Artifact Of Preservation Research Articles

Organic membranous skeleton of the Precambrian metazoans from Namibia

Unlike the celebrated Ediacara fossils, those from the roughly coeval localities of the Kuibis Quarzite of Namibia are preserved not as imprints on the sandstone bedding plane, but three-dimensionally, within the rock matrix. The pattern of deformation and the presence of sand in lower parts of the bodies of Ernietta , the most common and typical of those organisms, indicate that their three-dimensional preservation is a result of a density-controlled sinking of sand-filled organic skeletons within hydrated mud layers. Specimens of Ernietta have preserved various stages of migration across the mud beds. Their wall material, as documented by the mode of deformation, was not only flexible, but also elastic, which makes it unlike chitin. The walls thus seem to be proteinaceous, built probably of a collagenous fabric. The Ernietta skeleton was built of series of parallel chambers, which excludes the possibility that these were external body covers. The chambers apparently represent walls of hydraulic skeleton units, resembling the basement membrane of chaetognaths or the notochord sheath of primitive chordates. Such chambers are widespread among the earliest fossil animals represented by fossils preserved in sandstone. The rise and fall of the Ediacaran faunas thus seem to be partially preservational artifacts. The range of its occurrence is a result of two successive evolutionary events: the origin of an internal hydraulic skeleton enclosed by a strong basement membrane, and the appearance of decomposers with abilities to disintegrate such collagenous sheaths.

Mutations in BRCA1 from Fixed, Paraffin-Embedded Tissue Can Be Artifacts of Preservation

DNA isolated from paraffin-embedded tissues has been used for analysis of DNA alterations in disease states. Use of archival tissue can expedite the gathering of large numbers of specimens from rare disease subtypes that would take years to accumulate prospectively. Therefore, archival tissues from 70 ovarian cancer cases diagnosed before or at age 40 were retrieved for analysis of BRCA1 mutations. DNA was isolated from paraffin-embedded tissue of 70 ovarian cancer cases diagnosed before or at age 40. BRCA1 mutation analysis was conducted by single-strand conformation polymorphism analysis and DNA sequencing. Fifty-eight BRCA1 mutations were found in 34 of the 70 ovarian cancer cases. Twenty-two cases had one mutation each and 12 cases had multiple mutations. Multiple mutations found in histologically normal tissue of 2 cases were not present in matched tumor tissue. For another case, DNA from two separate blocks of normal tissue contained different mutations. These observations were anomalous and suggested that mutations detected in fixed tissues may be artifacts of tissue preservation and not present in the original unfixed tissues. To test this suggestion, blood was obtained from 2 patients for whom mutations were found in fixed, normal tissue. DNA from their unfixed lymphocytes did not contain the mutations found in fixed normal tissue. Thus, mutations found in fixed, paraffin-embedded tissues can be artifacts of tissue preservation. The reliability of DNA sequence data derived from such tissues must be questioned in the absence of corroborating data from unfixed tissues. This severely limits the use of fixed tissues as a source of DNA for retrospective research and for clinical genetic testing in families for which a disease-affected member is not alive.

Hiatuses on Vøring Plateau: sedimentary gaps or preservation artifacts?

The sediments recovered on ODP Leg 104 have been reported to be characterized by hiatuses. The hiatuses were defined by biostratigraphy and were believed to be caused by erosion related to temporary changes of bottom current composition and velocity. They have been associated with major paleoenvironmental changes, reorganization of global deep water production, and increased bottom water flows. Because of the importance of hiatuses for ongoing research, we decided to closely investigate the sedimentation history for the most significant Pliocene and Miocene biostratigraphic hiatuses by sedimentologic and geochemical means. The sedimentologic studies include clay mineral distributions, grain size data, and organic carbon concentrations. The geochemical studies include determination of 87 86 Sr ratios, 10Be and Ir concentrations. The results of the sedimentologic studies suggest either that paleoenvironmental changes associated with hiatuses are not represented in the preserved sediments, or that the hiatuses are an artifact of interpretation of the biostratigraphic data. Strontium isotopes indicate continuous sedimentation for the interval investigated at Site 642, an interpretation confirmed by the steady decline in 10Be. 87 86 Sr ratios in the interval from above and below proposed hiatuses H 2.2 2.3 and H2.1 2.2 at Site 643 display stronger changes with depth than expected by steady sedimentation. Ir data for this same interval indicate reduced sedimentation rates. Combining both, sedimentologic and geochemical evidence, the proposed hiatuses could not be confirmed and may represent preservation artifacts.

A Velociraptor wishbone

The ‘wishbone’ of birds comprises two clavicles fused into a structure1 known as a furcula. In an influential 1926 book on bird origins by Heilmann2, the furcula's supposed absence in dinosaurs was considered powerful evidence barring them from bird ancestry. A furcula has now been found in several theropod dinosaurs3,4, but its absence in other theropods, and the uncertainty of whether this absence is real or an artefact of preservation, obscures the evolutionary history of this structure5. Here we report the discovery of a furcula in Dromaeosauridae, a group posited to be the closest relative of birds6,7.

A Late Cretaceous nodosaurid ankylosaur (Dinosauria: Ornithischia) from marine sediments of coastal California

A partial nodosaurid ankylosaur skeleton, consisting primarily of the ilia, hindlimbs, posterior dorsal armor, plus partial forelimb elements and additional armor, was recovered from the marine Point Loma Formation, late Campanian age, north of San Diego, California. The specimen is similar to contemporaneous species ofPanoplosaurusandEdmontoniafrom terrestrial sediments of the western interior, but there are also similarities to the armor ofStegopelta landerensisfrom marine sediments of earliest Cenomanian age from Wyoming. Skeletal elements critical for generic determination are not preserved, and the specimen is identified as Nodosauridae, incertae sedis. An associated marine invertebrate fauna and nannoplankton flora have revealed some discrepencies in the correlation systems used for Upper Cretaceous marine rocks of coastal California. Hollow limb bones of the specimen are interpreted as a preservational artifact. Nodosaurids had broad ecological tolerances, and visited riparian and coastal environments more frequently than other dinosaurs. However, a review of morphologic and distributional evidence fails to support a theory of amphibious or aquatic habits for nodosaurids.

Seven tephra isochrones in Scotland

This paper reports the presence and geochemical characteristics of seven separate tephras in Scotland that fell in AD 1510, and about 450 BP, 2100 BP, 3600 BP, 3830 BP, 5600 BP and around 6000 BP. The results come from 14 peat bogs spread throughout the Highlands and Islands where 26 tephra deposits have been characterized using over 400 grain-discrete electron-probe micro-analysis. Firstly, tephras are identified and defined by major element characteristics at a reference site. Secondly, tephras are linked between sites on the basis of geochemistry, and dated. Informal names, based on British sites, are suggested for tephras not yet linked to precise sources in Iceland. Despite some apparent variation in colour, all analysed grains in these seven deposits are silicic or intermediate in composition (SiO2 >57%). This phenomenon does not appear to be an artifact of preservation for two reasons. First, small (<40 μm) basic glass shards in peat bogs in northern Iceland are found to have undergone minimal alteration over the last 6000 years; so it is unlikely that basic glass of this age or less in British peat bogs has dissolved since its deposition. Second, tests have shown that the extraction technique used to remove tephra from peat does not significantly alter the mass or particle-size distribution of these glass shards. This study stresses the advantage of using a geographical range of sites to build a regional tephrochro nology in distal areas of volcanic fallout, because there is frequently great local variability in tephra distribution. In the five years since tephras were first identified in northern Britain it has become apparent that tephrochronology can make significant and exciting new contributions to environmental studies throughout this region.

Additional Specimens of the Western Cottonmouth (Agkistrodon piscivorus leucostoma, Reptilia: Squamata) from Kansas

The collection of two Western Cottonmouths (Agkistrodon piscivorus leucostoma) from Cherokee County in 1991 is reported. The history of this taxon in Kansas is discussed and previous records are discredited. The collection of the latest specimens confirms the presence of this taxon in extreme east-central Cherokee County. The history of Agkistrodon piscivorus leucostoma in Kansas is a curious one. Although the taxon had been reported from the state as early as 1937 (Hall and Smith, 1947) and as recently as 1977 (Collins, 1977; Perry, 1977; Rundquist et al., 1978), no undoubtedly valid specimens have been collected from Kansas (Collins, 1982). We herein report on two specimens of A. p. leucostoma collected in 1991 that establish this species' existence in Kansas. On 14 September 1991, an adult male A. p. leucostoma (KU 218677) was taken by Shane Eckhardt at ca. 4.8 km E of the jct. of U.S. route 69 and Kansas route 160, Cherokee County, extreme southeastern Kansas. The animal was found in late afternoon at a litter-strewn parking area approximately 125 m W of the Spring River. Associated habitat is mixed hardwood deciduous forest (primarily Quercus and hickory) in the Spring River floodplain in the Cherokee Plain physiographic province (Kiichler, 1974). The specimen (KU 218677) is an adult male with an SV of 778 mm, TL 925 mm. Meristic characters are as follows: ventrals133, subcaudals-45, undivided subcaudals -24, supralabials-8-undet. (damaged), infralabials 9-10, scale row formula--23-25-21. The dorsal pattern is normal for A. p. leucostoma, as is the ventral pattern. There is damage to the right mandible and a portion of the venter in the anterior quarter of the specimen. As a consequence of news media publicity connected with KU 218677, This content downloaded from 157.55.39.208 on Fri, 14 Oct 2016 04:07:07 UTC All use subject to http://about.jstor.org/terms VOLUME 96, NUMBERS 1-2 149 another specimen found earlier was reported to us by Mr. Eckhardt. On 29 August 1991, an adult male A. p. leucostoma (KU 218780) was collected at 1.6 km S, 3.4 km E of the junction of U.S. route 69 and Kansas route 160, Cherokee County, Kansas, by Ken Outt. The animal was found near a small concrete culvert that runs under a gravel road. Associated habitat was cultivated fields to the north of the road and mixed hardwood deciduous secondary growth forest immediately to the south. While recovering this snake, Mr. Outt was bitten by it on the first phalange of his right index finger and was mildly envenomated. This second specimen is an adult male with an SV of 600 mm, TL 705 mm. Meristic characters are as follows: ventrals136, subcaudals-43, undivided subcaudals--17, supralabials--8-8, infralabials11-11, scale row formula 24-24-2 1. The dorsal pattern is somewhat more distinct than normally seen in living adult A. p. leucostoma, but this may be an artifact of preservation. The ventral pattern is normal. The specimen has sustained severe midbody trauma, apparently after having been run over by a vehicle. In addition, there are three major dislocating fractures of the vertebral column anterior to the midbody trauma.

Experimental taphonomy of embryo preservation in a Cenozoic brooding bivalve

To distinguish between alternative explanations for the presence of synchronous broods in the Miocene-Pliocene bivalve, Transenriella species. we performed in situ burial experiments of the Recent species T. corfusa. All Recent Transennella species are asynchronous brooders; a single brood contains all or most developmental stages. Specimens from Miocene—Pliocene deposits of California suggest that some members of this taxon were synchronous brooders, i.e., all the embryos of a brood develop simultaneously with only one developmental stage represented at any time. The presence of synchronous Transennella broods in the Miocene—Pliocene could indicate that an evolutionary change in mode of reproduction has occurred in this genus. Alternatively, asynchronous brooding in this taxon may he conservative and preferential preservation of later stages of development, or seasonal variation in reproduction, could result in a taphonomic overprint. Our burial experiments indicate that the earliest stages of development are almost entirely lost; however. there is enough preservation of the later stages of development to distinguish the two modes of reproduction. Additionally. we discovered a single fossil specimen with an asynchronous brood. Based primarily on this specimen, and observations from the burial experiments, we conclude that the fossil synchronous broods are an artifact of preservation and asynchronous brooding in Transenella is conservative.

Temporal distribution of carbonatites

About half of the 330 occurrences listed in the global inventory of carbonatites have been dated, mostly by the K-Ar method, and they fall into groups that appear to be temporally and spatially linked to major orogenic events. Overall, however, the temporal pattern is one of exponential increase in frequency with time. This secular trend has been interpreted as an increase in carbonatite activity in the course of terrestrial evolution. In an alternative interpretation, the observed trend may be viewed as an artifact of preservation, because the probability of preservation decreases exponentially with increasing age of the crustal segments. The secular trend for carbonatites since at least 1.8 Ga, the time that continental crust attained its near-modern extent, is compatible with a steady-state process of carbonatite generation and destruction that has a half-life of ∼445 m.y. The most acceptable model for the temporal distribution of carbonatites involves a combination of orogenic activity and erosional dispersal.

Time averaging and temporal persistence in chemoautotrophic molluscan-dominated death assemblages on the Louisiana continental slope

Petroleum seeps on the Louisiana continental slope produce luxurious communities based on chemoautotrophic symbiotic bacteria. Petroleum seeps are typical cold seep communities and generate classic autochthonous death assemblages. Fossil cold seep communities are well-known, common and widespread. Most fossil analogues are dominated by lucinids, but petroleum seeps are not. A combination of sample collection and in situ experimentation has been used to determine rates of taphonomy and time averaging in petroleum seep assemblages.Cores were obtained and sectioned from seep sites in Green Canyon lease blocks 272, 234, and 184 and Garden Banks lease block 386. Lucinids and thyasirids were selected for dating time-since-death to determine the importance of time averaging in these assemblages. Dating was accomplished by measuring the free amino acid content of the shells. Time-since-death became progressively older with depth; accordingly little time averaging had occurred in these autochthonous assemblages. Lucind and mussel shells were placed on the sea floor and recovered 3 yr later. Comparison of each species to the controls left on a laboratory shelf for 3 yr shows that taphonomic alteration was rapid. Mussels were more severely altered than lucinids. Mussels were more heavily dissolved, had more altered edges, were more prone to fragmentation and exhibited greater weight loss than lucinids. The rapid taphonomic loss of the mussels suggests that the preponderance of lucinids in the fossil record is an artifact of preservation. Taphofacies analysis suggests the same; thus verifying an important assumption of taphofacies analysis that taphonomic signatures record biases in preservation by identifying inter-species differences in the rates of important taphonomic processes. Significant variability in taphonomic rates exists between shells from locations 10 m apart indicating significant local variation in the taphonomic process. Local variability in taphonomic and community attributes is characteristic of many autochthonous assemblages.

Environmental Patterns in the Origins of Higher Taxa: The Post-Paleozoic Fossil Record

Of the 26 well-preserved orders of benthic marine invertebrates that have originated since the beginning of the Mesozoic, 20 first appear in onshore environments. This distribution differs significantly from that shown by well-preserved genera and families, and by the 16 poorly preserved orders. These discordances suggest that the pattern of preferential onshore origination is not an artifact of preservation or collection and that the origin of higher taxa cannot be regarded as a simple extrapolation of rates and patterns at lower levels. The onshore environment fosters production or enhances survivorship of species that initiate lineages tending to accumulate suites of derived characters and that thus are ultimately afforded high taxonomic rank.

Species recognition and sexual dimorphism in Proconsul and Rangwapithecus

The taxonomy of the early Miocene hominoid genera Proconsul and Rangwapithecus is re-examined. The recognition of sexual dimorphism and the identification of sex dimorphs among canine specimens is made an important element in the sorting of specimens into phena and the allocation of these to species hypodigms. I propose that specimens from Rusinga and Mfwangano previously identified as Proconsul africanus , including the well known 1948 skull, are actually females of P. nyanzae. P. africanus specimens from Songhor and the type site of Koru differ from Rusinga and Mfwangano specimens formerly placed in P. africanus . Accordingly, P. africanus sensu stricto is restricted to Koru and Songhor and Proconsul nyanzae is restricted to Rusinga and Mfwangano. Previous characterizations of P. africanus were based almost entirely upon specimens from Rusinga, and should therefore now be applied to female P. nyanzae instead; the paleobiology of P. africanus and P. nyanzae must now be reconsidered. As a result of the reassignment of the Rusinga Proconsul specimens, P. nyanzae exhibits degrees of postcanine metric variability exceeding those of living catarrhines. It is suggested that this reflects in part a true species characteristic, perhaps related to an exceptional amount of size dimorphism, and in part a preservational artifact relating to the time-accumulated nature of the fossil sample. These results have implications for the systematics of other Miocene hominoid species.

Taphonomy and ichnology of cephalopod shells in a Maastrichtian chalk from Western Australia

The post-mortem history of a prolific Maastrichtian ammonite and nautiloid fauna preserved as phosphatic steinkerns in chalk of the upper Miria Formation of Western Australia is described. Sediment infilling of phragmocones, required for their fossilisation, was accomplished by means of perforations in the shell wall induced by the activity of abundant endoliths. These include borings ascribed to clionid sponges (Entobia), thallophytes, polychaete worms (including Caulostrepis and probable Maeandropolydora), phoronids (Talpina and Gnathichnus) and others of conjectural origin. Sediment infilling by this mechanism is considered to be more applicable to the taphonomy of phragmocones in general than sediment entry through the siphuncle and to be indicative of low sedimentation rates for the hosting strata. Nonheteromorph ammonites, and the nautiloid Cimomia, are preserved almost exclusively as phragmocones in the upper Miria Formation, and are numerically subordinate to the heteromorph Eubaculites which, together with Glyptoxoceras, is preserved predominantly as body chambers. This phragmocone/body chamber preservational contrast is attributed to the influence of shell shape on pre-burial mechanical abrasion. The apparent dominance of Eubaculites is considered to be largely a preservational artifact and ascribed to the ease with which body chambers were infilled, and thereby favoured for steinkern formation, relative to phragmocones. Phragmocones, or parts thereof. not filled with sediment were eliminated from the fossil record by diagenetic aragonite dissolution. Many ammonites with open umbilici have the early whorls missing. This we attribute to the trapping of sediment in the umbilicus prior to burial, preventing endolith attack whereby the early whorls avoided a sediment infilling. However, the protected inner whorls of involute ammonites and Cimomia, which have closed umbilici, and the inner whorls of evolute ammonites where a cemented umbilical plug supported the mouldic cavity left by shell dissolution, were preserved and were commonly infilled with calcite spar later in diagenesis. The dearth of cephalopod fossils in chalk underlying the upper Miria Formation is ascribed to diagenesis in which aragonite dissolution was not preceded by cementation.

Accuracy and Precision in Anuran Morphometrics: Artifacts of Preservation

Lee,J. C. (Department of Biology, University of Miami, Coral Gables, Florida 33124) 1982. Accuracy and precision in anuran morphometrics: artifacts of preservation. Syst. Zool., 31: 266-281.-Univariate comparisons of twenty morphometric characters of sixty adult Bufo marinus recorded before preservation and six and 14 months after preservation reveal a statistically significant heterogeneity among group means of fourteen characters. After six months in preservative, the means of six of these 14 characters are significantly larger, and eight are significantly smaller, than for the same individuals prior to preservation. After eight. additional months in preservative, the means of five of these 14 characters differ significantly in the opposite direction of that initially detected. Linear regressions and analyses of covariance suggest that over the range of sizes considered here, preservation effects the elevations but not the slopes of the regressions between pairs of morphometric characters. Sexual size dimorphism in six characters is documented for the fresh material; certain of these intersexual differences are reduced or are undetectable in the preserved material, and in one instance a new, marginally significant, difference is apparent after 14 months in preservative. Not all characters are measurable with equal precision, and there is a highly significant rank correlation between precision and inter-individual character variance. Principal component analyses confirm and extend the univariate findings, and discriminant function analysis reveals differences between fresh and preserved specimens of roughly the same magnitude as those commonly used to justify taxonomic decisions. The assumption that preservation induces tolerably little morphological distortion in frogs appears to be substantially violated, but an unknown proportion of among-group character variation may be attributable to measurer bias. [Bufo marinus; univariate analysis; discriminant function analysis; principal component analysis; morphometrics; accuracy; precision.]

Accuracy and Precision in Anuran Morphometrics: Artifacts of Preservation

Accuracy and Precision in Anuran Morphometrics: Artifacts of Preservation

OBSERVATIONS ON THE FINE STRUCTURE OF CALLIXYLON WOOD

Secondary wood of three species of Callixylon of Lower Mississippian age, preserved by three different modes (fusinization, silicification, and phosphatization), have been studied and characterized in detail. Problems of interpretation at the SEM level of permineralized woods, both containing (silicified wood of Callixylon erianum) and essentially lacking (phosphatized wood of C. arnoldii) original organic cell wall material, are analyzed and discussed. In particular, it is concluded that the flat to curved surfaces showing no evidence of apertures, observed in bordered pit pairs, commonly represent pit membrane surfaces. It is accepted, however, that some concave surfaces might be the mineral accretion surfaces of incomplete pit cavity casts as proposed by Leo and Barghoorn (1976). Regions between groups of pits, previously interpreted as crassulae, may be artifacts of preservation. The fusinized wood has the general appearance of charcoal, but unlike commercially produced charcoal provides evidence of its original microfibrillar structure. The origin of fusain in the fossil record is discussed, and it is concluded that it probably had several origins, including forest fire. Since charcoal can be produced naturally in the absence of O2 (Brown and Davis, 1973), the suggestion that fusain (charcoal) in the geologic column provides a basis for “assessing oxygen levels in paleoatmospheres” (Cope and Chaloner, 1980) is not supported. Natural sites of fusain production in the absence of O2 are regions of vulcanism and organic sediments inhabited by anaerobic microorganisms. A circular pattern of crystal orientation in the pit borders of C. arnoldii is interpreted to represent the original microfibrillar pattern. Pit apertures in C. arnoldii are shown to be circular to slightly elliptical. Interpretive evidence of heterogeneous pit membranes in C. arnoldii suggests but does not prove the presence of a torus. The distinctive central region in some pit membranes of the fusinized wood of Callixylon sp. might represent accumulations of waste metabolites. It is argued that a torus would be highly adaptive in large pits with circular apertures.

PITTING IN PSILOPHYTON DAWSONII, AN EARLY DEVONIAN TRIMEROPHYTE

Tracheids of Psilophyton dawsonii Banks, Leclercq, and Hueber 1975 from calcareous pebbles of late Early Devonian age on the Gaspé Peninsula, eastern Canada, are shown to have scalariform bordered and circular bordered pits. Macerated tracheids embedded in Spurr epoxy resin and sectioned at 2 μm; peels, ground sections, SEM and petrographic observations all indicate that interconnections between the scalariform bars are composed of secondary wall material and that the patterns produced are not artifacts of preservation. The interconnections produce a wide range of patterns from simple vertical strands, to reticula, to more extensive interconnections that outline circular openings. The circular openings result in scalariform bordered pits that are considered to be multiaperturate. P. dawsonii is believed to illustrate the most complex pitting yet demonstrated conclusively in Early Devonian time. Among trimerophytes P. forbesii, tracheids of P. charientos, and Hostinella from Röragen, Norway lack only the circular bordered pits.

Pitting in Psilophyton dawsonii, an Early Devonian Trimerophyte

Tracheids of Psilophyton dawsonii Banks, Leclercq, and Hueber 1975 from calcareous pebbles of late Early Devonian age on the Gaspé Peninsula, eastern Canada, are shown to have scalariform bordered and circular bordered pits. Macerated tracheids embedded in Spurr epoxy resin and sectioned at 2 μm; peels, ground sections, SEM and petrographic observations all indicate that interconnections between the scalariform bars are composed of secondary wall material and that the patterns produced are not artifacts of preservation. The interconnections produce a wide range of patterns from simple vertical strands, to reticula, to more extensive interconnections that outline circular openings. The circular openings result in scalariform bordered pits that are considered to be multiaperturate. P. dawsonii is believed to illustrate the most complex pitting yet demonstrated conclusively in Early Devonian time. Among trimerophytes P. forbesii, tracheids of P. charientos, and Hostinella from Röragen, Norway lack only the circular bordered pits.

Surface Patterns on Selected Mississippian Conodonts

Four distinct surface patterns occur among Mississippian and some other Late Paleozoic conodonts; smooth surfaces, raised reticulate networks, parallel columns, and lamellar scarps. The bar surfaces of bar forms and platform surfaces of specimens of Gnathodus and Pseudopolygnathus appear smooth at magnifications of as much as 1300X. In contrast, specimens of other platform genera, such as Polygnathus, Siphonodella, and Palmatolepis, bear raised reticulate network on the oral surface. Reticulation appears to be an organic property of the conodont rather than an artifact of preservation, and closely resembles patterns resulting from ameloblastic secretion of mammalian dental enamel. Denticle sides are characterized by a pattern resembling the surface of irregularly bundled fibers. Similar patterns have been observed on fish teeth, but not on scolecodonts. The aboral surface possesses an attachment scar composed of concentrically arranged scarps and slopes. Each scarp is the edge of an individual conodont lamella and appears to be the point of juncture between the lamellae of the basal material and those of the conodont. The distinction between conodont lamellae and those of the basal material appears to be one of composition, crystal size, and durability within an otherwise continuous mass. This observation fails to support Gross9 (1960) concept of a cycle of secretion of conodont lamellae, their partial resorption, and the subsequent independent secretion of basal-material lamellae.