Tiiu Märss

Squamation of the thelodont agnathan Phlebolepis

ABSTRACT The study of more than 250 articulated specimens of the thelodont Phlebolepis elegans (Upper Silurian, Estonia) has shown how the morphology and sculpture pattern of scales depend upon their position on the body. Five morphological scale varieties are distinguished for several Silurian thelodonts. Phlebolepis ornata n.sp. from the Estonian Upper Silurian (lower Ludlow) is described.

EXOSKELETAL ULTRASCULPTURE OF EARLY VERTEBRATES

Abstract For the first time, systematic treatment of the ultrasculpture on the exoskeleton of Palaeozoic Agnatha (Thelodonti, Heterostraci, and Osteostraci) is given, supplemented with a discussion of the main published sources on Gnathostomata, such as Acanthodii, Chondrichthyes, and Osteichthyes. Seven main types of ultrasculpture have been determined: (1) fine longitudinal striation; (2) transverse lamellae; (3) short V-shaped slits and branching grooves; (4) polygons separated by walls; (5) polygons separated by intercellular grooves; (6) micronodules; and (7) microtubercles. Five of these types occur on the scales of thelodonts. No ultrasculpture has been found on sandiviiform and loganelliiform thelodonts, anaspids, some Ordovician taxa, and placoderms. A smooth surface might be the most primitive condition for the scale surface. Fine longitudinal striation, transverse lamellae, and polygons separated by walls are the features connected with enameloid tissue. Polygons separated by intercellular grooves may characterize both enameloid and enamel. Microtubercles (as has been shown earlier) characterize true enamel. All ultrasculpture types with certain different features occur in more than one higher taxonomic group. Among Thelodonti these types mirror the classification of the subclass into four higher-rank taxonomic groups. Relationships between thelodonts and chondrichthyans based on ultrasculpture characteristics are much more complicated than previously discussed. Four types of ultrasculpture, found among thelodonts, occur also on the scales of chondrichthyans and/or putative chondrichthyans. Different ultrasculpture types might also point to the polyphyletic nature of the Thelodonti.

A NEW CHONDRICHTHYAN FROM THE WENLOCK, LOWER SILURIAN, OF BAILLIE-HAMILTON ISLAND, THE CANADIAN ARCTIC

Abstract Kannathalepis milleri, gen. et sp. nov. is described from the Upper Sheinwoodian to Lower Homerian interval (Wenlock) of the Baillie-Hamilton Island section, Canadian Arctic. Based on several distinct features, such as the occurrence of several canals in the neck region, much smaller length of the wide and short base compared to the crown, presence of compound scales, character of dentine canals and tubules, and specific ultrasculpture of the crown, the new taxon Kannathalepis milleri, gen. et sp. nov. is assigned to Kannathalepididae fam. nov. of uncertain order of the subclass Elasmobranchii, class Chondrichthyes.

New and biostratigraphically useful thelodonts from the Silurian of the Mackenzie Mountains, Northwest Territories, Canada

Abstract Two new species of thelodont are described. A new thelodontiform is based on articulated skeletons and isolated scales from late Telychian (Llandovery) to earliest Sheinwoodian (Wenlock) shales of the Avalanche Lake sections, Mackenzie Mountains, N.W.T., Canada. The head is blunt; it and the anterior trunk are broad and dorsoventrally flattened, while the posterior trunk and tail are more slender and laterally compressed. Unlike most other articulated thelodonts, there is no evidence of fins apart from the caudal fin. The hypocercal tail displays no evidence of rays or intermediate secondary lobes. The scales are robust, with large bases, large pulp cavities, and strong crown ridges in a stellate pattern; scale crowns are almost unornamented along the presumed ventral margin of the trunk and tail. An articulated specimen from the uppermost Telychian of Baumann Fiord, Ellesmere Island, Canadian Arctic Archipelago, is considered conspecific because of similarities in scale size and ornament, and extends the stratigraphic range of the species from the late Telychian griestoniensis-sakmaricus Graptolite Zone to the early Sheinwoodian centrifugus-insectus Graptolite Zone/Pterospathodus procerus Conodont Superzone, and the geographic range from the Selwyn Basin of the Yukon and N.W.T. to the Franklinian Basin of the Canadian Arctic Archipelago, Nunavut. A new phlebolepidiform is based on scales only. It also occurs both in the Mackenzie Mountains and the Canadian Arctic Archipelago. The two new species contribute significantly to a preliminary vertebrate faunal succession scheme for both areas and are therefore considered to be biostratigraphically useful.

Buccopharyngo-branchial denticles of Phlebolepis elegans Pander (Thelodonti, Agnatha)

Abstract Denticles and denticulate platelets belonging to internal structures of Phlebolepis elegans Pander, along with two new types of scales, are described for the first time. Large fragments taken from seven articulated specimens of P. elegans were dissolved in acetic acid. The denticles were picked from the resulting residues where they were found adhering to the visceral surfaces of scales in samples taken from the anterior part of the body. The overall shapes of the tiny denticles vary from rhombic to elongate, and the sculpture on their surface consists of longitudinal ridges of different lengths and/or tubercles. Denticles are also covered with an ultrasculpture of longitudinal striations. The shape of each denticle varies depending on where it was situated in the body. Rhombic or roundish denticles were found together with rostral and short cephalo-pectoral scales, the latter often pierced by pore canals of the lateral line system; strongly elongate denticles were associated with long lateral scales. These newly described denticles covered the buccopharyngo-branchial region of P. elegans. Denticulate platelets also came from the branchial region. Two sets of scales in a slightly crescent-shaped arrangement are bordered on one side by very narrow and high scales. These crescent-shaped sets bordered openings, which are here interpreted as branchial openings. The openings were situated between long lateral scales, and each successive opening was placed slightly ventral to its more anterior neighbor. These internal denticles reinforce the idea that such structures are general features of thelodonts, and not specializations of restricted groups of thelodonts.

The earliest osteostracan Kalanaspis delectabilis gen. et sp. nov. from the mid-Aeronian (mid-Llandovery, lower silurian) of Estonia

ABSTRACT The Osteostraci are an early clade of jawless vertebrates widely regarded as the sister group of all jawed vertebrates. Despite their prominent role in the Silurian to Devonian marginal marine ecosystems, the early evolutionary history of this subclass remains poorly understood. Here we report the discovery of a head shield (part with broken right margin and counterpart) of Kalanaspis delectabilis gen. et sp. nov. from the Kalana Quarry in Central Estonia. Reliably dated to the Pranognathus tenuis Conodont Zone, Raikküla Regional Stage, mid-Aeronian, mid-Llandovery, the specimen is about 10 million years older than the previously known oldest record of the Osteostraci. Kalanaspis delectabilis gen. et sp. nov. is identified as belonging to the new Family Kalanaspididae fam. nov., Order Ateleaspidiformes. It has a relatively large dorsal head shield, which is somewhat wider than long, with very small and very closely placed orbits, short prepineal length, one pair of notably long lateral fields, and gently undulating posterior margin. It lacks a posteromedian dorsal ridge as well as cornual processes and pectoral sinuses. No impressions of nasohypophyseal and pineal features were detected. The preservation of the specimen is unusual; the energy-dispersive spectroscopy (EDS) elemental analysis revealed that calcium phosphate, the original major constituent of the dermal skeleton of vertebrates, has disappeared and has been substituted by carbonaceous matter.