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AN EMPIRICAL SYNTHETIC PATTERN STUDY OF GARS (LEPISOSTEIFORMES) AND CLOSELY RELATED SPECIES, BASED MOSTLY ON SKELETAL ANATOMY. THE RESURRECTION OF HOLOSTEI

2010; American Society of Ichthyologists and Herpetologists; Linguagem: Inglês

1938-5110

Lance Grande, Herpetologists,

Paleontology and Evolutionary Biology

The comparative osteology, phylogenetic relationships, and historical biogeography of all known fossil and living gars (Ginglymodi: Lepisosteiformes) are investigated in detail. Clearly documented comparative data on the skeletons of lepisosteiforms and closely related taxa are examined with regard to phylogeny, ontogeny, historical biogeography, stratigraphic paleontology, and paleoecology. This study of lepisosteiform fishes is also used as a platform to explore several fundamental methodological and philisophical concepts important to phylogenetic/evolutionary investigations (pp. 819-828). The seven extant species of gars are the sole surviving members of a formerly diverse division of fishes, Ginglymodi (here including Lepisosteiformes, †Semionotiformes, and †Macrosemiiformes). Even Lepisosteiformes (the ginglymodin subgroup that is the focus of this study) was much more diverse taxonomically and morphologically during Mesozoic and Paleogene times than it is today. It was also much more geographically widespread. Lepisosteiformes, as defined here, are those neopterygian fishes with opisthocoelous vertebral centra, tongue (a basihyal tooth plate consisting of bony plates or entoglossals), dorsal fin set far posterior on body (predorsal length of 75-88% of standard length) and several other uniquely derived morphological characters. To set the style and terminology for description of the fossil taxa, and to present a fresh look at the osteology and development of living gars, the skeletal anatomy of Lepisosteus osseus and Atractosteus spatula is redescribed in great detail, and the other five extant gars in somewhat less but still significant detail, all using modem methods of preparation and illustration. Such detailed study of the extant species was necessary to adequately interpret probable intraspecific variation and ontogenic variation in fossil taxa. Individual variation of both bone morphology and developmental timing in Lepisosteus osseus is also examined. The order Lepisosteiformes (gars) is classified into two families here: Lepisosteidae (including Lepisosteus, Atractosteus, †Cuneatus gen. nov., and †Masillosteus) and †Obaichthyidae fam. nov. (including †Obaichthys and †Dentilepisosteus gen. nov.). The genera †Cuneatus gen. nov. and †Masillosteus show a diversity of extinct very short-snouted lepisosteid gars that lived in the Eocene. One group (†Masillosteus) had very few sharply pointed teeth, but the coronoids and upper jaw bones were covered with large flat tipped (styliform) teeth, suitable for crushing snails or other shelled organisms. †Obaichthyidae fam. nov., represents a family of South American/African gars that unlike Lepisosteidae have a well-developed interopercle, an intercalar fused to the exoccipital, and other features showing basal lepisosteiform morphology to be quite different than was previously thought. Acid transfer preparations of †obaichthyids from the Upper Cretaceous Santana Formation of Brazil show many previously undescribed morphological details of those taxa that are key to basal neopterygian phylogenetic studies. One of the results of new information on basal lepisosteiform morphology is that (rather than Halecostomi) appears to be a monophyletic group based on morphological data (reported briefly in Grande, 2005). Previous phylogenetic studies regarding the classic gar-Amia-teleost problem are reviewed (i.e., the vs. Holostei controversy) and the characters used to support in those studies are evaluated. With the morphological evidence now pointing to Holostei, morphological and molecular evidence are now in agreement on the issue. The Superdivision (as used here) includes the Divisions Halecomorphi and Ginglymodi. Ginglymodi here includes at least the orders Lepisosteiformes, †Semionotiformes (†Semionotus elegans as used here for outgroup comparison), and †Macrosemiiformes (†Macrosemiidae sensu Bartram, 1977). The order †Semionotiformes is still in need of rigorous review and revision to determine which taxa can be included with the genus without making the order nonmonophyletic. Accurate resolution of the vs. Halecostomi problem helps calibrate the basal interrelationships of Neopterygii, a taxon containing nearly half of all living vertebrate species. Establishing this pattern of relationship will have ramifications for the general phylogenetic studies within Neopterygii, and for studies that use Neopterygii for outgroup comparison. This study also demonstrates the power of a fossil taxon to help overturn a phylogenetic pattern derived from the morphology of extant species. Much of what tipped the scale away from a monophyletic Halecostomi and toward a monophyletic in this study is the morphological evidence described here for the Cretaceous †Obaichthyidae (fam. nov.). †Obaichthyids are clearly lepisosteiforms based on numerous characters, but they have a well-developed interopercle and other features that falsify previously proposed synapomorphies for the monophyly of Halecostomi. These critical characters were missed in previous descriptions of †Obaichthys because of a lack of adequately prepared specimens. †Obaichthyids represent a phylogenetic link in the truest sense of the term. Numerous general methodological, philosophical, and theoretical concepts are discussed in this paper (which can be located using either the Subject Index or the Table of Contents), including the concept of the effective type specimen, the concept of the effective type genus for taxonomic names above family level rank, the use of quotation marks to indicate uncertain taxonomic designation, philisophical roots of the comparative technique, the importance of stages of disarticulation in interpreting the morphology of fossil taxa, anatomical terminology as a tool, relative empiricism, missing data, data partitioning, and the meaning of cladograms. Once again, I apply what was previously (Grande and Bemis, 1998) termed an empirical synthetic pattern to an investigation of a basal neopterygian group (Lepisosteiformes), although in the end, that approach is nothing more than a fancy name for good comparative biology.