Harding Sandstone Research Articles

The oldest three-dimensionally preserved vertebrate neurocranium

The neurocranium is an integral part of the vertebrate head, itself a major evolutionary innovation1,2. However, its early history remains poorly understood, with great dissimilarity in form between the two living vertebrate groups: gnathostomes (jawed vertebrates) and cyclostomes (hagfishes and lampreys)2,3. The 100 Myr gap separating the Cambrian appearance of vertebrates4–6 from the earliest three-dimensionally preserved vertebrate neurocrania7 further obscures the origins of modern states. Here we use computed tomography to describe the cranial anatomy of an Ordovician stem-group gnathostome: Eriptychius americanus from the Harding Sandstone of Colorado, USA8. A fossilized head of Eriptychius preserves a symmetrical set of cartilages that we interpret as the preorbital neurocranium, enclosing the fronts of laterally placed orbits, terminally located mouth, olfactory bulbs and pineal organ. This suggests that, in the earliest gnathostomes, the neurocranium filled out the space between the dermal skeleton and brain, like in galeaspids, osteostracans and placoderms and unlike in cyclostomes2. However, these cartilages are not fused into a single neurocranial unit, suggesting that this is a derived gnathostome trait. Eriptychius fills a major temporal and phylogenetic gap in our understanding of the evolution of the gnathostome head, revealing a neurocranium with an anatomy unlike that of any previously described vertebrate.

Provenance of Devonian–Carboniferous strata of Colorado: The influence of the Cambrian and the Proterozoic

ABSTRACTWe report new LA-ICP-MS U–Pb detrital zircon ages and sedimentary petrology of silty to sandy limestones and dolostones, as well as calcareous to dolomitic sandstones of the Devonian–Carboniferous (Mississippian) Chaffee Group. We also report new detrital zircon ages from the late Cambrian Sawatch Quartzite, and a U–Pb zircon crystallization age on a late Mesoproterozoic (1087.9 ± 13.5 Ma) granitoid of underlying basement from the Eagle Basin of northwest Colorado. Grain populations in the Chaffee Group are mostly bimodal. More than 84% of zircons centered around a Paleoproterozoic (ca. 1.78 Ga) mode typical of the Yavapai province that forms much of the basement of Colorado and an early Mesoproterozoic (ca. 1.42 Ga) mode typical of A-type granites that intrude this region. A notable late Mesoproterozoic (ca. 1.08 Ga) mode exists in some Chaffee samples, giving those samples a trimodal detrital zircon age distribution. These bipartite or tripartite detrital zircon age modes exist in Cambrian, Devonian, and Carboniferous strata from paleogeographically adjacent successions, but the correlation between the Chaffee zircons is highest with the region’s basal Cambrian sandstones of the Sawatch Quartzite, Flathead Sandstone, and Ignacio Quartzite, which have similar (ca. 1.08 Ga, 1.43 Ga, 1.70 Ga, respectively) zircon populations, and a paucity of > 1.8 Ga grains. This similarity suggests that most grains in the Chaffee Group derive from recycling of these basal sandstones, and that little sediment was derived directly from thenexposed Precambrian basement highs, from the Wyoming craton to the north, or from Paleoproterozoic arcs and orogens to the west and northeast. Minor Mesoarchean to early Paleoproterozoic (ca. 3.00 to 2.40 Ga) grains exist in the Chaffee Group, an attribute shared by the Late Ordovician Harding Sandstone of Colorado’s Front Range, but that is absent from the region’s underlying Cambrian sandstones—suggesting some recycled mixture of Cambrian and Ordovician sedimentary rocks. No near-depositional age grains are present in the Chaffee Group. The youngest grain is Early Devonian (~417 Ma), > 45 million years (m.y.) older than these strata. Additionally, Paleozoic grains are extremely uncommon (< 0.1%; n = 2,927 grains).

The systematics of the Mongolepidida (Chondrichthyes) and the Ordovician origins of the clade.

The Mongolepidida is an Order of putative early chondrichthyan fish, originally erected to unite taxa from the Lower Silurian of Mongolia. The present study reassesses mongolepid systematics through the examination of the developmental, histological and morphological characteristics of scale-based specimens from the Upper Ordovician Harding Sandstone (Colorado, USA) and the Upper Llandovery–Lower Wenlock Yimugantawu (Tarim Basin, China), Xiushan (Guizhou Province, China) and Chargat (north-western Mongolia) Formations. The inclusion of the Mongolepidida within the Class Chondrichthyes is supported on the basis of a suite of scale attributes (areal odontode deposition, linear odontocomplex structure and lack of enamel, cancellous bone and hard-tissue resorption) shared with traditionally recognized chondrichthyans (euchondrichthyans, e.g., ctenacanthiforms). The mongolepid dermal skeleton exhibits a rare type of atubular dentine (lamellin) that is regarded as one of the diagnostic features of the Order within crown gnathostomes. The previously erected Mongolepididae and Shiqianolepidae families are revised, differentiated by scale-base histology and expanded to include the genera Rongolepisand Xinjiangichthys, respectively. A newly described mongolepid species (Solinalepis levis gen. et sp. nov.) from the Ordovician of North America is treated as family incertae sedis, as it possesses a type of basal bone tissue (acellular and vascular) that has yet to be documented in other mongolepids. This study extends the stratigraphic and palaeogeographic range of Mongolepidida and adds further evidence for an early diversification of the Chondrichthyes in the Ordovician Period, 50 million years prior to the first recorded appearance of euchondrichthyan teeth in the Lower Devonian.

The Sequence Stratigraphic and Environmental Context of Primitive Vertebrates: Harding Sandstone, Upper Ordovician, Colorado, USA

The depositional environment of the Upper Ordovician Harding Sandstone of central Colorado has been debated since Charles Walcott first described its pteraspidomorph agnathans. Although most previous workers have favored a marine setting for the Harding based on the presence of marine invertebrates and burrows, some have argued for a fluvial or estuarine habitat for these primitive vertebrates. This study presented here is the first regional, comprehensive analysis of the depositional environments and sequence stratigraphy of the Harding Sandstone. The Harding contains deposits from an array of coastal environments, including transition zone, shoreface, bay/lagoon, washover fan, and bayhead delta. Features of these environments suggest that the Harding was deposited on a microtidal coast with low wave energy. Within the study area, two depositional sequences are developed, and each apparently lacks a lowstand systems tract. The Harding contains numerous flooding surfaces, which preclude any attempt to interpret the Harding as a single depositional environment. Flooding surfaces commonly display evidence of stratigraphic condensation, including burrowed firmgrounds, concentrations of pteraspidomorph plates, and dolomitic cementation. Pteraspidomorph remains are associated most commonly with shoreface facies, are uncommon in bay/lagoon facies, and are absent from bayhead-delta facies. Collectively, these occurrences argue for a shallow-marine habitat for these primitive vertebrates.

Late Ordovician vertebrates from the Bighorn Mountains of Wyoming, USA

Abstract: Late Ordovician vertebrate faunas occur in clastic sedimentary units along the length of the Rocky Mountains from Colorado to Montana, and across the border into Canada. Most research has, however, been conducted on localities in the southern part of the outcrop belt, particularly the Harding Sandstone Formation of Colorado. Micropalaeontological sampling of the coeval South Piney Member (Winnipeg Formation) in the Bighorn Mountains of Wyoming has revealed an abundant vertebrate fauna. The vertebrate assemblage includes a low‐abundance fauna of 13 conodont taxa that together indicate an undatus Chronozone age (mid‐Mohawkian; mid‐Caradoc; Late Ordovician). The pteraspidomorphs Astraspis desiderata Walcott and Eriptychius americanus Walcott are also present together with one new taxon, Eleochera glossa gen. et sp. nov., which is interpreted as a derived stem‐gnathostome on the basis of its scale histology and morphology. The fauna bears a strong similarity to that of the Harding Sandstone but is of lower diversity. In particular, it lacks the fine‐grained, deeper water component of the Harding Sandstone that contains, inter alia, thelodonts and stem‐chondrichthyans.

A review of placoderm scales, and their significance in placoderm phylogeny

ABSTRACT An historical review of the literature relating to placoderm scales preserved in association with articulated dermal plates, or as isolated units in microvertebrate assemblages, is followed by a discussion of their relevance in phylogenetic analyses of the Placodermi. The dentinous tissue forming the tubercles of Early Devonian acanthothoracid scales and dermal bone is similar to that of the dermal bone ornament of some osteostracans, and denticles of the vertebrate Skiichthys from the Ordovician Harding Sandstone. This similarity supports the proposition that the gnathostomes are the sister-group of the Osteostraci, with the Placodermi branching earliest within the gnathostomes, and the Acanthothoraci branching earliest within the Placodermi. The meso-semidentine in acanthothoracid tubercles, rather than semidentine (sensu stricto), is most likely to be synapomorphic for the Placodermi.

Scales of thelodont and shark-like fishes from the Ordovician of Colorado

THE phylogeny of primitive vertebrates has been investigated over the past fifteen years1–3, and there has been a notable increase in the amount of data by which such phylogenies can be tested4–7. Stratigraphic data should provide constraints to such analyses, and we record here descriptions of putative shark and thelodont scales from the Harding Sandstone (Caradoc, Late Ordovician) of Colorado. The fossil record of sharks is extended back from the Llandovery (lower Silurian) by some 25 million years and that of thelodonts by 10 million years from the Ashgill (Late Ordovician). The new data indicate that a major radiation of lower vertebrates took place during the Ordovician.

Les vertébrés des grès de harding: Ce que Vaillant a pu observer

In a short note published in 1902, Vaillant briefly described, without illustrations, “true” bone tissue with osteocytic lacunae in small vertebrate fragments from the Harding Sandstone (Ordovician of Colorado). Recent examination of the actual thin sections used by Vaillant allows to recognize, in addition to Astraspis and Eriptychius, occurence of “vertebrate inderteminate A” of Denison (1967), which confirms the value of Vaillant's observations. Moreover, other fragmentary remains observed in the thin sections may correspond to other taxa recently described or currently beeing investigated by Dr. Moya M. Smith and her coworkers. Our observations confirm the need to keep the old histological sections, obviously in Paleontology but also in Biology, as valuable documents both for scientific and patrimonial purposes. Organisation of “histothèques” taking full advantage of current progress in computerized recording and transportation of information should fullfil this need.

The harding sandstone revisited -a new look at some old bones

Although the Harding Sandstone (basal Franklinian, Caradoc, Ordovician) of Canon City, Fremont County, Colorado, U.S.A., has been known for over one hundred years, recent fieldwork in the type area and the preparation of new material has yielded a substantial amount of new data on the vertebrate fauna and palaeoecology of the unit. The preliminary results of this work include a revision of the diversity of fish present during the Lower Caradoc, with major implications for the timing of the first diversification of the fishes.

A new locality of fossiliferous Harding Sandstone: evidence for freshwater Ordovician vertebrates

ABSTRACT The Ordovician Harding Sandstone has played an important role in studies of the environment of the earliest types of “ostracoderms” (armored jawless fish-like vertebrates). Such studies have concentrated on fossil collections from the type locality, a quarry near Canon City, Colorado, which contains a mixed fossil fauna of “ostracoderms” and marine invertebrates. This co-occurrence has been used in the past as evidence that the vertebrates from the Harding Sandstone were marine animals. Fossil associations alone, however, do not provide sufficient data for paleoenvironmental interpretations. It is also necessary to analyze depositional facies. A previously unpublished locality of the Harding Sandstone allows interpretations of depositional environments by sedimentary facies analysis. Vertebrate fossils of the Harding Sandstone are restricted mainly to supratidal and, to a lesser extent, intertidal deposits. They are most abundant in channelized, granular conglomerates, units devoid of marine inve...

A Reassessment of Astraspis desiderata , the Oldest North American Vertebrate

The fossil evidence for the early evolution of vertebrates consists of the remains of agnathans, jawless vertebrates, from the Ordovician of Australia, North America, and Bolivia. Because of the fragmentary nature of the material, it has not been possible to reconstruct these animals sufficiently well to understand their relation to each other and to later vertebrates. Though two genera have been known from the Harding Sandstone of North America since 1892, there are still only three articulated specimens known, two of Astraspis desiderata and one of Eriptychius americanus. The most recently found specimen of Astraspis was reexamined and found to show the orbit, a series of eight branchial openings and a complete tail, structures hitherto undescribed in any Ordovician vertebrate. A reconstruction ofAstraspis shows, on the basis of the series of branchial openings, that it is a primitive craniate and not a heterostracan as previously thought.

The palaeoecology of the Ordovician Harding Sandstone (Colorado, U.S.A.)

The Middle Ordovician Harding Sandstone in Colorado, U.S.A. is the oldest formation known to have an environment rich in vertebrate fossils. In order to study the palaeoecology of these oldest vertebrates, a detailed analysis of both the sediments and the associated invertebrate fauna was made. Undulatory quartz, sandstone mineralogy, geochemistry, trace fossils, and ecological implications of the invertebrate fossils have been studied. The type of environment is not a typical one, but was probably a shallow-water marine (intertidal, or shallow subtidal), in a lagoon-like or estuarine—deltaic habitat, with changing salinity, and a generally warm climate.

An early Ordovician vertebrate

HETEROSTRACAN fishes are the oldest known fossil vertebrates, and well preserved examples are frequently found in rocks of late Silurian and Devonian age, but in older rocks their remains are fragmentary and rare. Their oldest occurrence yet described is in the Harding Sandstone, Colorado (middle Ordovician1,2), where two genera occur; one of these has subsequently been recognised in marine limestones of similar age from Ontario, Canada3.

Middle Ordovician Fish-bearing Beds from the St. Lawrence Lowlands of Quebec

Ostracoderm tubercles were recovered from the lower portion of two Black River Group sections between Montreal and Quebec City. Some of these fish remains seem identical to tubercles of Astraspis desiderata from the Harding Sandstone of Colorado. The age of the Quebec remains is questionably earliest Blackriveran or basal Caradocian of the European standard. The remains were found in sandy carbonates probably laid down in the supratidal to shallow marine environments. It is concluded that these remains may represent part of a continent-wide, biostratigraphically useful vertebrate fauna and that bulk sampling and acid residuing might be a technique whereby sparse, fragmentary, earliest Paleozoic fish remains could be found.

A Second Specimen of Youngquistina mitteni from the Harding Sandstone (Ordovician) of Colorado

Introduction Yogngqgi.rtina was described by Miller (1969) as a new genus of undetermined chordate. Miller compared the specimen to Cullison's (1938) genus Archaeognvzhs, and concluded that the specimens differed generically, but were related enough to be included in the Family Archaeognathidae Miller 1969, and that the comb-like archaeognathids had an as yet undetermined relationship to conodonts and other Paleozoic chordate remains. In that four archaeognathid specimens comprise the only known remains of a rare, and largely unknown early chordate group, the authors feel that the discovery of this specimen and its description warrant publication.

Youngquistina mitteni, New Genus and Species of Chordate from the Harding Sandstone (Ordovician) of Colorado

Youngquistina mitteni, New Genus and Species of Chordate from the Harding Sandstone (Ordovician) of Colorado

Paleozoic stratigraphy of the Sawatch Range, Colorado

The Sawatch Range, a high and rugged mountain uplift in central Colorado, separates the Arkansas River valley from the plateau region to the west. It includes some of the highest peaks in Colorado, with summit elevations around 14,400 feet. Very little of the region lies below 9000 feet. The rocks range from pre-Cambrian to Recent but consist mainly of pre-Cambrian granite and metamorphics and Paleozoic sedimentary rocks. All are cut by Tertiary intrusives. The Paleozoic sedimentary rocks represent every period except the Silurian. The Cambrian system is represented by the Sawatch quartzite, which attains considerable thickness locally and can be roughly subdivided into four members. The Ordovician rocks include three formations—the Manitou dolomite, Harding sandstone, and Fremont limestone. The Manitou, consisting of more or less siliceous dolomite and dolomitic limestone, is widespread and varies greatly in thickness. The Harding sandstone, represented by quartzite in part of the area, is thin and found only along the southern end of the range and along Cement Creek. The Fremont limestone is predominantly fossiliferous dolomite and is found only in the southern part of the Sawarch Range province. Unconformities occur at the top of each of the Ordovician formations, and the formations thin or wedge out to the north. The Devonian sedimentary rocks included in the Chaffee formation are widespread and include the shaly or sandy Parting member below and the Dyer dolomite member above. The Devonian sediments are discussed at length as many new data are presented. The Leadville limestone of Mississippian age unconformably overlies the Chaffee and is an important ore-bearing formation at Aspen and in other mining districts along the range. Another unconformity separates the Leadville from the Pennsylvanian Weber (?) formation above. The latter generally consists of shales in the lower parts, above which is a series of limestones and shales, overlain by sandy shales or sandstones. These beds grade upward into a series of red sandstones, grits, and conglomerates with some interbedded limestones, known as the Maroon formation, which includes both Pennsylvanian and Permian deposits. At present the boundary between the Pennsylvanian and Permian beds of this region has not been definitely drawn on either lithologic composition or color. The lower beds contain abundant fossils, but fossils become scarcer at higher stratigraphic positions. The information available suggests that the Pennsylvanian series may be 2000 to 2500 feet thick. Devonian fish fragments were obtained from the lower member of the Chaffee including the basal layers. A coral fauna, obtained from the upper Leadville limestone at several localities, suggests the presence of some Mississippian beds younger than those hitherto reported. Structures suggesting growths of calcareous algae were noted in the Manitou and the Chaffee, and well-preserved fossil algae were obtained from the upper Leadville limestone and lower Weber (?) formation. The paper includes 24 measured sections.

The Harding sandstone of Colorado

The Harding sandstone of Colorado