Richard J. Aldridge

Conodont affinity and chordate phylogeny.

Current information on the conodonts Clydagnathus windsorensis (Globensky) and Promissum pulchrum Kovács‐ Endrödy, together with the latest interpretations of conodont hard tissues, are reviewed and it is concluded that sufficient evidence exists to justify interpretation of the conodonts on a chordate model. A new phylogenetic analysis is undertaken, consisting of 17 chordate taxa and 103 morphological, physiological and biochemical characters; conodonts are included as a primary taxon. Various experiments with character coding, taxon deletion and the use of constraint trees are carried out. We conclude that conodonts are cladistically more derived than either hagfishes or lampreys because they possess a mineralised dermal skeleton and that they are the most plesiomorphic member of the total group Gnathostomata. We discuss the evolution of the nervous and sensory systems and the skeleton in the context of our optimal phylogenetic tree. There appears to be no simple evolution of free to canal‐enclosed neuromasts; organised neuromasts within canals appear to have arisen at least three times from free neuromasts or neuromasts arranged within grooves. The mineralised vertebrate skeleton first appeared as odontodes of dentine or dentine plus enamel in the paraconodont/euconodont feeding apparatus. Bone appeared later, co‐ordinate with the development of a dermal skeleton, and it appears to have been primitively acellular. Atubular dentine is more primitive than tubular dentine. However, the subsequent distribution of the different types of dentine (e.g. mesodentine, orthodentine), suggests that these tissue types are homoplastic. The topology of relationships and known stratigraphic ranges of taxa in our phylogeny predict the existence of myxinoids and petromyzontids in the Cambrian.

ORIENTATION AND ANATOMICAL NOTATION IN CONODONTS

Abstract All aspects of conodont paleontology rely on the identification and description of homologous anatomical units or elements. But the current schemes of anatomical notation and terms for orientation were formulated at a time when little was known of conodont anatomy or skeletal architecture, resulting in some confusion and difficulties in their application. With improving knowledge of conodonts, these problems are becoming increasingly acute. In an attempt to address current problems, we introduce new terms for orientation in conodonts and their elements, and a modified scheme of anatomical notation. The principal axes of the conodont body are identified as rostrocaudal, dorsoventral, and mediolateral, with opposite lateral sides designated dextral and sinistral. Anatomical notation is defined according to topological relationships between elements with reference to the principal axes of the body and takes the form of letters with numeric subscripts (e.g., P1, P2, S0-S4). The ozarkodinid apparatus serves as a standard, but the Pn-Sn scheme can be applied rigorously to all taxa that are known from natural assemblages or where an hypothesis of topological homology can be inferred from secondary morphological criteria.

Early Silurian oceanic episodes and events

Biotic cycles in the early Silurian correlate broadly with postulated sea-level changes, but are better explained by a model that involves episodic changes in oceanic state. Primo episodes were characterized by cool high-latitude climates, cold oceanic bottom waters, and high nutrient supply which supported abundant and diverse planktonic communities. Secundo episodes were characterized by warmer high-latitude climates, salinity-dense oceanic bottom waters, low diversity planktonic communities, and carbonate formation in shallow waters. Extinction events occurred between primo and secundo episodes, with stepwise extinctions of taxa reflecting fluctuating conditions during the transition period. The pattern of turnover shown by conodont faunas, together with sedimentological information and data from other fossil groups, permit the identification of two cycles in the Llandovery to earliest Wenlock interval. The episodes and events within these cycles are named: the Spirodden Secundo episode, the Jong Primo episode, the Sandvika event, the Malmøykalven Secundo episode, the Snipklint Primo episode, and the Ireviken event.

Ludlow (late Silurian) oceanic episodes and events

Biotic and sedimentological cycles in the late Silurian are closely similar to those recognized in the early Silurian. They are explained here by alternations between primo and secundo conditions, the former being characterized by cold dense oceanic bottom waters and high nutrient supply, and the latter by salinity-dense bottom waters and diminished nutrient supply. The lithological pattern, together with the turnover of conodont faunas and of other fossil groups, permits the identification of a sequence of episodes and events in the Ludlow following the Klinte Secundo Episode, which developed in the latest Wenlock: The Sproge Primo Episode, the Linde Event, the Etelhem Secundo Episode, the Havdhem Primo Episode, the Lau Event, the Hoburgen Secundo Episode and the Klev Secundo–Primo Event, continuing into the earliest Přídolí. Testing of this model against others that invoke a primary sea-level control is marred by the current imprecisions of biostratigraphical correlation.

87Sr/86Sr,143Nd/144Nd and REEs in Silurian phosphatic fossils

The feasibility of using the chemistry of phosphatic fossil material to trace palaeo-seawater composition for the Silurian has been evaluated. Contrary to Recent material, fossil fish are shown to be unreliable for Sr isotope stratigraphy. Conodonts, however, may be used by employing 0.2M HNO3 dissolution, except for samples showing degrees of thermal maturation greater than colour alteration index (CAI) values of 2.5. Samples from the same stratigraphic levels from the U.K., Czechoslovakia, Gotland and the U.S.A. gave similar87Sr/86Sr ratios, suggesting that they have recorded the original seawater value. Significant inter-specific differences occur in REE concentrations and Nd isotopic compositions of conodont elements, and Ce and Eu anomalies vary with REE content. This suggests that artefacts are introduced at the same time that large amounts of REEs are incorporated into the apatite. Preliminary results for the Sr isotopic composition of Silurian seawater are presented. Values are generally lower than previously suggested, showing an increase in87Sr/86Sr from ∼ 0.70825 at 435 Ma rising to ∼ 0.70875 at 418 Ma and levelling off in the Late Silurian. The overall rate of increase in seawater ratio is about2–3 × 10−5 Ma−1 and can be accounted for by the widespread decrease in volcanism compared with the Ordovician; a proportionally greater flux of Sr to the oceans from continental erosion of only ∼ 10% would account for the long-term increase in seawater87Sr/86Sr ratio throughout the Silurian.

Wenlock (Silurian) oceanic episodes and events

Biotic and sedimentological cycles in the Wenlock are related to episodic changes in oceanic state. These may be explained by alternations between primo and secundo conditions, the former being characterized by cold oceanic bottom waters and high nutrient supply, and the latter by salinity-dense bottom waters and diminished nutrient supply. The lithological pattern, together with the turnover of conodont faunas and of other fossil groups, permits the identification of a sequence of episodes and events in the Wenlock, following the extinctions associated with the earliest Wenlock Ireviken Event: the Vattenfallet Secundo Episode, the Sanda Primo Episode, the Boge Event, the Lansa Secundo Episode, the Allekvia Primo Episode, the Valleviken Event, the Hellvi Secundo Episode, the Mulde Event and the Klinte Secundo Episode. The Mulde Event differs from others in separating two secundo episodes, and may represent an interval when deep waters remained saline and vertical circulation virtually ceased. The effects of the events and episodes were stronger in the early and late Wenlock than in the middle part of the epoch.

New evidence on the anatomy and phylogeny of the earliest vertebrates

We report the discovery of a new agnathan specimen from the Lower Cambrian Chengjiang Lagerstätte of China and thereby provide new evidence on the myomeres (V–shaped), the branchial apparatus (gill filaments and arches), the dorsal fin and the gonads (24–26) of the earliest vertebrates. The new specimen and the co–occurring Myllokunmingia fengjiaoa and Haikouichthys ercaicunensis represent a single species, which is a primitive member of the crown group craniates (vertebrates) and post–dates the origin of the myxinoids (hagfish). The origin of the vertebrate clade is at least as old as Early Cambrian.

The interrelationships of ‘complex’ conodonts (Vertebrata)

Synopsis Little attention has been paid to the suprageneric classification for conodonts and existing schemes have been formulated without attention to homology, diagnosis and definition. We propose that cladistics providesan appropriate methodology to test existingschemes of classification and in which to explore the evolutionary relationships of conodonts. The development of a multielement taxonomy and a concept of homology based upon the position, not morphology, of elements within the apparatus provide the ideal foundation for the application of cladistics to conodonts. In an attempt to unravel the evolutionary relationships between ‘complex’ conodonts (prioniodontids and derivative lineages) we have compiled a data matrix based upon 95 characters and 61 representative taxa. The dataset was analysed using parsimony and the resulting hypotheses were assessed using a number of measures of support. These included bootstrap, Bremer Support and double‐decay; we also compared levels of homoplasy to those expected given the size of the dataset and to those expected in a random dataset. The dataset was analysed in three hierarchical tranches, representing three levels of certainty concerning multi‐element reconstructions and positional homologies. There is much agreement between the results derived from the three partitions, but some inconsistency, particularly in the precise composition of the three main evolutionary grades traditionally recognised (Prioniodontida, Prioniodinina, Ozarkodinina). This is considered to result from (a) the progressive inclusion of data that is increasingly uncertain and (b) the inclusion of increasingly distantly related taxa, introducing spurious hypotheses of homology. We tested for these by partitioning the dataset into the three main evolutionary grades and in each instance resolution was seen to increase substantially, especially among prioniodinins. Our concluding scheme of relationships is a tree derived from a compilation of the three component subtrees, which is directly compatible with the most‐parsimonious trees derived from the initial second tranche analysis with the exception of the position of Hibbardella. This is compared in detail to the main extant schemes of suprageneric classification. A formal scheme of suprageneric classification is presented and the distribution of characters with respect to component clades is considered as a basis for identifying diagnostic characters.

Conodonts with preserved soft tissue from a new Ordovician Konservat-Lagerstätte

A newly-discovered Konservat-Lagerstätte in the Upper Ordovician of South Africa has yielded giant conodont apparatuses, some of which are associated with preserved soft tissues of the conodont animals. Lobate structures located to the anterior of the conodont apparatus in several specimens are interpreted as sclerotic cartilages surrounding the eyes, comparing closely with those of the Silurian agnathan Jamoytius. One specimen also displays a possible trunk trace.

The conodont controversies

The discovery of fossilized conodont soft tissues has led to suggestions that these enigmatic animals were among the earliest vertebrates and that they were macrophagous, using their oropharyngeal skeletal apparatus to capture and process prey. These conclusions have proved controversial. There is now a consensus that conodonts belong within the chordates, but their position within the clade is hotly debated. Resolution of these questions has major implications for our understanding of the origin of the vertebrates and the selective pressures that led to the development of the vertebrate skeleton.