Gloria Arratia

Palatoquadrate and its ossifications: Development and homology within osteichthyans

The palatoquadrate and associated dermal bones have significant evolutionary transformations among teleostomes and provide numerous features that characterize teleostomian subgroups. The palatoquadrate forms the upper part of the mandibular arch and is present as a single cartilaginous element in the early ontogeny of teleostomes, except for some advanced teleosts such as siluroids where it is divided into pars autopalatina and pars pterygoquadrata. During ontogeny, the palatoquadrate may ossify as a unit, with a pars autopalatina (absent in Acanthodii), pars quadrata, and pars metapterygoidea in teleostomes (e.g., primitive acanthodians and actinopterygians, onychodonts, and rhipidistians). However, the palatoquadrate may remain cartilaginous (e.g., chondrosteans) or it may ossify as separate elements (e.g., autopalatine, metapterygoid, and quadrate) as occurs in advanced acanthodians, Polypterus and advanced actinopterygians, and advanced actinistians. From the single‐unit pattern, separate autopalatine, metapterygoid, and quadrate evolve in parallel in the three teleostomian subgroups. Therefore, it is necessary to distinguish between actinopterygian and actinistian autopalatines and among acanthodian, actinopterygian, and actinistian metapterygoids and quadrates. A palatoquadrate fused with the neurocranium occurs in parallel in dipnoans.

The urohyal : development and homology within osteichthyans

The formation of the unpaired structure ventral to the basibranchial region, the so‐called urohyal, differs within osteichthyans. A cartilaginous preformed, unpaired “urohyal” is present in sarcopterygians. A three‐tendon ossification is present in Polypterus. An “urohyal” or urohyal is absent in both Amia and Lepisosteus. The urohyal formed as an unpaired ossification of the tendon of the sternohyoideus muscle is a feature of teleosts. A new structure, the parurohyal, arises as a double ossification of the tendon of the sternohyoideus muscle in siluroids; during ontogeny an anterodorsal crest or cup‐like structure derives from the anterior basibranchial region and the tendon bone; therefore, the parurohyal is compound in origin. Judging from their formation and their distribution within osteichthyans the cartilaginous preformed “urohyal” and the teleostean urohyal are nonhomologous, whereas the urohyal and parurohyal are homologous. The urohyal is connected by ligaments with the ventral hypohyals in most teleosts, whereas it articulates with the ventral hypohyals in teleosts such as Anguilla and Chanos. The parurohyal is a synapomorphy of siluroids. The parurohyal in siluroids is articulated with both ventral and dorsal hypohyals, and with the basibranchial region in catfishes such as diplomystids and ictalurids, whereas it articulates only with the ventral hypohyals in other catfishes such as trichomycterines. The passage of the hypobranchial artery through the hypobranchial foramen of the parurohyal is a unique feature of siluroids, like the absence of the basihyal bone.

Reevaluation of the caudal skeleton of certain actinopterygian fishes: III. Salmonidae. Homologization of caudal skeletal structures

The ontogenetic development of caudal vertebrae and associated skeletal elements of salmonids provides information about sequence of ossification and origin of bones that can be considered as a model for other teleosts. The ossification of elements forming the caudal skeleton follows the same sequence, independent of size and age at first appearance. Dermal bones like principal caudal rays ossify earlier than chondral bones; among dermal bones, the middle principal caudal rays ossify before the ventral and dorsal ones. Among chondral bones, the ventral hypural 1 and parhypural ossify first, followed by hypural 2 and by the ventral spine of preural centrum 2. The ossification of the dorsal chondral elements starts later than that of ventral ones. Three elements participate in the formation of a caudal vertebra: paired basidorsal and basiventral arcocentra, chordacentrum, and autocentrum; appearance of cartilaginous arcocentra precedes that of the mineralized basiventral chordacentrum, and that of the perichordal ossification of the autocentrum. Each ural centrum is mainly formed by arcocentral and chordacentrum. The autocentrum is irregularly present or absent. Some ural centra are formed only by a chordacentrum. This pattern of vertebral formation characterizes basal teleosts and primitive extant teleosts such as elopomorphs, osteoglossomorphs, and salmonids.

Reevaluation of the caudal skeleton of some actinopterygian fishes: II. Hiodon, Elops, and Albula

The vertebral centra of Hiodon, Elops, and Albula are direct perichordal ossifications (autocentra) which enclose the arcocentra as in Amia. An inner ring of ovoid cells forms in late ontogeny from the intervertebral space inside the autocentrum. The chordacentrum is reduced or completely absent in centra of adult Elops, whereas it forms an important portion of the centra in adult Hiodon. The posterior portion of the compound ural centrum 3+4+5 is partially (Hiodon) or fully formed by the chordacentrum (Elops, Albula). The haemal arches and hypurals are fused medially by cartilage or bone trabecles of the arcocentrum with the centra, even though they appear autogenous in lateral view in Elops and Albula. The composition of the caudal skeleton of fossil teleosts and the ontogeny of that of Hiodon, Elops, and Albula corroborate a one‐to‐one relationship of ural centra with these dorsal and ventral elements. The first epural (epural 1) of Elops relates to ural centrum 1, whereas the first epural (epural 2) of Hiodon and Albula relates to ural centrum 2. In Albula, the first ural centrum is formed by ural centrum 2 only. With 4 uroneurals Hiodon has the highest number within recent teleosts. Juvenile specimens of Hiodon have eight, the highest number of hypurals within recent teleosts; this is the primitive condition by comparison with other teleosts and pholidophorids. Reduction of elements in the caudal skeleton is an advanced feature as seen within elopomorphs from Elops to Albula. Such reductions and fusions occur in osteoglossomorphs also, but the lack of epurals and uroneurals separates most osteoglossomorphs (except Hiodon) from all other teleosts.

THE SISTER-GROUP OF TELEOSTEI: CONSENSUS AND DISAGREEMENTS

Abstract Several hypotheses supporting monophyly of the Teleostei on the basis of synapomorphies have been produced over the last 30 years. The concept of Teleostei sensu Patterson (1977) and Patterson and Rosen (1977), with halecomorphs as the sister-group and †Pachycormiformes and †Aspidorhynchiformes at the base, has been questioned recently. A new hypothesis has been suggested (Arratia, 1999, 2000a) with †Pholidophorus as the basal taxon. Whereas the monophyly of Teleostei has been supported by numerous investigations based on morphological evidence of fossil and living forms and on molecular data, the sister-group of Teleostei is still unresolved. Possible sister-groups are the Amiiformes, Lepisosteiformes, †Dapedium, †Pycnodontiformes, †Pachycormiformes, and †Aspidorhynchiformes. Their relative positions in the cladogram changes when different outgroups are used. The large actinopterygian clade comprising the stem-groups of teleosts and the Teleostei (including fossil and extant members) and excluding the Halecomorphi (Amia and relatives) and the Ginglymodi (Lepisosteus and relatives) is formally named Teleosteomorpha. The name Teleostei is reserved here for the apomorphy-based taxon Teleostei that includes the fossil basal teleosts and the Teleocephala (crown-group). The monophyly of Teleostei is supported by one uniquely derived character and numerous homoplasious derived characters. The available information reveals that there is no correlation between the age of the probable sister-groups of Teleostei and their primitiveness. All of them appeared at the same time as the Teleostei (e.g., †Pycnodontiformes) or are younger (e.g., Amiiformes, Lepisosteiformes, †Dapedium, †Pachycormiformes, and †Aspidorhynchiformes). The closest living (Amia or Lepisosteus) and the closest fossil sister-group of teleosts remains unknown.

Development and diversity of the suspensorium of trichomycterids and comparison with loricarioids (Teleostei: Siluriformes)

Studies of ontogenetic series of trichomycterids and other catfishes reveal that the suspensorium of siluroids is highly specialized; several synapomorphies separate siluroids from other teleosts. In siluroids, the palatoquadrate is divided into pars autopalatina and pars pterygoquadrata and both are usually connected by the autopaiatine‐metapterygoid ligament. The pterygoquadrate is broadly joined to the dorsal limb of the hyoid arch, forming a cartilaginous hyomandibular‐symplectic‐pterygoquadrate plate in early ontogeny. This produces a special alignment of the hyomandibula and quadrate which is characteristic of siluroids. A symplectic bone is absent. The interhyal is absent in trichomycterids and astroblepids. Dorsal and ventral limbs of the hyoid arch are connected by a ligament. A rudimentary interhyal and this ligament are present in primitive siluroids such as diplomystids and nematogenyids as well as loricariids. The metapterygoid arises as an anterior ossification of the pars pterygoquadrata in siluroids. The formation and position of the metapterygoid exhibit two patterns: (1) the metapterygoid develops as an ossification of a cartilaginous projection positioned between the future hyomandibula and quadrate in primitive catfishes (e.g., Diplomystes) as well as in Nematogenys, callichthyids, loricariids, and astroblepids; (2) the metapterygoid arises as an ossification of the cartilaginous projection (pterygoid process) positioned just above the articular facet of the quadrate for the lower jaw. An ossified anterior chondral pterygoid process of the complex quadrate is present in trichomycterids, whereas the process is absent (simple quadrate) in catfishes such as diplomystids, nematogenyids, callichthyids, and loricariids. The anterior membranous process of the quadrate of Astroblepus is non‐homologous with the chondral pterygoid process of trichomycterids; both structures arose independently within the loricarioids. Despite topological relationships, the origin and development of bones reveal the presence of a chondral hyomandibula which develops a large meinbranous outgrowth during ontogeny and a chondral metapterygoid in trichomycterids. The presence of a compound hyomandibula + metapterygoid or a compound metapterygoid + ectopterygoid + entopterygoid have no developmental support in trichomycterines or other siluroids. The “entopterygoid” of Nematogenys and Diplomystes arises as an ossification of a ligament. The dermal entopterygoid of other ostariophysans and the “entopterygoid” are homologous. An ectopterygoid or tendon bone “ectopterygoid” is absent in loricarioids. The suspensorium is an important structural system which has significant evolutionary transformations which characterize loricarioid subgroups; however, no character, of the suspensorium supports the monophyly of the loricarioids.

Reevaluation of the caudal skeleton of actinopterygian fishes: I. Lepisosteus and Amia

The centra of Lepisosteus are perichondral ossifications of arcualia (i.e., arcocentra), whereas those of Amia are direct perichordal ossifications (i.e., autocentra) that enclose the arcualia. The preural centra of Lepisosteus are monospondylous, whereas the ural centra are formations of inter‐ and basidorsal arcualia. In contrast, the preural centra of Amia are diplospondylous, whereas preural centrum 1 (and sometimes preural centrum 2) and ural centra are monospondylous. The ural centra of Lepisosteus are expansions of dorsal arcualia, but those of Amia are expansions of the basiventral autocentrum. This explains the fusion of the neural arches with the ural centra and the presence of autogenous hypurals in Lepisosteus, in contrast to the situation in Amia in which the compound ural neural arch (the fused ural neural arches) is free, and the hypurals are fused to the ural centra. Lepisosteus possesses true epurals, which are modified neural spines, whereas in Amia the “epurals” are positioned between the neural spines like radials. Lepisosteus and Amia possess a polyural caudal skeleton with a one‐to‐one relationship between ural centra and hypurals; the number of hypurals may be reduced in adult Lepisosteus.

New teleostean fishes from the Jurassic of southern Germany and the systematic problems concerning the ‘pholidophoriforms’

ZusamenfassungDie neue GattungSiemensichthys aus dem Oberen Jura Süddeutschlands wird beschrieben. Sie umfaßt die zwei ArtenS. macrocephalus (Agassiz) (ursprünglich in die GattungPholidophorus gestellt) undS. siemensi n. sp., die einige wenige Synapomorphien gemeinsam haben (z. B. nur ein Supramaxillare, das den dorsalen Rand des Maxillare bedeckt). Beide Arten werden beschrieben und ihre phylogenetische Stellung analysiert. Die phylogenetischen Analysen von 27 Taxa und 141 Merkmalen zeigen, daßAnkylophorus aus dem Kimmeridgium von Cerin undSiemensichthys sowieEurycormus aus dem Solnhofener Plattenkalk Bayerns eine monophyletische Gruppe bilden. Diese neue ausgestorbene Gruppe (vorläufig alsSiemensichthys- Gruppe bezeichnet) ist die Schwestergruppe vonPholidophorus s. str. und weiter fortgeschrittener Teleosteer. Das Schwestergruppen-Verhältnis ist durch 8 Merkmale belegt (z. B. Besitz eines Supraoccipitales; das Supraoccipitale reicht vorwärts in das Dach der Oticalregion; Articulare verschmolzen mit Angulare und Retroarticulare; Besitz eines verlängerten posteroventralen Fortsatzes des Quadratum; Besitz von dorsalen Fortsätzen an der Basis der innersten caudalen Flossenstrahlen des oberen Schwanzlobus; bewegliches Prämaxillare). Vergleiche mit Arten vonPholidophorus s. str. eröffnen ein neues Verständnis der GattungPholidophorus. Mindestens vier Synapomorphien werden aufgezeigt, die die Monophylie vonPholidophorus unterstützen. Daraus folgt, daß europäische spätjurassische Arten, die früher zu den Pholidophoridae und zur GattungPholidophorus (z. B. ‘Ph.’armatus, ‘Ph.’ falcifer, ‘Ph.’ micronyx, ‘Ph.’ microps) gestellt wurden, neu untersucht werden müssen, denn sie gehören weder in die Familie noch in die Gattung. Die Ordnung PholidophoriformesBerg ist keine monophyletische Gruppe. Die sogenannten ‘Pholidophoriformes’ bedürfen einer Revision.AbstractA new genus,Siemensichthys, from the Upper Jurassic of southern Germany is described. The new genus includes two species,S. macrocephalus (Agassiz) which was formerly in the genusPholidophorus, andS. siemensi n. sp. The two species share synapomorphies such as only one supramaxillary bone covering the dorsal margin of the maxilla. Both species are described, and their phylogenetic position is analyzed. The phylogenetic analyses, based on 27 taxa and 141 characters, show thatAnkylophorus from the Kimmeridgian of Cerin,Siemensichthys andEurycormus from the Solnhofen Lithographic Limestone of Bavaria, form a monophyletic group. The new extinct clade (preliminarily identified as theSiemensichthys- group) is proposed as the sister-group ofPholidophorus s. str. plus more advanced teleosts. This sister-group relationship is supported by eight characters (e.g., supraoccipital bone extending forward in the roof of the otic region; articular bone fused with both the angular and retroarticular; presence of an elongated posteroventral process of quadrate; presence of dorsal processes at the base of the innermost caudal rays of upper lobe; mobile premaxillary bone). Comparisons with species ofPholidophorus s. str. provide a new understanding of the genusPholidophorus. At least four synapomorphies are proposed to support the monophyly ofPholidophorus. As a consequence of this new interpretation, the European Late Jurassic species previously assigned to the Pholidophoridae and to the genusPholidophorus (e.g., ‘Ph.’armatus, ‘Ph.’ falcifer, ‘Ph.’ micronyx, ‘Ph.’ microps) should be reexamined because they do not belong to the family nor to the genus. The order PholidophoriformesBerg is not a monophyletic group as currently constructed. Therefore, all so-called pholidophoriforms are in need of revision.

Jaws and Teeth of American Cichlids [Pisces: Labroidei]

The morphology of the upper, lower, and pharyngeal jaws is very similar among American cichlids. Common conditions are: (1) the presence of a premaxillary dentigerous arm shorter than the ascending arm (exceptions are Astronotus, Cichla, and Crenicichla semifasciata), (2) a narrow coulter area; in contrast, a broad coulter area is found in the Crenicichline Group, in certain chaetobranchines, and in Apistogramma, (3) the mandibular sensory canal exists to the skin through five or six simple pores; in contrast, it exits through numerous small pores that increase in number during ontogeny in the Chaetobranchine Group, certain crenicichlines, such as Cichla, Crenicichla lepidota, Crenicichla proteus, and Crenicichla vittata, and certain genera of the Cichlasomine Group A, such as Caquetaia, Petenia, Neetroplus, and “Cichlasoma,” and (4) the premaxilla and dentary of American cichlids commonly bear unicuspid, conical teeth with a few exceptions such as Neetroplus (with scraping blade teeth) and “Cichlasoma” facetum, “C.” cyanoguttatum, “C.” guttulatum, and “C.” spilurum (with bicuspid [hooked] teeth). In contrast to the near uniformity of the upper and lower jaws, the upper and lower pharyngeal jaws present a great diversity of tooth shapes. At least seven types are found in American cichlids; usually, several types exist on a single tooth plate, but the combination of tooth types differs among some genera.

LATE JURASSIC FISHES FROM LONGING GAP, ANTARCTIC PENINSULA

Abstract Few records of Late Jurassic fishes have been reported previously from Antarctica. They include an indeterminate teleost from the Ameghino (Nordenskjöld) Formation at Longing Gap and two incomplete aspidorhynchiforms from James Ross Island, all of presumed Late Jurassic age. New fish material recently recovered in the Upper Jurassic of Longing Gap is described. The new material consists of one piece of body squamation, which, based on the structure of the scales, corresponds to a new genus and species (Ameghinichthys antarcticus gen. et sp. nov.) of an indeterminate actinopterygian family; one aspidorhynchiform identified as Vinctifer sp. due to the structure and distribution of the scales; and numerous specimens of a new ichthyodectiform, Antarctithrissops seymouri gen. et sp. nov. This new genus differs from European ichthyodectiforms in the shape of the preopercle, the presence of long sensory preopercular branches almost reaching the posterior margin of the bone, and the uncommon structure of the scales, with a fine layer of bone obscuring the circuli. The presence of Vinctifer in the Antarctic is consistent with its other Gondwanan records. The Ichthyodectiformes, previously known from four European genera, extends the distribution of the group to the southernmost part of the Southern Hemisphere during the Late Jurassic. In contrast to most non-teleostean fishes, the known Late Jurassic teleosts apparently are species endemic to restricted areas in the Southern Hemisphere.