Head Shield Research Articles

New data on two rare East Asian cladocerans, Coronatella trachystriata (Chen, Zhang & Liu, 1994) and Nedorhynchotalona chiangi Kotov & Sinev, 2011 (Cladocera: Anomopoda: Chydoridae)

Two rare East Asian cladocerans, Coronatella (Coronatella) trachystriata (Chen, Zhang et Liu, 1994) and Nedorhynchotalona chiangi Kotov & Sinev, 2011, were found in Qili Lake on the border of Anhui and Jiangsu Provinces of China. Morphology of the species was studied by scanning electron microscopy for the first time, revealing unique morphological features: valve sculpture as a system of ridges in C. (C.) trachystriata and head shield with only lateral head pores present in N. chiangi. Both species have numerous adaptations for a benthic mode of life. They were found in a pelagic plankton sample during strong wind action, but were absent in littoral samples. Our data suggest that both species inhabit bottom sediments in a deeper zone of lakes at some distance from the shore, usually not sampled during cladoceran faunistic surveys. Morphology and distribution of the species are discussed.

A non‐monophyly of ‘crowned’ Daphnia (Ctenodaphnia) Dybowski et Grochowski, 1895 (Cladocera: Daphniidae): From genomes to morphology

AbstractDaphnia (Crustacea: Cladocera) has been frequently used as a model taxon for studying prey antipredator defences. Among numerous representatives of this genus, there are several taxa within the subgenus Daphnia (Ctenodaphnia) with a morphological innovation unique for these cladocerans, a head plate. In some populations, the margin of this anterior projection of carapace into the head shield is adorned with a remarkable ‘crown of thorns’, which has been shown to be an antipredator adaptation against tadpole shrimps (Notostraca). This structure is phenotypically plastic, dependent on the presence of these omnivorous crustaceans in the respective water bodies. We aimed to evaluate the monophyly of Eurasian ‘crowned’ Daphnia species (i.e., those forming the ‘crown of thorns’ under some circumstance) based on genomic phylogenies and morphology. For this study, we have individually sequenced the genomes of five daphniids, four representing taxa able to form ‘crowns’ (two specimens of different clades of the D. (C.) atkinsoni complex and two specimens of D. (C.) triquetra from distant populations), and D. (C.) mediterranea distantly related to D. (C.) atkinsoni that lacks the head plate. We analysed them along with genomes obtained from GenBank, focusing on either full mitochondrial or partial nuclear datasets (BUSCO). Our main hypothesis on a monophyly of all ‘crowned’ daphnids was rejected. Genomic analyses confirmed existence of two independent lineages able to express this phenotypic trait in the Palaearctic: (1) a monophyletic D. (C.) atkinsoni s. lat. and (2) D. (C.) triquetra, formally redescribed here. These lineages form a well‐supported clade together with several other species lacking a head plate (including D. (C.) mediterranea). Genomic analyses indicate that D. (C.) atkinsoni s. lat. is closely related to D. (C.) tibetana; mitochondrial markers also suggest a close relationship of D. (D.) triquetra with D. (C.) studeri, both D. (C.) tibetana and D. (C.) studeri, are lacking this morphological feature. Molecular clock estimated the time of the differentiation of the major clade containing both ‘crowned Daphnia’ to the Late Mesozoic, confirming an antiquity of the head plate as antipredator defence.

Placoderms

For over 70 million years, during the Paleozoic, the placoderms (Greek for 'plated skin'), an extinct group of armoured fishes, were the most abundant and diverse vertebrates on our planet. Some of the first placoderm fossils discovered - such as Bothriolepis with its bone-covered pectoral fins - seemed so bizarre that they were thought to represent turtles or ancient beetles. All placoderms bear thick overlapping dermal plates around the head (called the 'head shield') and an area of the body covered in similar overlapping plates enveloping the pectoral to anal region (called the 'trunk-shield'). Placoderm fossils (Figure 1) are known from every continent on Earth ranging from the early Silurian (∼438 million years ago) to the end Devonian (∼359 million years ago) when they became extinct.

Morphological identification of the whizzling sand snake: <i>Psammophis sibilans</i> in Zaria, Northwestern Nigeria

Background: Despite extensive research on snakes, although subject to some variation and ignored by many authors, scarce information is available on the morphology of Psammophis Sibilans.
 Objectives: This research aimed to study the morphological characterization of Psammophis Sibilans in Zaria, Northwestern Nigeria, as a basis for its morphological identification which may be extrapolated or used as baseline data to show the reliability of scale count in species determination of Psammophis Sibilans.
 Methods: Active capture methods were used on free-range snakes and scales were counted to distinguish between the families, generic and specific levels.
 Results: The result showed that on the head shield, the loreal [1.0±0.00], pre-nasal[2.0±0.00], internasal[2.0±0.00],preocular[4.0±0.00],supraocular[2.0±0.00],frontal[1.0±0.00], parietal[2.0±0.00] were all the same for all the population studied (n = 20). The dorsal scale [18.00±0.00], and anal plate [1.00±0.00] were also the same for all the snakes studied while the ventral scales [173±2.83], and tail scales [191.30±1.76] showed significant differences within the group. For the zoometric parameters, the body length [85.33±0.44], mid-circumference [7.40±0.10], head length [3.50±0.29], and tail length [32.17±0.17] all showed significant differences within and between the population except for the head width [1.00±0.00].
 In conclusion: The head shields are more reliable in the identification of Psamophis sibilans than the zoometric parameters studied, except for the head width. We also discovered a significant variation in the loreal head shield which was just one for all the population studied as opposed to studies from Egypt and Ethiopia where a range of one to two was reported.

Early developmental stages of a Lower Ordovician marrellid from Morocco suggest simple ontogenetic niche differentiation in early euarthropods

Early developmental stages of euarthropods are exceptionally rare in the fossil record. This hampers our understanding of the biology, phylogeny, and development of this extremely diverse metazoan group. Herein, we use classical paleontological methods in combination with synchrotron X-ray microtomography to explore the morphology in ca. 480 million-year-old early developmental stages of the Lower Ordovician Fezouata Shale marrellid euarthropod. These stages range between 3.8 and 5.3 mm in length and are characterized by three distinct pairs of gently curved spines that projects from the head shield. The first pair of cephalic appendages are represented by uniramous antenullae of a sensory function. The second pair of cephalic appendages is robust, and had an anchoring or stabilizing function. The third cephalic appendage pair is composed of long cylindrical podomeres and was used for walking. The trunk appendages are biramous and consist of an endopod and a lamellate exopod. Two anterior trunk endopods are composed of long slender podomeres and were used for walking, while the more posterior trunk endopods bear robust endites and associated setae and were used for food gathering. The trunk of the earliest developmental stages is composed of thirteen segments, in contrast to more than 22 segments in the adult trunk. The similar appendage morphology and differentiation along the body is evident in adult individuals of the Fezouata marrellid, suggesting these different developmental stages shared similar methods of locomotion and food processing. Given that adults and juveniles are often preserved in the same or nearby sites, the niche differentiation between these life stages would be the result of the absolute smaller appendage size in immature stages compared to larger adults, effectively differentiating the size of food resources consumed by each. In addition, the delicate setae present in the posterior trunk appendages of early developmental stages might have been used to capture smaller food particles. This simple mode of ontogenetic niche differentiation might have been common in the early diverging euarthropod groups.

Development of the muscular and nervous systems during the larval ontogeny of the stalked barnacle, Octolasmis angulata Aurivillius 1894 (Cirripedia: Thoracicalcerea: Poecilasmatidae)

The advancements in microscopic techniques have stimulated great interest in the muscular and neural architectures of invertebrates, specifically using muscle and neural structures to infer phylogenetic relationships. Here, we provide the data on the development of the muscular and nervous systems during the larval development of stalked barnacle, Octolasmis angulata using the phalloidin F-actin and immunohistochemical labelling (e.g. acetylated α-tubulin and serotonin) and confocal laser scanning microscopy analysis. All naupliar stages shared the same muscle and neural architectures with only the discrepancy in size. The nauplii have a complex muscle arrangement in their feeding apparatus and naupliar appendages. Most naupliar muscles undergo histolyse during the cyprid metamorphosis. The cyprid muscles form beneath the head shield at the end of nauplius VI. The naupliar and cyprid central nervous systems exhibit the typical tripartite brain comprising the protocerebrum, deutocerebrum and tritocerebrum. The serotonin-like immunoreactivity is mainly found in the naupliar brain, mandibular ganglia, cyprid brain and posterior ganglia. Our study revealed that numerous muscle and neural architectures in the naupliar and cyprids have phylogenetic significance, but future studies on the myoanatomy and neuroanatomy of other barnacle species are necessary to determine the homology of these structures.

Brighter nights risk extinguishing glow-worm twinkle

The bright lights of big cities are wonders of the modern world, intended to help us work, stay safe and enjoy the world around us long after the sun has set. Although artificial light has been great for increasing human productivity, some nocturnal animals, and even people, pay a price for this illumination. From increasing the amount of time that predators are active to disrupting migrations, light pollution affects many animals; but how do animals that use their own luminescence to lure food or attract mates fare against this new, brighter background? Female common glow-worms (Lampyris noctiluca) emit a green glow from their abdomen to attract flying males, but they are unable to fly themselves to new locations to escape light pollution. Because of this, Estelle Moubarak, Sofia Fernandes, Alan Stewart and Jeremy Niven of the University of Sussex, UK, wondered how hard it is for male common glow-worms to find mates in an ever-brighter environment.After collecting glow-worms at night from the South Downs, UK, the researchers transported them back to the lab, before beginning the tricky task of transferring the male insects to a Y-shaped ‘maze’ without exposing them to artificial light. The team placed the male glow-worms at the bottom of the Y-maze and a green LED, which mimicked a female's glow, at the top of one of the arms, which the male had to walk towards. They then recorded if and how long it took the males to find the fake female. Then, the team switched on a white light above the maze, ranging from 25 lx (25 times brighter than moonlight) to 145 lx (equivalent to the light beneath a streetlamp). Although all of the glow-worms found the LED in the dark, only 70% found the LED at the dimmest levels of white light, and just 21% of the insects found their potential mate in the brightest light.Not only did the white light affect the glow-worm's ability to find a female, but it also caused them to take longer to reach the LED. In the dark, the worms took ∼48 s to reach the female-mimicking LED; however, it took the same glow-worms ∼60 s to reach the LED in the lowest white light levels. Illuminating the maze also caused the male glow-worms to spend more time in the bottom part of the maze without moving towards a female. In the dark, the insects only spent ∼32 s in the bottom of the Y-maze, whereas they spent ∼81 s in the bottom of the maze in the brightest conditions.Moubarak suggests that male glow-worms were unable move towards the females when dazzled by white light because they cover their compound eyes with a head shield, which acts like a pair of sunglasses, reducing the amount of bright light they see. In fact, when the white light illuminated the area with the fake female LED, the glow-worms shaded their eyes for ∼25% of the trial compared with only ∼0.5% of the time when the maze was in the dark. ‘Keeping their eyes beneath their head shield shows male glow-worms trying to avoid exposure to the white light, which suggests that they strongly dislike it,’ says Niven. So, although our bright night-time world has helped give rise to our modern society, it has had a drastic impact on male glow-worms and their ability to find mates. If this trend persists, meadows and heaths across Europe and Asia that have lit up with the twinkling of the female glow-worms for millions of years will fall dark.

Artificial light impairs local attraction to females in male glow-worms.

The negative effects of artificial lighting at night (ALAN) on insects are increasingly recognised and have been postulated as one possible cause of declines in insect populations. Yet, the behavioural mechanisms underpinning ALAN effects on insects remain unclear. ALAN interferes with the bioluminescent signal female glow-worms use to attract males, disrupting reproduction. To determine the behavioural mechanisms that underpin this effect of ALAN, we quantified the effect of white illumination on males' ability to reach a female-mimicking LED within a Y-maze. We show that as the intensity of illumination increases, the proportion of males reaching the female-mimicking LED declines. Brighter illumination also increases the time taken by males to reach the female-mimicking LED. This is a consequence of males spending more time: (i) in the central arm of the Y-maze; and (ii) with their head retracted beneath their head shield. These effects reverse rapidly when illumination is removed, suggesting that male glow-worms are averse to white light. Our results show that ALAN not only prevents male glow-worms from reaching females, but also increases the time they take to reach females and the time they spend avoiding exposure to light. This demonstrates that the impacts of ALAN on male glow-worms extend beyond those previously observed in field experiments, and raises the possibility that ALAN has similar behavioural impacts on other insect species that remain undetected in field experiments.

A possibly deep branching artiopodan arthropod from the lower Cambrian Sirius Passet Lagerstätte (North Greenland)

AbstractArtiopoda was a diverse group of Palaeozoic euarthropods that proliferated in the early Palaeozoic, epitomized by the ubiquitous trilobites. Their possible phylogenetic position outside mandibulates and chelicerates offers the potential for understanding the evolution of euarthropods in more detail. However, this opportunity remains unexploited given that identification of deep‐splitting artiopodans remains to be fully explored. Here, we describe a new non‐trilobite artiopodan from the lower Cambrian Sirius Passet Lagerstätte, North Greenland. Thulaspis tholops gen. et sp. nov. is a large species with a broad, domed head shield, followed by a trunk consisting of 15 thoracic tergites and a small pygidium, giving the body an ovoid appearance when viewed dorsally. Thulaspis is distinctive with its rounded genae and anterior thoracic pleural tips, as well as short pleural spines posteriorly. A heart‐shaped hypostome with an anterior lobe is present. Appendages, partly obscured by the tergal skeleton, have many moderate length gnathobasic spines, and large flap‐like exopods with a fringe of small setae. Cladistic analyses recover Thulaspis as the sister taxon to Squamacula, a genus found in the Chengjiang and Emu Bay Shale biotas, in either a polytomy with a number of artiopodan taxa or as a sister group to all other artiopodans, indicating an important role in understanding the roots of artiopodan anatomy and evolution.

A revision of Gerrhopilus inornatus (Squamata: Gerrhopilidae) reveals a multi-species complex

Melanesian blindsnakes of the genus Gerrhopilus have been little collected or researched. I examined specimens assigned in museums to Gerrhopilus inornatus and found considerable morphological diversity among them that indicates the presence of multiple species. I redescribe G. inornatus (Boulenger) based on the holotype and one additional specimen, and I describe six new species among specimens currently subsumed under that name from Papua New Guinea: Gerrhopilus flavinotatus sp. nov., Gerrhopilus lorealis sp. nov., Gerrhopilus papuanorum sp. nov., Gerrhopilus polyadenus sp. nov., Gerrhopilus slapcinskyi sp. nov., and Gerrhopilus wallachi sp. nov. Each species is currently known from only 1-3 specimens, and all but two are known only from single localities. In addition to traditional information on scale counts, habitus, and color patterns, I found the numbers and distributions of epidermal glands among the head shields to be especially useful for discriminating among species. The number of recognized Melanesian Gerrhopilus has increased tremendously in recent years, but the region has been poorly sampled for these snakes, and it is to be expected that additional species will be identified at such time as surveys can more effectively target these cryptic snakes.

Characterization of Read/Write Head Shield Asperities in Hard Disk Drives

Characterization of Read/Write Head Shield Asperities in Hard Disk Drives

New ptyctodontid placoderm from the Famennian (Upper Devonian) of European Russia

Neruchella eichwaldi gen. et sp. nov. is erected on the basis of a head shield found in the Famennian Lebedyanian Regional Stage of the Orel Region, Russia. The new taxon differs from all known ptyctodontids, and is characterized by the central plates in contact along the skull midline, the meeting of the sensory-line canals on the centrals and the loss of the nuchal plate. The structures of the head shields of two Famennian ptyctodontids Neruchella and Chelyophorus are completely different from the general skull bone pattern of older ptyctodontid taxa.

Larvae of two parasitic barnacles, Parasacculina pilosella (Van Kampen et Boschma, 1925) (Rhizocephala: Polyascidae) and Sacculina pugettiae Shiino, 1943 (Rhizocephala: Sacculinidae) studied by scanning electron microscopy

The complete larval development of Parasacculina pilosella (Van Kampen et Boschma, 1925) and Sacculina pugettiae Shiino, 1943 including five naupliar stages and one cypris stage is described and illustrated using SEM. P. pilosella and S. pugettiae have a sacculinid type of development. Nauplii possess a naupliar eye, short frontolateral horns with terminal processes, and a ventral process between the furcal rami. Larvae lack a flotation collar, seta 6 on the antennule and a seta on the antennal basis. Cyprids have a nearly straight LO2. Breakage zone and a spinous process are present only in male larvae. Nauplii of the two species differ by the morphology of the furca: in P. pilosella, the furcal rami are longer and not drowned into cuticular sockets. Naupliar antenna of S. pugettiae has a lateral seta on the endopod which is lacking in P. pilosella. Dorsal head shield setae 1 and 2a are present in S. pugettiae nauplii and not found in P. pilosella larvae. In P. pilosella, all dorsal setae have subterminal pores, whereas in S. pugettiae, pores of the setae 2 are shifted proximally. It is possible that the presence/absence of setae 1 and 2a is the distinctive feature of nauplii of the families Sacculinidae and Polyascidae.

Development and Evaluation of a Thimble-Like Head Bolus Shield for Hemi-Body Electron Beam Irradiation Technique

Background: The hemi-body electron beam irradiation (HBIe<sup>–</sup>) technique has been proposed for the treatment of mycosis fungoides. It spares healthy skin using an electron shield. However, shielding electrons is complicated owing to electron scattering effects. In this study, we developed a thimble-like head bolus shield that surrounds the patient’s entire head to prevent irradiation of the head during HBIe<sup>–</sup>.Materials and Methods: The feasibility of a thimble-like head bolus shield was evaluated using a simplified Geant4 Monte Carlo (MC) simulation. Subsequently, the head bolus was manufactured using a three-dimensional (3D) printed mold and Ecoflex 00-30 silicone. The fabricated head bolus was experimentally validated by measuring the dose to the Rando phantom using a metal-oxide-semiconductor field-effect transistor (MOSFET) detector with clinical configuration of HBIe<sup>–</sup>.Results and Discussion: The thimble-like head bolus reduced the electron fluence by 2% compared with that without a shield in the MC simulations. In addition, an improvement in fluence degradation outside the head shield was observed. In the experimental validation using the inhouse- developed bolus shield, this head bolus reduced the electron dose to approximately 2.5% of the prescribed dose.Conclusion: A thimble-like head bolus shield for the HBIe<sup>–</sup> technique was developed and validated in this study. This bolus effectively spares healthy skin without underdosage in the region of the target skin in HBIe<sup>–</sup>.

Ventral organization of Jianfengia multisegmentalis Hou, and its implications for the head segmentation of megacheirans

AbstractMegacheirans, or great‐appendage euarthropods, have featured prominently in discussions regarding the early evolution of the head region in total‐group Euarthropoda. However, several aspects of the ventral morphology of most representatives remain incompletely known given the loss of data associated with fossil compression, coupled with the rarity of some of these taxa. Here, we describe the ventral aspect of head organization of the jianfengiid megacheiran Jianfengia multisegmentalis using micro computed‐tomography, and explore its evolutionary significance. The head consists of a pair of stalked eyes, a pair of great appendages, and four pairs of biramous appendages, all covered by a dorsal head shield. The rostral portion of the head bears a median projection, which we interpret as an anterior sclerite akin to that observed in several other Cambrian euarthropods. The anterior sclerite in J. multisegmentalis articulates with robust stalks with rounded projections in a more adaxial position, and bears the compound eyes. Critically, the ventral side of the head of J. multisegmentalis features a prominent lobe‐shaped hypostome/labrum complex located immediately behind the great appendages, and between the first pair of biramous appendages. This situation is consistent with that observed in Leanchoilia illecebrosa and suggests a conserved, and possibly ancestral, pattern of a six‐segmented head (eyes, great appendages and four biramous limb appendages) with a hypostome/labrum complex for Megacheira. The existence of the hypostome/labrum complex in J. multisegmentalis falsifies the hypothesis that the anterior projections on the head of jianfengiids might be homologues of the euarthropod labrum.

A new species of honeycombed Chydorus Leach, 1816 (Cladocera: Anomopoda: Chydoridae) from tundra of North-East Russia.

A new cladoceran of the faviformis-group of the genus Chydorus Leach, 1816 (Cladocera: Chydoridae), characterized by a honeycomb-like sculpture of valves and head shield, is described from tundra lakes and ponds in North-East Russia. Morphology of the new species was studied using optical and scanning electron microscopy. Chydorus izvekovae sp. nov. differs from all other species of the faviformis-group in a narrow labral keel with elongated apex, and in a greater number of meshes on the valves and head shield. Other species of the faviformis-group in the Western Hemisphere are distributed in the tropical-subtropical zone, they also inhabit mostly shallow waters with well-developed macrophyte zone.

X-ray protective materials from barium sulfate/amorphous cellulose/natural rubber composites

In this study, barium sulfate/amorphous cellulose (Ba/AC) was incorporated into natural rubber to produce X-ray shielding materials. Ba/AC was prepared by in-situ regeneration of amorphous cellulose from eucalyptus pulp and office waste paper with barium chloride. The shielding sheets were prepared by mixing vulcanized natural rubber with Ba/AC using both office waste paper (NR-W) and purified eucalyptus pulp (NR-E) and were compared to barium sulfate powder (NR-P) used as a conventional material. NR-W and NR-E exhibited a predominant attenuation property compared to that of NR-P. The amorphous cellulose surrounded by barium sulfate is essential in distributing and reducing the aggregation of barium sulfate particles. This behavior results in most incident X-ray beams being effectively absorbed. Subsequently, prototypes were fabricated using NR-W and NR-E with a thickness of 0.25-mm Pb in accordance with the quality standards of medical diagnostic X-ray machines in Thailand. The head, thyroid, and breast shields were produced to protect from direct X-ray beams. Additionally, a lab coat was prepared to block scattered beams. Finally, the fabricated X-ray shielding prototypes are eco-friendly, cost-effective, and nontoxic.

Differing radiation exposure in scrub technicians and rotating staff in cardiac catheterization laboratory: occupation matters

Abstract Funding Acknowledgements Type of funding sources: None. Background Radiation exposure of cardiac catheterization lab personnel is an evolving area of concern, due to long term, repetitive exposure causing deterministic and stochastic effects(1,2). Current radiation protection measures are based on increasing distance, shielding from fluoroscopy source and depend on primary operator behavior(3) Effective dose(ED) in micro Sieverts( μSv) as a measure of individual absorbed radiation dose provides feedback as to occupational exposure and risks. Dosimeter badges worn over equipment measure ED correlating with radiation exposure of areas unprotected by standard lead apron such as head, neck, lens and upper limbs. (4) Most studies focus on primary operator exposure (5,6,7), overlooking non physician staff comprising scrub technicians working as assistants to primary operator physicians, and rotating staff involved in peri-procedural care of patients. Purpose This study aimed to find out ED acquired by scrub technicians(ED.S) and rotating staff(ED.R) in interventional cardiology and invasive electrophysiology procedures, utilizing external dosimeters worn over lead aprons. Methods We prospectively collected radiation data over forty six invasive Cardiology procedures from 15th March 2021 to 1st June 2021 comprising fourteen diagnostic coronary and graft angiograms, seventeen combined diagnostic and interventional cardiology procedures, nine sole percutaneous coronary interventions and six electrophysiology procedures. Nine staff members: two female and seven male members participated as scrub technicians and rotating staff, provided with external dosimeter badges. ED was obtained by logging in dosimeter badges into pre calibrated monitors. Results Significantly high mean ED was acquired by rotating staff as compared to scrub technicians, mean ED.R:21.04 μSv( SD: +/-39.64) and mean ED.S: 7.54 μSv( SD:+/- 17.23), this three fold difference in radiation exposure was statistically significant with a p-value of 0.03. This was consistent across procedure groups, with rotating staff acquiring higher ED compared to their scrub technician colleagues in diagnostic, interventional and electrophysiology cases. Statistically significant differences in radiation exposure of both staff groups were demonstrated in diagnostic and interventional cardiology procedures, with p-values of 0.04 and 0.01 respectively.(See Table) Conclusion This small, single center study highlights occupational role as a significant factor in varying radiation exposure amongst non-physician staff . Higher ED was attributed to rotating staff duties involving proximity and mobility around fluoroscopy source and patient, devoid of protective lead shields. Based on these findings, avoiding fluoroscopy use while staff performs duties involving proximity to patient such is mandatory. Other essential measures should include accessory left sided leads shields, mandatory protective lens and head shields for rotating staff. Abstract Table

A demineralized osteostracan fossil from the Silurian Kalana Lagerstätte of Estonia: revealing its internal anatomy and uncovering a unique type of fossilization

The anatomy of a dorsal head shield of an agnathan fossil Kalanaspis delectabilis, belonging to class Osteostraci from the Aeronian (early Silurian) Kalana Lagerstätte of Estonia was studied using the X-ray computer-tomography. Scanning exposed shallow superficial relief on the dorsal surface of the head shield, which unmasked the internal anatomy of the specimen. The Kalanaspis fossil displays a mosaic of morphological characters: the overall shape of the cephalothoracic head shield and the main structures, like lateral fields, are typical for the group, but instead of the open pineal foramen, common to most osteostracans, the organs in the pineal region are concealed by the shield. A combination of characters that are common to osteostracans, along with unique features known among other early vertebrates, refer to the position of K. delectabilis within the stem group of Osteostraci. The carbonaceous mode of preservation of the head shield of the Kalanaspis fossil, previously undocumented among vertebrate fossils, contradicts the conventional fossilization pattern and refers to an unusual taphonomic history. This anomalous type of preservation appears to be the key to unlock its fossilization history and the complex taphonomic conditions of the whole Kalana Lagerstätte. The main processes for the atypical fossilization of the specimen of K. delectabilis are most likely related to the microbially controlled dissolution of apatitic bone tissues and replacement with microbial biofilms, possibly by the invasion of collagenolytic bacteria.

НЕКОТОРЫЕ ВОПРОСЫ ТЕРМИНОЛОГИИ ЭЛЕМЕНТОВ ПАНЦИРЯ ПАЛЕОЗОЙСКИХ РЫБ (PLACODERMI, ANTIARCHI)

The terminology and morphology of plates of the posterior part of the antiarch head shield (Placodermi, Antiarchi) are discussed. The terms «zatilochnaya» and «bokovaya (kraevaya) zatilochnaya» was previously accepted in antiarch skulls and are suggested for use in Russian-language literature. These terms are more correct and clearly define a position and development of these plates in the head shield of antiarchs. The titles «zagrivkovaya (nuchalnaya)» and «bokovaya zagrivkovaya (paranuchalnaya)», recently applied to them in Russian-language literature may indicate a connection in development with an anterior part of a trunk. A similar term is used for acipenserid exoskeleton. The acipenserid «nuchalnaya kost’» is located posterior to the «verchnezatilochnaya» (after Gurtovoi), and embryonically developed in an anterior part of the trunk over basidorsals and bones of the shoulder girdle. The name «pervaya spinnaya plastinka» (first dorsal scute: after Hilton and others) is also use for this bone. The term «zagrivkovaya plastinka» is used in other vertebrate skeletons, for example, in turtles; this name denotes the unpaired element of a carapace (postcranial skeleton). Using the terms «zagrivkovaya (nuchalnaya)» and «bokovaya zagrivkovaya (paranuchalnaya)» in the morphology of antiarchs and other placoderms may lead confusion in the terminology of skeletal elements at early vertebrates, incorrect conclusion of their homology, structure and development of the head shield of these unusual fishes.