The evolutionary affinities and composition of the Gasterosteiformes have been the subject of much debate; presently they are considered to be closely related to the teleost fish orders Synbranchiformes (swamp eels), Elasso-matiformes (pygmy sunfish, Elassoma), Mugiliformes (mullets), and the collective Atherinomorpha (silversides, needlefishes, killifishes, and allies) and are placed with them in the larger group Smegmamorpha. Smegmamorphs are characterized by a unique configuration of the first vertebra and its associated intermuscular bone. Within this group, gas-terosteiforms may be more closely related to the Syn-branchiformes, but relationships within smegmamorphs are still the subject of controversy. The smegmamorphs, in turn, are closely related to the largest of all teleost groups, the Per-comorpha. This evolutionary arrangement is the most recent and best-supported hypothesis based on morphological evidence, but alternative schemes of relationship have been proposed (including molecular studies), and a true consensus has not yet emerged.
The Gasterosteiformes, as described in this chapter, comprises two suborders, the Gasterosteoidei and the Syng-nathoidei. Other authors sometimes have recognized these as separate orders, but evidence exists that they are each other's closest relative and therefore warrant being classified together. Gasterosteiformes share various specializations, such as pelvic bones without anterior processes, absence of Baude-lot's ligament (a stout ligament connecting the shoulder girdle to either the posterior cranial base or an anterior vertebra—also absent in synbranchiforms), certain features of their branchial and caudal skeletons, and a particular configuration of their scales, which are represented by enlarged scutes, or plates. (Scales are absent in Hypoptychus, Aulostomus has small ctenoid scales, and Fistularia has embedded spines).
The extent to which these features (and others) are truly indicative of a common ancestry for Gasterosteiformes is not completely understood.
The Gasterosteoidei includes the following families: Hy-poptychidae (for the sand eel, Hypoptychus dybowskii), Aulorhynchidae (tubesnouts, two monotypic genera), Indos-tomidae (Indostomus, with three species), and Gasterosteidae (some five genera and, conservatively, seven spp.). The Syn-gnathoidei comprises the families Syngnathidae (seahorses and pipefishes, with about 52 genera and 220 spp.), Aulosto-midae (trumpetfishes, Aulostomus, with some three spp.), Fis-tulariidae (cornetfishes, Fistularia, with four spp.), Macroramphosidae (snipefishes, three genera with 12 spp.), Centriscidae (shrimpfishes, two genera with four spp.), Solenostomidae (ghost pipefishes, Solenostomus, with some four spp.), and Pegasidae (seamoths, two genera with five spp.). Altogether, the order Gasterosteiformes is represented by 11 families, 70 genera, and at least 265 species, but unde-scribed species have been discovered (including some 20 species of pipefishes and seahorses), and numerous nominal species presently in synonymy, in fact, may be valid (such as for sticklebacks). Much work remains to be done concerning their taxonomy, and the phylogenetic position of Indostomus is still debated.
The fossil record of the Gasterosteiformes is extensive and dates back at least some 75 million years to the Calcare di Mellissano deposits near Nardo, in southeastern Italy (Apulia). This early fossil, Gasterorhamphosus zuppichinii, is known from skeletal remains and is similar to modern snipefishes. Additional gasterosteiform fossils, known from more or less complete skeletons, have been described from the extensive Monte Bolca beds of northeastern Italy (dating back some 52 million years). These include representatives of the Syng-nathidae (at least five genera), Solenostomidae (some three
genera), Centriscidae (three genera), Aulostomidae (four genera), and Fistulariidae (some three genera and four spp.). The actual number of species from Monte Bolca is difficult to estimate with precision, but this formation represents the greatest extinct diversity of the order. The gasterosteiform species present in both the Nardo and Monte Bolca deposits were inhabitants of the former Tethys Ocean, which separated the extinct continents Laurasia and Gondwana during much of the Cretaceous and Tertiary periods. Other fossils, mostly represented by fragmentary material and allied to the Aulostomidae and Fistulariidae, are known from Turkmenia, in deposits almost contemporaneous with Monte Bolca. Fossil sticklebacks have been found in the Tertiary of Califor-
nia (Monterey Formation) and Siberia. Tertiary pipefishes have been described from the Modelo and Puente Formations of southern California as well as from the Caucasus and Carpathian Mountains of eastern Europe.
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