Araripichthys axelrodi, MAISEY & MOODY, 2001

MAISEY, JOHN G. & MOODY, JOHN M., 2001, A Review of the Problematic Extinct Teleost Fish Araripichthys, with a Description of a New Species from the Lower Cretaceous of Venezuela, American Museum Novitates 3324, pp. 1-27 : 5-14

Araripichthys axelrodi ; the holotype, P­907 MBLUZ (entire specimen) .

in the present family/genus diagnosis. Two characters previously considered diagnostic of the genus are known only in A. castilhoi and A. corythophorus , but have not yet been observed in A. axelrodi ; that is, presence of an edentulous vomer, and an opening between the pterosphenoid and sphenotic connecting the orbit and dilatator fossa. These characters are provisionally retained here as diagnostic for the family Araripichthyidae . Another character previously regarded as diagnostic is the extent of the median (dorsal, anal) fins rostrocaudally. The character is present in A. castilhoi and A. axelrodi but it cannot be determined in the available material of A. corythophorus , although its median fins were evidently extremely long in the rostrocaudal axis and may well have been as extensive as in the other species (Cavin, 1997a, 1997b). This character has also been retained in the family/genus diagnosis given above.

Some other features (including several listed in the diagnosis of Araripichthys presented by Maisey and Blum, 1991: 208) are unknown in both A. corythophorus and A. axelrodi . Such characters may be diagnostic of the genus, only the type species, or a combination of the type and one other species. In view of this uncertainty these characters were omitted from the diagnoses presented above, as well as from the phylogenetic analysis outlined below. Characters falling into this category include:

(i) Fusion of vertebral arches and centra. In A. castilhoi all the neural and haemal arches and parapophyses of pleural ribs are fused to their respective centra (including those of preurals 1 and 2). Not all the centra are visible in specimens of A. corythophorus and A. axelrodi , and it is uncertain whether all three species agree in this respect. A. castilhoi is also the only species in which epineural bones have been observed (see below).

(ii) Tripartite occipital joint. See phylogenetic discussion below.

(iii) Shape of the caudal fin. This is deeply forked, but with rounded fin lobes in A. castilhoi . The caudal fin is still unknown in A. corythophorus and A. axelrodi .

(iv) Meristic data for A. castilhoi (about 86 scales along lateral line, 33 scales in vertical series at deepest point above lateral line, 18–20 below; D:53; A:32; P:12+; V0).

Within the revised concept of the genus presented here, three species are distinguished. Cavin (1997a, 1997b) noted differences between Araripichthys corythophorus and A. castilhoi in the size and extent of the epioccipital and in the arrangement of circumorbital bones. The shape of the posterior skull margin differs in all three species, particularly the shape of the parietal and pterotic margins (fig. 5). These are much straighter in the Moroccan form than in either A. castilhoi or A. axelrodi . In A. castilhoi , the parietal posterior margin has a distinctly angular profile just above the pterotic, a feature that may be correlated with deepening of the subepiotic fossa. The epioccipital separates the subepiotic fossa (which lies medially) from the posttemporal fossa (which lies laterally), but its exposed lateral margin appears smaller in A. axelrodi and A. corythophorus than in A. castilhoi . The parietal posterior margin is marked by a distinct process in A. castilhoi and (to a lesser extent) in A. axelrodi . Unfortunately, the collective extent of variation in epioccipital shape in Araripichthys encompasses practically the entire range of that seen in other elopomorph fishes, and the primitive condition is uncertain. For example, in Tarpon, Notelops brama, and Rhacolepis buccalis the epioccipital forms a distinct posterior process, whereas in Elops and Osmeroides latifrons it is less prominent (Forey, 1973).

The identity of the bone Cavin (1997a: fig. 1.9) called the antorbital in Araripichthys corythophorus is problematic because the antorbital is not known in the other species and because the circumorbital series is incompletely preserved in the Moroccan form. Cavin (1997a) identified a series of five bones (antorbital plus four infraorbitals) in A. corythophorus , the same number as in the infraorbital series of A. castilhoi , but in his illustration (fig. 5C here) he shows a gap that may have been occupied by the dermosphenotic (identified as the last infraorbital by Maisey and Blum, 1991). Thus, the circumorbital series in Araripichthys may have included an antorbital (unknown in A. castilhoi ), followed by three infraorbitals and a dermosphenotic (not known in A. corythophorus ). Alternatively, the ‘‘antorbital’’ of A. corythophorus may correspond to the first infraorbital in A. castilhoi (Maisey and Blum, 1991: 210; = lachrymal of Silva Santos, 1985a: pl. III), while its last ‘‘infraorbital’’ may represent an incomplete dermosphenotic, in which case the circumorbital series may have consisted of four infraorbitals plus the lachrymal. No supraorbital has been identified in any species of Araripichthys . A single anamestic suborbital has been identified in A. castilhoi , but it has not been found in the other species.

PHYLOGENETIC AND BIOGEOGRAPHIC RELATIONSHIPS

The maxilla is excluded from the superior border of the mouth in all three species of Araripichthys (figs. 2–6), but the suspensorial arrangement of A. axelrodi is apparently the least specialized of the three taxa. In Araripichthys axelrodi the ascending process of the premaxilla is certainly weaker than in A. castilhoi . In the type specimen of A. corythophorus the ascending process is broken, but as restored by Cavin (1997a) the process was long. The maxilla has a condyle and a rodlike anterior process in both A. castilhoi and A. corythophorus , but neither seems to be present in A. axelrodi (cf. figs. 3, 6). Presence of a strong premaxillary ascending process and a maxillary condyle are unusual features for a nonacanthomorph elopocephalan (Maisey and Blum, 1991); indeed, it was partly this elaboration of the jaws in Araripichthys that originally led Silva Santos (1983, 1985a) to suggest it as a primitive acanthomorph.

On the basis of these slight differences in jaw morphology, Araripichthys castilhoi and A. corythophorus are hypothesized to be more closely related to each other than to A. axelrodi from Venezuela (fig. 7A). The deeply emarginated shape of the posterior skull roof (formed by the epioccipital and parietal) in A. castilhoi and A. axelrodi is regarded tentatively as a plesiomorphic similarity, while the straighter margin in A. corythophorus may represent an autapomorphy of that species. Alternatively, if the straight posterior margin of the posterior skull roof is regarded as a primitive condition, the polarity of this character would be reversed, and the deep emargination would then unite A. castilhoi and A. axelrodi . There are several other distinctive and unusual morphological features of A. castilhoi that at present cannot be compared in one or both of the other species.

The stratigraphic range of the family Araripichthyidae extends from the Aptian to the Turonian. According to the phylogenetic hypothesis presented here (fig. 7A), the earliest form is also the most primitive. The minimum age for the origin of Araripichthys is lower Aptian (approx. 124 mybp), while divergence of A. castilhoi from the hypothetical lineage leading to A. corythophorus dates from at least the lower Albian (112 mybp). An area cladogram based on these relationships shows Venezuela as a sister area to Brazil (Araripe basin) and Morocco (fig. 7B). At present, the most plausible paleobiogeographic scenario is that Araripichthys originated in the Pacific or western Tethys during the Aptian or slightly earlier and spread subsequently southeastward (either along the southern margin of Tethys or via an epicontinental seaway), reaching NE Brazil by the Albian and Morocco by the Turonian.

INTERRELATIONSHIPS OF THE ARARIPICHTHYIDAE

The phylogenetic relationships of the Araripichthyidae have not been clearly established. When Araripichthys was first described, it was identified as a beryciform, which would make it the earliest known acanthopterygian (Silva Santos, 1983, 1985a). In subsequent studies, however, the genus has been relegated to a lower phylogenetic position within teleosts (e.g., as Elopocephala incertae sedis; Maisey and Blum, 1991; Cavin, 1997a) principally because its caudal fin skeleton resembles that of other generalized elopocephalan fishes and it lacks apomorphic characters found in higher teleosts (fig. 8). Patterson (1993: 627) suggested that Araripichthys is a pachyrhizodontid, despite many profound differences (especially in its body form and jaws) from other pachyrhizodontids such as Rhacolepis, Pachyrhizodus, Greenwoodella, and Goulmimichthys. Cavin (1997a: 67) compared pachyrhizodontids and araripichthyids but was unable to identify any apomorphic characters with which to unite them as a monophyletic group.

Silva Santos (1985a) claimed that several supposed beryciform or acanthopterygian characters are present in Araripichthys , including presence of spiny rays in the dorsal and anal fins; a stegural (first uroneural with an anterodorsal outgrowth of membrane bone; an elevated occipital crest; absence of supraorbitals; presence of a mobile, sliding premaxilla with an extensive ascending process; modified maxillary­vomerine articulation; exclusion of the maxilla from the superior margin of the mouth; and absence of

Fig. 6 .

pelvic fins. Unfortunately, most of these features are not acanthomorph synapomorphies and were discarded as characters uniting Araripichthys and acanthomorphs by Maisey and Blum (1991: 215). They suggested, however, that the absence of pelvics and those characters involving the jaws offer ‘‘tempting but not compelling support for a relationship’’ between Araripichthys and lampri­ Continued.

diforms, and that lampridiforms may have arisen from basal acanthopterygians ‘‘prior to the major radiation of the group during later Cretaceous and Early Tertiary times.’’ Only the more derived modern lampridiform taxa (i.e., those with a ribbonlike body form) lack pelvics, however, whereas these are present in more basal members of the group. Pelvics are also absent in modern eels, as well as in many tselfatioids. Thus, the phylogenetic position of Araripichthys is unresolved even quite fundamentally within teleosts, and therefore presents a situation that invites further discussion.

1. Is Araripichthys related to the Ferrifronsidae ?

Acanthichthys and Ferrifrons are two late Cretaceous teleosts from North America. According to Arratia and Chorn (1998), Acanthichthys and Ferrifrons are closely related, and both these genera were placed within a new family Ferrifronside . They also identified three of Johnson and Patterson’s (1993) acanthomorph characters in Acanthichthys (characters 1, 6 and 7; that is, spiny dorsal fin­rays, with the condition in the anal fin being unknown; medial pelvic process ossified medially; and first vertebral centrum with facets for exoccipitals). Far less evidence exists to place Ferrifrons within acanthomorphs. Arratia and Chorn (1998) identified spiny fin­rays in the anal fin, but the dorsal fin is unknown, as are the pelvics (which are assumed, by comparison with Acanthichthys , to have been positioned in an abdominal position, although one could just as readily argue that they were absent, as in Araripichthys ). No other acanthomorph characters listed by Johnson and Patterson (1993) were documented by Arratia and Chorn (1998) in Ferrifrons .

Arratia and Chorn (1998: 313) found differences between Acanthichthys and Araripichthys that ‘‘clearly separate both genera.’’ They did not explore the possibility of a relationship between Araripichthys and Ferrifronsidae any further, but instead side­stepped the issue by reiterating the opinions of Maisey and Blum (1991) and Patterson (1993) that Araripichthys is not an acanthomorph. Both Ferrifrons and Acanthichthys nevertheless resemble Araripichthys in their general body form, sinusoidally curved vertebral column, head with a steeply sloping frontal region, presence of exoccipital facets in the occipital region, shape and arrangement of mouthparts (especially exclusion of the maxilla from the superior border of the mouth), edentulous jaws, expanded (compound?) first anal pterygiophore, and scales covering the bases of the dorsal and anal fins.

The occipital joint and pelvic process are both unknown in Ferrifrons , and the only putative acanthomorph­like feature identified by Arratia and Chorn (1998) is the presence of spiny fin­rays in the anal fin. Although they claimed that Araripichthys differs from the Ferrifronsidae in lacking median fin spines and pelvic fins, in fact the anal fin is Fig. 8. Caudal endoskeleton in an acid­prepared specimen of Araripichthys castilhoi, AMNH 11944.

unknown in Acanthichthys , and the pelvics and dorsal fin are unknown in Ferrifrons , so the supposed difference has not yet been convincingly demonstrated. True fin spines are unsegmented median structures, with no trace of fusion between left and right lepidotrichia. Dorsal and anal spines with a median internal cavity not only occur in acanthopterygians, but also in notacanths and the otophysan Lipogenys (Johnson and Patterson, 1993) . The presence of these spines is therefore not a reliable indicator of acanthopterygian affinity.

Scale morphology supposedly differs in Araripichthys and Ferrifronsidae (Arratia and Chorn, 1998) . In Araripichthys , the scales are almost all cycloid, whereas both Acanthichthys and Ferrifrons have spinoid scales (sensu Roberts, 1993). Nevertheless, this distinction is reduced by the observation that, in the region between the pelvic and anal fins of Acanthichthys , the scales also ‘‘seem to be cycloid’’ (Arratia and Chorn, 1998: 309). While the presence of spinoid scales may unite Acanthichthys and Ferrifrons , it does not clearly distinguish them from Araripichthys .

The presence of a tripartite occipital arrangement (in which the exoccipitals are incorporated into the occipital joint, with distinct facets for the first vertebra) was regard­ ed by Arratia and Chorn (1998) to be an important character uniting Acanthichthys and acanthomorphs. A similar arrangement is clearly present in Araripichthys (at least in the type species). While the tripartite occipital condyle in Araripichthys and Acanthichthys is similar to that of neoteleosts, its value as an acanthomorph character is questionable. Separate articular facets are present on the exoccipitals of salmoniform teleosts (e.g., Thymallus ; Norden, 1961), and Rosen (1985) also recognized a tripartite occipital arrangement involving the exoccipitals in non­neoteleostean teleosts, noting that in many cases the appearance is related to fusion between the first vertebral centrum and basioccipital. He also noted Patterson’s (1975: 318) suggestion that growth of an ‘‘osteoid plug’’ within the notochordal canal of the basioccipital in pholidophorids is responsible for excluding the exoccipitals from the posterior face of the occiput.

These findings collectively led Rosen (1985: 11) to propose that ‘‘the tripartite arrangement of bones on the posterior face of the braincase is a primitive feature that has been restored in neoteleosts and some primitive euteleosts by a means so far unknown.’’ Rosen (1985: 14) concluded that ‘‘the tripartite joint of neoteleosts is a very old feature and the primitive and widespread presence of an accessory neural arch is inferred to be the remains of an ontogeny that had incorporated vertebral fusion with the occiput.’’ Actually, there is some paleontological support for even greater antiquity, because a similar tripartite arrangement also occurs sporadically outside teleosts, for example in some extinct halecomorphs (e.g., Oshunia , ‘‘ Aspidorhynchus ’’; Maisey, 1999: figs. 1, 12).

Rosen (1985: 22) nevertheless also outlined a phylogenetic­developmental scenario for the origin of the triple joint in acanthomorphs, involving several presumably derived and successively acquired characters, which included (1) development of a gap between occiput and first centrum, and exposure of exoccipitals and basioccipital as attachment or articular surfaces; (2) loss of accessory neural arch (all myctophoids and acanthomorphs); (3) ligamentous attachment of exoccipitals to dorsolateral part of first centrum; (4) extension of exoccipitals into this ligamentous network, accompanied by growth of autocentral prezygapophyses and closure of gap between basioccipital and first centrum; and (5) bone­to­bone condylar articulations between occiput and facets on first centrum.

Collectively, these observations, interpretations, and scenarios only seem to compound the problem; a tripartite occipital joint may be a primitive teleostean (or halecostome) character, which may show up as an occasional homoplasy within teleosts, yet in neoteleosts several apparently apomorphic features of the joint are distinguishable. It remains to be seen if some or all of these supposed derived features are absent in non­neoteleosteans possessing a triple occipital joint, and unfortunately no ontogenetic data are available for the fossils.

There are, however, paired prezygapophyses on the anterior neural arches in Araripichthys castilhoi . These appear to be functionally similar to (although perhaps not homologous with) the autocentral prezygapophyses of acanthomorphs (Rosen, 1985) in bracing the occipital connection.

Figures

Fig. 6. Detail of the head in two acid­prepared specimens of Araripichthys castilhoi from the Santana Formation of Brazil showing the well­developed premaxillary process and articular condyle on maxilla. (A) AMNH 11943; (B) AMNH 13968 (compare with fig. 3).