the megatracksite surface in the upper tongue of the Summerville Formation. The small theropod ichnofacies is reminiscent of other early Mesozoic dune facies ichnofaunas from the Wingate and Navajo formations (Late Triassic and Early Jurassic) where small theropod tracks occur in association with other small tetrapod footprints. All such examples evidently represent a recurrent dune facies ecosystem dominated by diminutive vertebrates. Because the small theropod ichnofacies is one of three ichnofacies found in a thin stratigraphic sequence (<20 m) that contains no body fossils, it is clear that vertebrate tracks play an important role in providing insight into the paleoecology of units previously considered devoid of any useful fossil evidence. The three successive ichnofacies represent a transgressive transition from sand dunes, through sandy shoreline to shallow marine environments, each with its quite distinct vertebrate fauna.

Sequence stratigraphic controls on synsedimentary cementation and preservation of dinosaur tracks: Example from the lower Cretaceous, (Upper Albian) Dakota Formation, Southeastern Nebraska, U.S.A.

A thin cemented sandstone bed in the Upper Albian Dakota Formation of southeastern Nebraska contains the first dinosaur tracks to be described from the state. Of equal importance to the tracks are stable-isotope (C, O) analyses of cements in the track bed, especially in the context of data derived from generally correlative strata (sandstones and sphaerosiderite-bearing paleosols) in the region. These data provide the framework for interpretations of paleoenvironmental conditions, as well as a novel approach to understanding mechanisms of terrestrial vertebrate track preservation. High minus-cement-porosity (> 47%) and low grain-to-grain contacts (∼2.5) in the track bed indicate early (pre-compaction) lithification. Although phreatic cements dominate, the history of cementation within this stratigraphic interval is complex. Cathodoluminescence petrography reveals two distinct calcite zones in the track-bearing horizon and four cement zones in stratigraphically equivalent strata from a nearby section. The earliest calcite cements from both localities are likely coeval because they exhibit identical positive covariant trends ( δ 18O values of − 9.89 to − 6.32‰ and δ 13C values of − 28.01 to − 19.33‰ VPDB) and record mixing of brackish and meteoric groundwaters. All other calcite cements define meteoric calcite lines with δ 18O values clustering around − 9.42‰ and − 8.21‰ VPDB from the track-bearing horizon, and − 7.74‰, − 5.81‰, and − 3.95‰ VPDB from the neighboring section. Distinct meteoric sphaerosiderite lines from roughly correlative paleosols serve as a proxy for locally recharged groundwaters. Back-calculated paleogroundwater δ 18O estimates from paleosol sphaerosiderites range from − 7.4 to − 4.2‰ SMOW; whereas, meteoric calcite lines from the track horizon are generally more depleted. Differences in cement δ 18O values record changes in paleogroundwater recharge areas over time. Early calcite cements indicate mixing of fresh and brackish groundwaters during the syndepositional lithification of the track horizon. Later calcite cements, however, indicate recharge from a larger catchment basin that extended far inland. Therefore, the cements likely record a rise and subsequent fall in relative sea level. We conclude that scrutiny of the cement isotope geochemistry of genetically significant surfaces, especially track beds, can provide new data for interpreting sea level change.

A NOVEL CONTEMPORARY FLUVIAL ICHNOCOENOSE: UNIONID BIVALVES AND THE SCOYENIA-MERMIA ICHNOFACIES TRANSITION

Abstract A transitional Scoyenia–Mermia ichnocoenose from the Saint John River, Fredericton, New Brunswick, Canada, is dominated by elements of the Mermia ichnofacies, with traces comparable to Curvolithus, Helminthopsis, Gordia, Spirophycus, and Lockeia. Environmental characteristics are, however, more typical of the Scoyenia ichnofacies, with an emersion event providing conditions favorable to viewing traces preserved in a sand-softground substrate. Observation of in situ trace-making behavior allowed traces to be attributed to their progenitors, which include unionid and sphaeriid bivalves. An omission assemblage of vertebrate tracks also was present, comprising gull, raven, and mink. Oichnus-like borings were observed in some unionid shells. The shallow-tier trace assemblage created in a high-energy river channel may be expected to have a poor preservational potential, with loss of trace definition observed at the water margin during emersion and subsequent deterioration by eolian sediment transport.

Newly Discovered Sauropod Dinosaur Tracks with Skin and Foot-Pad Impressions from the Upper Jurassic Morrison Formation, Bighorn Basin, Wyoming, U.S.A

A recently discovered dinosaur tracksite from the Upper Jurassic Morrison Formation, Bighorn Basin, Wyoming, contains abundant sauropod tracks that exhibit varying degrees of preservation. Most of these tracks appear as indistinct bulges on the bottoms of sandstone beds, but several are well preserved and show foot-pad and skin impressions. Three track morphotypes are recognized: a sauropod pes print, a Brontopodus -like manus print, and a diplodocid manus print. The Brontopodus -like manus print most likely represents the footprint of a brachiosaur. This morphotype also contains evidence of phalangeal nodes—the first reported for a sauropod manus. The diplodocid manus print is unique because it contains impressions of a substantial ungual on digit I and a heel pad. A partial sauropod track cast also contains an impression of interlocking, polygonal scales. This is only the second known North American sauropod footprint that contains skin impressions. The spectrum of preservational quality of the tracks and associated trace fossils is used to infer the relative moisture content of the original substrate. Moisture content of the original substrate is estimated to have been moist to borderline saturated. Observations of the tracks at the study areas also are used to establish a list of features that can be used to distinguish deep vertebrate tracks from load casts resulting from gravity-induced soft-sediment deformation.

Lithofacies distribution of invertebrate and vertebrate trace-fossil assemblages in an Early Mesozoic ephemeral fluvio-lacustrine system from Argentina: Implications for the Scoyenia ichnofacies

This paper discusses the lithofacies distribution and ichnotaxonomic assignment of invertebrate, vertebrate and plant trace fossils in an Early Mesozoic ephemeral fluvio-lacustrine system from northwest Argentina. The overall palaeoenvironment is envisaged as a distal, low-gradient fluvio-lacustrine setting under semi-arid climate, similar to a terminal fan. Depositional lobes (facies association A) are composed of distributary ribbon channels (facies A1), and associated pond and sheetflood deposits (facies A2). Lowlands between depositional lobes (facies association B) contained poorly drained floodplain zones similar to wetlands (facies B1) and shallow closed lakes (facies B2). Depositional lobes were fringed by playa-lake settings (facies association C), including mudflats (facies C1) that are partially eroded by ephemeral fluvial channels (facies C2). The ichnofauna of the Santo Domingo Formation displays high ichnodiversity (30 trace fossil types) and includes 5 trace fossil assemblages that are linked to sedimentary facies. These trace fossil assemblages are highly distinctive as half of the recognised trace fossil types are assigned to different assemblages. The ephemeral fluvial channel assemblage (facies A1 and C2) displays the lower ichnodiversity ( n = 6) and includes Skolithos linearis, Diplichnites sp., Palaeophycus tubularis, Taenidium barretti, Dicynodontipus sp. and footprints in cross-section. The floodplain pond assemblage (facies A2) shows the higher ichnodiversity ( n = 15) and is composed of Diplichnites sp., Helminthoidichnites tenuis, P. tubularis, Rusophycus carbonarius, S. linearis, Spongeliomorpha carlsbergi, T. barretti, appendage marks, burrows with brush-like features, epichnial bilobed traces, hypichnial bilobed ridges, bird-like footprint type A, tridactyl footprints, footprints preserved in cross-section, and small epichnial rounded pits. The poorly drained floodplain assemblage (facies B1) is of moderate ichnodiversity ( n = 10) and consists of H. tenuis, S. carlsbergi, ? Spongeliomorpha sp., imbricated backfilled burrow fillings, root trace type A, Tetrasauropus sp., bird-like footprint type A, large pentadactyl footprints, tridactyl footprints, and footprints preserved in cross-section. The high ichnodiversity ( n = 13) nearshore lacustrine assemblage (facies B2) is the only assemblage that lacks vertebrate traces, and is composed of Cochlichnus anguineus, Cruziana problematica, H. tenuis, Palaeophycus heberti, P. striatus, P. tubularis, Scoyenia gracilis, S. carlsbergi, T. barretti, epichnial bilobed traces, hypichnial bilobed ridges, root trace type B, and scratch marks. The intermediate ichnodiversity ( n = 9) mudflat assemblage (facies C1) is typified by the largest ichnodiversity of vertebrate tracks and includes H. tenuis, P. tubularis, S. carlsbergi, Dicynodontipus sp., Tetrasauropus sp., bird-like footprints of types B and C, large pentadactyl footprints, and tridactyl footprints. The trace fossil assemblages from Santo Domingo Formation are typical representatives of the Scoyenia ichnofacies. The distinctiveness of the studied trace fossil assemblages suggests that a future division of the Scoyenia ichnofacies would be possible, provided that occurrences of invertebrate, vertebrate and plant trace fossils in definite sedimentary facies are considered. The subdivision of the Scoyenia ichnofacies into recurrent groups of trace fossils (ichnosubfacies) with a palaeoecological and palaeoenvironmental meaning can be based on two approaches: distinction of associations of vertebrate tracks and recognition of ichnocoenoses related to substrates with different degrees of water saturation and firmness. The first approach can be based on an evaluation of the recurrent vertebrate ichnoceonoses used to recognise vertebrate ichnofacies. In this way, some of the candidate vertebrate ichnofacies are proposed as subdivisions of the Scoyenia ichnofacies. The second approach is based on the identification of crosscutting relationships between trace fossils and between trace fossils and sedimentary structures within some trace fossil assemblages. Using this methodology, it is possible to define distinct assemblages of trace fossils: a “pre-desiccation suite” with structures lacking ornamentation and developed in a soft substrate and a “desiccation suite” typified by striated traces that crosscut the former. These trace-fossil assemblages help to characterise the changes in organism–sediment interaction during progressive desiccation of a water-lain substrate. Additional neoichnological studies are necessary to assess the exact meaning of different trace fossil features in terms of substrate consistency.

Calculation of Dynamic Spinal Ligament Deformation

Objective. Previous methods to determine spinal ligament deformation have included either custom-designed transducers or computational methods using rigid body transformation of kinematic data. Goals of the present study were to describe a computational methodology to determine dynamic deformations of an arbitrarily oriented ligament in a spine specimen and its associated errors. Methods. Calculation of ligament deformation in a spinal segment with vertebral motion tracking flags utilized digital stereophotography, lateral neutral posture radiograph, and detailed quantitative anatomy to develop geometrical relationships between flag markers and ligament attachment points. A custom jig, consisting of two flags each with four markers, was constructed to quantify errors associated with computed ligament deformation, flag marker translation, and flag rotation. Results. Average error in ligament deformation was dependent upon motion direction and ranged between 0.03 mm (SD 0.45 mm) and 0.28 mm (SD 0.18 mm). Average error for flag marker translation ranged between 0.02 mm (SD 0.14 mm) and 0.11 mm (SD 0.39 mm), and for flag rotation ranged between -0.06° (SD 0.17°) and 0.07° (SD 0.12°). Conclusions. Accuracy of the present technique was equivalent to or greater than that of previous methods. The present technique utilized relatively cost-effective digital stereophotography, and may be used to calculate strain in ligaments not readily accessible for transducer application. The methodology has wide-spread applicability for analyses of dynamic or static spinal or other ligament strains, and may be used to determine spinal canal and intervertebral foramen narrowing and area reduction.