Development of the exoskeleton in osteostracans (Agnatha, vertebrata): New evidence of growth

Abstract

In the past decade, considerable progress has been made towards a better understanding of the development of the dermal skeleton in osteostracans. Based on the study of the fine shield structure in Thyes tes verrucosus Eichwald, 1854 from the Silurian of Saaremaa Island (Estonia), a model for the formation of its exoskeleton was proposed (Afanassieva, 2002(Afanassieva, , 2004b. New information on well preserved juvenile Super ciliaspis gabrielsei (Dineley et Loeffler, 1976) from the Lower Devonian of Canada, North West Territories, allowed the determination of ontogenetic patterns of the dermal skeleton in the tessellated taxon (Hawthorn et al., 2008). A comparison of the data on shield development in consolidated Thyestes and tessellated Super ciliaspis and also additional study of the material of Thyestes verrucosus corroborated the previous conclu sions and displayed new features of the formation of exoskeleton in this species. A different formation pattern of the exoskeleton was established in Paraungulaspis arctoa (Afanassieva et Karataj t Talimaa, 1998) from the Lower Devonian of October Revolution Island of the Severnaya Zemlya Archipelago, Russia (Afanass ieva, 2011;Afanassieva and Karataj t Talimaa, 2013). A comparison of the exoskeleton structure and growth of the hard covering in Thyestes verrucosus and Paraungulaspis arctoa corroborated the conclusion that these species belong to different phyletic lineages of osteostracans.

The dermal skeleton of the jawless vertebrateTremataspis mammillata(Osteostraci, stem‐Gnathostomata)

“…Based on different developmental stages in Tremataspis and the lack of evidence for extensive resorption in many osteostracans, Denison (, ) inferred that the headshield mineralised in a single event with centripetal addition of matrix in a superficial to basal direction, leading to the conclusion that growth was determinate and unipolar sensu Afanassieva (). The concentric arrangements of superficial and middle layer lamellae described above (Section 3.4.2) suggest centripetal deposition of matrix toward the superficial and lower vascular networks and centrifugal deposition away from the mesh canals.…”

“…In addition, juvenile growth stages of Parameteoraspis show that tesserae were initially separate and subsequently fused on reaching maximum size (Janvier, ). Bipolar growth (Afanassieva, ) has been found in the thyestiidans Procephalaspis (Janvier, ; Ørvig, ) and Tremataspis milleri (Märss et al, ) which show superposition of odontodes. A collection of thyestiid fragments described by Gross () and later named Tahulaspis (Märss et al, ) show initial ontogenetic stages with ridge like tubercles of primary dentine and grooves between them containing sieve plates.…”

“…This demonstrates that unlike T. mammillata , some thyestiids were capable of increasing the thickness of the dermal skeleton superficially and suggests that Tremataspis growth is not representative of the general thyestidian condition. Based on the pattern of tubercles and radiating canals, Thyestes has been suggested to have engaged in marginal growth of unfused tesserae and then fusion at adulthood with addition of odontodes around tubercles at the polygonal centres (Afanassieva, ). Marginal growth of tesserae almost certainly occurred in the derived zenaspidians which show separate tesserae and ontogenetic stages (Hawthorn et al, ), however, the only evidence for marginal growth in forms with consolidated headshields is the polygonal patterning of dermal skeletal freatures and concentric patterns of odontodes.…”

Biodiversity of the Silurian osteostracans of the East Baltic

“…The osteostracans from Saaremaa Island have been an important source for many researchers around the world, and these collections have been distributed to many research institutes and museums, promoting work by Wängsjö (1944, study based on his own collection in 1938), Obruchev (1964), Janvier (1974Janvier ( , 1975Janvier ( , 1978Janvier ( , 1985a, Janvier & Lelièvre (1994), Afanassieva (1985Afanassieva ( , 1986Afanassieva ( , 1991Afanassieva ( , 1996Afanassieva ( , 2004) and many others. These papers mainly address the shields, whereas the microstructure of the exoskeletons has been more thoroughly studied by Denison (1947Denison ( , 1951a, Gross (1956Gross ( , 1961Gross ( , 1968b, Afanassieva (1985Afanassieva ( , 1986Afanassieva ( , 1991Afanassieva ( , 1995Afanassieva ( , 2004Afanassieva ( , 2014, and Afanassieva & Märss (1997, 2014. In two monographs on the osteostracans of Russia and adjacent countries, Afanassieva (1991Afanassieva ( , 2004 devoted considerable attention to the exoskeleton of Estonian forms, and used features of the external morphology and internal structure at various levels of generalisation for diagnoses of the various rank taxa.…”

The exoskeleton of vertebrates: the geometric regularities of the formation of armor relief in early jawless vertebrates (Agnatha, Vertebrata)

“…Разнообразие в строении экзоскелета достигалось за счёт различной степени развития слоёв, которые его слагают, и комбинации альтернативных признаков, характеризующих экзоскелет, а именно наличия или отсутствия поровых полей, перфорированных септ, радиальных каналов, сосудистых отверстий и т. п. [7]. В скульптуре наружного скелета остеостраков, сложенной поверхностным дентиновым и/или средним костным слоями, представлены различные по форме и размеру бугорки и валики, сетевидные и пористые структуры, у ряда таксонов поверхность панциря гладкая.…”