ACANTHODIAN HISTOLOGY : SOME SIGNIFICANT ASPECTS IN TAXONOMICAl, AND

PHYLOGENETICAL RESEARCH

JUOZAS V A L I U K E V I ( ~ I U S

VALIUKEVICIUS J. 1995. Acanthodian histology : some significant aspects in taxonomical and phylogenetical research. [Histologie des Acanthodiens quelques aspects impertants pour les recherches taxonomiques et phylog~- n~tiques]. GEOBIOS, M.S, n ° 19 : 157-159.

ABSTRACT

Four histologic types are determined based on acanthodian scale structure. Nostolepis type (Climatiiformes) : scale crowns composed of simple or oriented mesodentine and base of cellular bone. Diplacanthus type (Diplacanthifor- rues): crown composed of simple mesodentine and base of acellular bone containing vascular canals. Acanthodes type (Acanthodiformes) : crown composed of dentine and base of acellular bone with narrow vascular (?) canals. Poracanthodes type (Ischnacanthiformes) : crown composed of dentine, simple mesodentine or both and containing pore canals, opening superficially or on the neck; the base is composed of both cellular and acellular bone.

KEY-WORDS : DENTINE, MESODENTINE, BONE, ENAMEL, VASCULAR CANALS.

RI~SUMI~

Selon la structure des ~cailles, on distingue quatre types histologiques chez les Acanthodiens. Le type Nostolepis (Climatiiformes) : couronne de m~sodentine simple ou orient~e, et base de tissu osseux cellulaire. Le type Dipla- canthus (Diplacanthiformes) : une couronne de m~sodentine pure, base de tissu osseux acellulaire avec des canaux vasculaires. Le type Acanthodes (Acanthodiformes) : couronne de dentine, base d'os acellulaire avec de fins canaux vasculaires (?). Le type Poracanthodes (Ischnacanthiformes) : couronne de dentine, de m~sodentine simple ou des deux, avec des canaux dont les pores s'ouvrent ~ la surface de la couronne ou sur le collet, base des ~cailles constitute de tissu osseux cellulaire ou acellulaire.

MOTS-CL]~S : DENTINE, M]~SODENTINE, OS, I~MAIL, CANAUX VASCULAIRES.

INTRODUCTION Four his tological types are obse rved in k n o w n a n d enough s tud i ed acan thod ians , based on scale m i c r o s t r u c t u r e : Nostolepis, Diplacanthus, Acan- thodes a n d Poracanthodes.

Nostolepis t ype (Gross 1971). The crown is com- posed of m e s o d e n t i n e (s imple or o r ien ted -- S t r a n g g e w e b e a f t e r Gross), a n d the base of cellu- l a r bone. This type is cha rac te r i s t i c of acan tho- d ians be long ing to the Orde r Cl imat i i fo rmes , in- c luding the fol lowing g e n e r a : Nostolepis PANDER 1856, Cheiracanthoides WELLS, 1944, Watsona- canthus VALIUKEVICIUS, 1979, Canadalepis VIETH, 1980, Wetteldorfia and Eiffelepis VIETH- SCHREINER, 1983, Mimioracanthus VALIUKEVI- CIUS, 1985, Laliacanthus KARATAJUTE-TALIMAA,

1986, Hanilepis WANG & DONG, 1989, Tareyacan- thus and Taimyrolepis VALIUKEVI~IUS, 1994 and Endemolepis (Valiukevi~ius in press) .

The nostolepidid a c a n t h o d i a n s a re divided in two groups according to the p re sence or absence of o r ien ted mesoden t ine as a f e a tu r e of the h ighes t t axonomic value . The or ien ted meso- den t ine is c lear ly shown in Tareyacanthus, Ende- molepis and Cheiracanthoides (with only one ex- cept ion in the l a t t e r : C. borealis VALIUKEVI~IUS, 1994). I t is ab sen t in the monospeci f ic g e n e r a Watsonacanthus, Wetteldorfia, Eiffelepis, Lalia- canthus, Taimyrolepis, Canadalepis, Hanilepis and Minioracanthus. The r e p r e s e n t a t i v e s of Nos- tolepis are d i s t r ibu ted into two groups : a lmos t a h a l f (9 out of 22) his tological ly s tud ied nostole- pids conta in this t i s sue in the crown w h e r e a s the

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other 13 are characterized by its absence, such as N. alta (M~irss 1986), N. guangxiensis (Wang 1992) and the whole group of species from Tai- myr (Valiukevi~ius 1994). Oriented mesodentine composes already the primordial scale (the oldest growth lamella) in all genera except Cheiracan- thoides, thus it starts with the youngest lamellae.

P A L E O H I S T O L O G I C A L T Y P E S I N A C A N T H O D I A N S C A L E S

Nostolepis type most of the representatives of Nostolepis have cell cavities in the mesodentine of the crown (sometimes even in the youngest la- mellae) and lack enamel in superficial layers. Un- doubtedly cell spaces are present in the crowns of Taimyrolepis, Canadalepis and Endemolepis whe- reas in Watsonacanthus and Tareyacanthus they are present in the primordial scale only. Lalia- canthus, Wetteldorfia, Eiffelepis and Minioracan- thus contain no such cavities in the crown.

The greatest diagnostic character of the vascular canals of mesodentine are the radial ones direc- ted centripetally and placed in the scale neck above the base. The Nostolepis species, as a rule, and the representat ives of Laliacanthus, Watso- nacanthus, Wetteldorfia, Eifellepis, Taimyrolepis and Minioracanthus as well, are distinguished by small, thin, networked or entirely not expressed radial canals. Only 8 nostolepids are charac- terized by large, wide, mult ibranched (robusta, arctica, alta and taimyrica) or monobranched (striata, gracilis, kervanensis and minima) canals. On the contrary, the Cheiracanthoides species have large and branching radial canals. Long, wide, circular and clear branched ascending vas- cular canals are characteristic of Cheiracanthoi- des, Tareyacanthus, Canadalepis and Endemole- pis, whereas Laliacanthus, Watsonacanthus, Wet- teldorfia, Eiffelepis, Taimyrolepis, Hanilepis and Minioracanthus stand out by their small, winding and networked ones.

4-6 growth lamellae on the crown are charac- teristic of most of the genera of this histological type ; there are seldom 8-12 such lamellae. Ex- t remely numerous and thin lamellae are known in Nostolepis multicostata (Vieth 1980). The sca- les of all genera grow superpositionnally except those of two nostolepids (robusta and spina) which grow in an areal way.

Diplacanthus t ype . (Valiukevi~ius 1985). The crown is made of simple mesodentine and the base of acellular bone pierced by multibranched vascular canals. These taxa are grouped in the Order Diplacanthiformes, including the genera

Diplacanthus AGASSIZ, 1844, Rhadinacanthus TRAQUAIR, 1888 and Ptychodictyon GROSS, 1973. Representatives of these genera have long, multi- branched (but with clear main branches), often bushy horizontal vascular canals which contain characteristically upwardly directed dense den- tine tubules. In the Diplacanthus and Rhadina- canthus species (Gross 1973) arcade connexions of the horizontal canals are placed anteriorly in the crowns, whereas they are absent in Ptycho- dictyon. The two lat ter species have long bran- ched ascending vascular canals. Winding, networ- ked canals are very characteristic of Diplacan- thus. The latter and Ptychodictyon have lacunae, which are lacking in Rhadinacanthus. Repre- sentatives of these three genera are distinguished by enlarged, widened and complex central canals in the primordial scales. The Ptychodictyon spe- cies have pore and slime canal nets forming anew in each growth lamella, contrary to Poracantho- des. In the elder lamellae they are changed into rounded denteons. The enamel-like t issue (thin layers) is present in Rhadinacanthus only. The scales of all genera grow superpositionally with 3-6 growth lamellae in Rhadinacanthus, 4-9 in Diplacanthus and 6-10 in Ptychodictyon.

Acanthodes t y p e (Gross 1971). The crown is composed of dentine and enamel like tissue, and the base of thin-layered compact acellular bone with short narrow vascular (?) canals. This is ex- clusive of acanthodians of the Order Acanthodi- formes, including the following genera - Cheiran- canthus AGASSIZ, 1845, Acanthodes? AGASSIZ, 1833, Haplacanthus AGASSIZ, 1845, Devononchus GROSS, 1940, Gomphonchus GROSS, 1971 (G.hop- pei excluded), Ectopacanthus VALIUKEVICIUS,

Vahukevaclus in 1979 (E. pusiUus excluded, ' "*" press) and Markacanthus VALIUKEVI~IUS, 1985. (Isodendracanthus Y VALIUKEVICIUS, 1979 may be- long to this type, but it is not included here be- cause of our paucity of histologic data).

The studied taxa can be divided in two groups based on histologic features : the first one inclu- des Cheiracanthus and Markacanthus and the se- cond comprises all the other mentioned genera.

Cheiracanthus (9 species) and Markacanthus (3 species) are the best characterized by features of the crown vascular canals. Both genera are dis- t inguished by long, multibranched ascending, ho- rizontal (with clear main branches), radial (pla- ced over the base as well as at the turning of ascending growth lamellae into horizontal ones) and large, widened circular canals. Both genera have complex, enlarged dentine canals in primor- dial scales.

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The Ectopacanthus, Haplacanthus, Devononchus, Acanthodes? and Gomphonchus species conta in long and s o m e w h a t b r anchy ascending vascu la r canals, as wel l as small, na r row and less c lear circular vascu la r canals. Hor izonta l and radia l canals are m a in ly long, a l i t t le b r anchy wi th clear ma in b ranches (except the Gomphonchus species the hor izonta l canals of which mak e a dense ne twor k and the radia l ones small net- works). Thick layers of enamel- l ike t i ssue in crowns are charac te r i s t i c of all t hese genera (con- t r a r y to t h in layers in Markacanthus and a few species of Cheiracanthus).

The scales grow superpos i t iona l ly : the mos t fre- quen t n u m b e r of growth lamel lae is 8 to 12, t he m a x i m u m is 22 in Acanthodes?.

Poracanthodes t y p e (Valiukevi~ius in press). The crown is composed of dent ine , s imple meso- dent ine or both, wi th pore canals opening superfi- cially and on the neck, or on the l a t t e r only. The base is m a d e of both cel lular and acel lular bone. This type is found in the following genera of the Order I schnacan th i fo rmes : Poracanthodes BROW- ZEN, 1934, Ectopacanthus VALIUKEVI~IUS, 1979, (E. pusillus only), Lietuvacanthus VALIUKEVICIUS (in press) and Gomphonchus (G. hoppei GROSS, 1947).

The mos t s t r ik ing histologic f ea tu re of these sca- les is the p resence of a pore canal sys tem functio- n ing as a long living organ. I t consists of a var ia- ble n u m b e r of radia l (2 to 12, m a x i m u m in Pora- canthodes menneri, Valiukevi~ius 1992), a rcade (absent in P. porosus and Gomphonchus hoppei) and superf ic ia l openings or ien ted in rows or uno- r iented. Lietuvacanthus and Ectopacanthus have openings only on the neck,

The crowns show dent ine only (like P. punctatus and G. hoppei), or t rue mesoden t ine (P. menneri), or both (Lietuvacanthus, E. pusillus). Cellular (P. subporosus), acel lu lar (E. pusillus and G. hoppei) or both s t r uc tu r e s of bone (P. menneri and Lietu- vacanthus) are observed in the base. The species of Poracanthodes are d is t inguished by long, bran- ched ascend ing and rad ia l vascu la r canals centr i- pe ta l ly p laced over the base, whe reas Lietuvacan- thus, E. pusiUus and G. hoppei have narrow, win- ding and ne tworked ones. The las t th ree only have layers of enamel- l ike t i ssue in the crown.

The scales grow superposi t ional ly , except in the th ree species of Poracanthodes (porosus, menneri and gujingensis (Wang & Dong 1989), w i th a

charac ter is t ic areal one-sided growing. The num- ber of growth lamel lae var ies f rom 6 (Lietuvacan- thus) to 12 (Poracanthodes).

Undoubtedly , the use of histologic inves t igat ions to de te rmine t axa is of g rea t impor tance . Accor- dingly to the observed acan thodians , t he dispat- ching of the Ectopacanthus and Gomphonchus species into two different histologic types should be dismissed. Based on t r u e obvious differences in the r ep resen ta t ives of these genera, the scales like those of Gomphonchus hoppei and Ectopa- canthus pusillus per t a in p robab ly to a single new taxon of generic level.

R E F E R E N C E S

GROSS W. 1971 - Downtonische und Dittonische Acan- thodier-Reste des Ostseegebietes. Palaeontographi- ca, A, Stuttgart, 136 : 1-82.

GROSS W. 1973 - Kleinschuppen, Flossentacheln und Z~me von Fischen aus europ~iischen une nordame- rikanischen Bonebeds des Devons. Palaeontogra- phica, Stuttgart, A, 142 : 51-155.

MARSS T. 1986 - [Silurian Vertebrates of Estonia and West Latvia]. Tallinn. (in Russian with English summar~v) : 104 p.

VALIUKEVI~IUS J. 1985 - Acanthodians from the Narva Regional stage of the Main Devonian Field. Vilnius, Mosklas (in Russian with English summary).

VALIUKEVICIUS J. 1992 - First articulated Poracantho- des from the Lower Devonian of Svernaya Zemlya. In MARK-KURIK E. (ed.) : Fossil fishes as living ani- mals. Academia, 1. Tallinn : 193-213.

VALIUKEVI~IUS J. 1994 - [Acanthodians and their stra- tigraphic significance]. In CHERKESOVA S.V., KARA- TAJUTE-TALIMAA V.N. & MATUKHIN R.G. (eds) : stra- tigraphy and fauna of the Lower Devonian from Ta- reya key section (Taimyr)]. St.Petersbourg : 131- 197 and 236-243 (in Russian).

VALIUKEV'I~IUS J. in press - Acanthodians and zonal stratigraphy of Lower and Middle Devonian in Bal- tic and Belarus. Palaeontographica, A, Stuttgart.

VIEWS J. 1980 - Thelodontier, Acanthodier und Elas- mobranchier Schuppen aus dem Unter Devon der kanadischen Arktis (Agnatha, Pisces). G6ttinger Ar- beiten ~r Geologie und Pali~ontologie, 23 : 69 p.

WANG N.-Z. & DONG Z.-Z. 1989 - Discovery of Late Silurian microfossils of Agnatha and Fishes from Yunnan, China. Acta Palaeontologica Sinica, 28 (2): 192-206 (in Chinese with English summary).

WANG N.-Z. 1992 - Microremains of Agnathans and Fishes from Lower Devonian of Central Guanxi and Eastern Yunnan, South China. Acta Palaeontologi- ca Sinica, 31 (3) : 280-303 (in Chinese with English summary).

J. VALIUKEVICIUS Institute of Geology

Sevcenkos 13 2600 Vilnius, Lithuania