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Evolution of the Premaxillary in the Primitive Fossil Actinopterygians

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Evolution of the Premaxillary in the Primitive Fossil Actinopterygians Evolution of the premaxillary in the primitive fossil actinopterygians Cécile POPLIN Laboratoire de Paléontologie - URA 12 du CNRS, Muséum national d'Histoire naturelle, 8 rue de Button, F-75231 Paris cedex 05 (France) Richard LUND Department of Environmental Studies, Adelphi University, Garden City, NY 11530 (USA) Poplin C. & Lund R. 1997. — Evolution of the premaxillary in the primitive fossil actinoptery­ gians. Geodiversitas 19 (3) : 557-565. ABSTRACT The different conditions of the premaxillary in primitive actinopterygians are individualized, or fused to adjacent bones. Their dental field can also be divi­ ded and fused to neighbouring bones, the premaxillaries may be separated across the midline, ot, finally, they have disappeared. This separation and absence can result in a rostral notch observed in rhadinichthyids, and presu­ med in other groups such as the redfieldiiformes. It can be sug­ gested that the plesiomorphous gnathostome dermal skeletal condition is micromeric, that the primitive actinopterygian larval snout condition derived from that condition was mesomeric, and that heterochronic changes during early actinopterygian evolution gave rise during development (1) of mesome­ ric adult primitive actinopterygians through neoteny, (2) of macromeric KEYWORDS adult primitive actinopterygians through fusions of bones, (3) to the condi­ Vertebrata, Actinopterygii, tion of actinopterygians lacking a premaxillary through its loss. The neopte- comparative anatomy, rygian condition may have arisen through paedomorphosis from either evolutionary trend, >remaxillary, mesomeric primitive fossil actinopterygians, or directly by heterochrony Jermal snout. from the larval primitive condition. • 1997 • 19 (3) 557 Poplin C. & Lund R. RÉSUMÉ Les différentes dispositions du prémaxillaire chez les actinoptérygiens primi­ tifs sont : différencié ou fusionné à des os adjacents, le territoire dentaire peut être divisé et fusionné à des os voisins, les deux prémaxillaires être séparés sans contact l'un avec l'autre, enfin le prémaxillaire peut avoir disparu. Cette séparation ou cette absence se traduisent le plus souvent par une lacune osseuse rostrale observée chez des rhadinichthyidés et supposée dans plusieurs autres groupes dont les redfieldiiformes. Il est suggéré que la disposition plé- siomorphe du squelette dermique des gnathostomes est micromérique, que la disposition larvaire du museau des actinoptérygiens dérivée de cette disposi­ tion plésiomorphe était mésomérique et que les changements hétérochro- niques au début de l'évolution des actinoptérygiens ont donné pendant le développement, (1) par néoténie, des actinoptérygiens primitifs adultes MOTS CLES mésomériques, (2) par des fusions osseuses, des actinoptérygiens primitifs Vertebrata, adultes macromériques, (3) par disparition, des formes dépourvues de pré­ Actinopterygii, anatomie comparée, maxillaire. Les néoptérygiens seraient issus par pédomorphose, soit des acti­ tendance évolutive, noptérygiens primitifs fossiles mésomériques, soit directement, par prémaxillaire, museau dermique. hétérochronie de la disposition primitive larvaire. INTRODUCTION THE DIFFERENT STATES OF THE PRE­ MAXILLARY IN LOWER ACTINOPTERY­ The snout in teleostome fishes is a region of the GIANS dermal skull concentrating elements sensitive to adaptations and evolution, as it bears teeth, In order to avoid confusion owing to the many narial openings and the anteriormost part of the bone nomenclatures used by authors, which lateral sensory system. The variations of its ana­ somehow darkened the understanding of the der­ tomy have been often dealt with in the past mal snout evolution in the past, the "premaxilla­ (Westoll 1937; Pehrson 1947, 1958; Gardiner ry" is defined here as the most anterior paired 1963; Wenz 1967; Pearson & Westoll 1972; upper anamestic toothbearing bone of the rim of Patterson 1975; Pearson 1982; Schaeffer 1984; the mouth. When it is fused to other bony terri­ Long 1988). More recently the evolutionary tories, compound names are used (Poplin & implications of some of its bony units in lower Lund 1995). As a whole, the five following states actinopterygians were discussed (Poplin & Lund of the premaxillary have been observed among 1995). In the present work, analysis is focussed primitive actinopterygians. on the area of the premaxillary among the der­ mal bones of the snout which underwent the 1. Individualized typical premaxillae are rather most numerous modifications of any area of rare in lower fossil actinopterygians: for instance, skull bones. Therefore, it is potentially one of the in Lower Carboniferous, fishes from Bear Gulch most significant complexes of bones and lateral (Montana, USA), the Triassic Pteronisculus line canals for phylogenetic purposes in lower (Nielsen 1942), Perleidus (Fig. 1A; Lehman actinopterygians. Because of difficulties in obser­ 1952). According to Hutchinson (1978), it is vation and preservation in fossil fishes, accurate also present in monophyletic lineages such as the descriptions and reconstructions of the snout are Triassic Brookvaliidae and Schizurichthyidae. In rather rare so that we refer here to the best recent forms, separate premaxillaries are present known taxa, even if reappraisials of them should in larval Polypterus (Fig. IB; Pehrson 1947, be eventually desirable. 1958) and in all the neopterygians, where they 558 GEODIVERSITAS • 1997 • 19(3) Evolution of the premaxillary in actinopterygians may even be larger than the maxillae (Fig. 1C). resulting in compound bony units such as "ros- tro-premaxillo-antorbitals", "rostro-premaxilla- 2. More often, in lower fossil actinopterygians, ries" and "antorbito-premaxillaries" (Fig. ID, E; the premaxillary is fused with adjacent bones Poplin & Lund 1995). In adult Polypterus the FIG. 1. — Dermal snouts: A-G, left lateral view; H, front view. A, Perleidus piveteaui (after Lehman 1952, fig. 86); B, Polypterus, 9.6 mm larva (after Pehrson 1958, fig. 8); C, Pomatomus saltatrlx (after Gregory 1933, fig. 177); D, Birgerla groenlandica (after Nielsen 1949, fig. 69); E, Moythomasia nitida (after Jessen 1968, fig. 1); F, Polypterus adult (after Daget 1958, fig. 1791); G, Watsonichthys pectinatus (after Gardiner 1963, fig. 1); H, Howqualepis rostridens (after Long 1988, fig. 14B). APx, antorbito-pre- maxillary; AoPxR, antorbito-premaxillo-rostral; IMx, Infraorbito-maxillary; Mx, maxillary; Na, nasal; Px, premaxillary; Pr, postrostral; RAo, rostro-antorbital; RPx, rostro-premaxillary; RPrPx, rostro-postrostro-premaxlllary. GEODIVERSITAS • 1997 • 19(3) 559 Poplin C. & Lund R. premaxillary is fused to the lateral rostral and to in Lower Carboniferous paleoniscoids from Bear the antorbital (Fig. IF; Pehrson 1947, 1958; Gulch (Montana, USA; Poplin & Lund 1995). Lehman 1958: 2092). The absence of the premaxillary results either in 3. The territory of the premaxillary dental field the anterior development of the maxillaries may also be divided into two units that are fused which then meet each other above the mouth to neighbouring bones (Moy-Thomas 1934; Ten (e.g. Aeduella, Fig. 2C), or in the snout skeleton Cate 1985). This is the case for instance in having a median notch above the mandibular Watsonichthys pectinatus (Fig. 1G) which shows symphysis. Such a notch is observed in Lower an antorbito-premaxillary together with a rostro- Carboniferous rhadinichthyids (Fig. 2A, B) like premaxillary. The Devonian Moythomasia durga- those from Montana (Poplin & Lund 1995) and ringa and Howqualepis rostridens (cf. Gardiner Scotland (Moy-Thomas & Dyne 1938). We sus­ 1984; Long 1988) are more complex and ques­ pect the presence of this notch in many other tionable cases (Fig. 1H); their anterior marginal paleoniscoid forms which lack the characteristic teeth are borne by three bones: paired antorbito- features of the premaxillary and show, in side premaxillaries (= Gardiner's and Long's "pre- view, a typically protruding snout above the maxillaries") and a median rostro-postrostro- aperture of the mouth: for instance Palaeoniscus premaxillary (= "dentigerous rostro-postrostral"). (Fig 2D; Aldinger 1937), Cycloptychius, Elonich- According to our interpretation, the territories of thys (Moy-Thomas & Dyne 1938), Boreosomus the right and left premaxillaries (or their dental (Nielsen 1942; Lehman 1952), Dicellopyge fields) are divided into four units: the median (Brough 1931). ones are fused to the single rostro-postrostral and the lateral ones are fused to the paired antorbitals Schaeffer's description suggests that the (Poplin & Lund 1995). Redfieldiidae (Fig. 2E) probably also had this anterior notch (1967: 307, 308): "As the man­ 4. In some cases, such as in some Bear Gulch dibles are about the same length as the infraorbi­ taxa, both premaxillaries are so weak and so loose tal ramus of the maxilla, the toothlike denticles that they are not in contact with each other. This along the ventral margin of the rostral and the situation results in a small median rostral notch antorbital could not have functioned in seizing quite like that described below. prey. Furthermore, it has not been possible to reconstruct [...] the snout [...] in a way that 5. The last state of the premaxillary is encounte­ would permit the rostral teeth to meet the man­ red in a number of primitive fossil actinoptery- dibular ones, even if the lower jaws were actually gians: its disappearance revealed by the complete longer. Part of the space between antorbitals absence of the characteristic features of its terri­ must have been occupied
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