Diplomystus brevissimus (de Blainville, 1818)

CHANG, MEE-MANN & MAISEY, JOHN G., 2003, Redescription of † Ellimma branneri and † Diplomystus shengliensis, and Relationships of Some Basal Clupeomorphs, American Museum Novitates 3404 (1), pp. 1-36 : 27-31

† Diplomystus brevissimus ( Patterson, 1967; now † Armigatus, Grande, 1982a ).

The relationships between the two Recent clupeiform suborders (Denticepitoidei, Clupeioidei) and some putative clupeomorph fossils have been investigated by Forey (1975) and Grande (1982a, 1985a), while the problem of clupeomorph relationships generally within teleosts has been discussed by Patterson and Rosen (1977) and Arratia (1996). † Diplomystus , † Armigatus , and † Ellimmichthys all seem to be close extinct relatives of modern clupeomorphs (i.e., Clupeiformes sensu Grande, 1985a ; in his phylogeny there is an unresolved trichotomy between † Armigatus , the group † Diplomystus + † Ellimmichthys , and Clupeiformes ). There are differences of opinion regarding the inclusion of certain other extinct taxa within clupeomorphs. † Ornategulum was included in clupeomorphs by Forey (1975), Patterson and Rosen (1977), and Grande (1982a), but was excluded by Grande (1985a). † Erichalcis was placed in the Clupeomorpha by Forey (1975) and Grande (1985a), but was regarded as the sister taxon of Esox + salmonids by Arratia (1996). Part of the uncertainty is due to the incomplete preservation of the material, and in the case of † Erichalcis the type series may include more than one taxon (Mark Wilson and Lance Grande, personal commun., 2000). † Santanaclupea shares at least two synapomorphies with clupeiforms, but cannot be placed unambiguously within any of the modern clupeiform groups ( Maisey, 1993).

In Grande’s (1985a) revision of Recent and fossil clupeomorphs, † Diplomystus and † Ellimmichthys form a monophyletic group (order † Ellimmichthyiformes ) supported by a single character (dorsal scutes broader than long or of rectangular shape). Clearly this character also unites † Ellimma branneri with these genera, although it is developed only in the posterior part of the scute series in this form.

Chang and Grande (1997) referred † Paraclupea chetungensis to an equivalent monophyletic group (family Paraclupeidae = Ellimmichthyidae ). They concluded that † Paraclupea is more closely related to † Ellimmichthys than to † Diplomystus , and should be included with † Ellimmichthys in the sub­

family † Paraclupeinae . Our investigation of † Ellimma branneri from Brazil suggests that it is also a paraclupeine, in which case the ornamentation of dorsal scutes becomes a synapomorphy of the subfamily. † Ellimma , † Ellimmichthys , and † Paraclupea form an unresolved trichotomy in our phylogenetic analysis, but meristic data for † Ellimma and † Ellimmichthys are similar whereas those for † Paraclupea are different. Our conclusions regarding † Diplomystus shengliensis agree with those of Chang and Chow (1978) and Zhang et al. (1985) that it is most closely related to † D. dentatus from North America.

Our analysis (fig. 13) supports the main conclusions reached by previous workers, especially those of Grande (1985a), although some aspects of our results require critical comment. First, our morphological evidence that the † Ellimmichthyiformes form a monophyletic group is not as strong as we would wish. Second, some extinct ‘‘double­armored’’ clupeomorph taxa (e.g., † Scutatospinosus, † Ezkutuberezi carmeni ) were excluded from our analysis, because we were unable to examine material or retrieve sufficient data from the published descriptions; their inclusion in future phylogenetic analyses is clearly desirable and may provide a useful test of the conclusions reached here. Third, although the trichotomy formed by † Armigatus , † Ellimmichthyiformes , and Clupeiformes in Grande’s (1985a) cladogram seems to be resolved here, our grouping of † Armigatus and † Diplomystus is actually supported by ambiguous characters (3, 9, 10, and 26 in our character list). Even the most convincing synapomorphy (a posteriorly expanded third hypural, leaving no gap between hypurals 2 and 4) also occurs in some clupeiforms not included in our analysis. For all these reasons the phylogenetic hypothesis presented here is considered tentative, as it reflects current uncertainties concerning paraclupeine relationships.

Two interesting paleobiogeographical puzzles involving China have emerged from this work. First, an apparent sister­group relationship has emerged between † Paraclupea and Early Cretaceous paraclupeines of western Gondwanan. Second, the presence of very similar † Diplomystus fossils in China and North America during the Eocene is curious. According to our analysis, paraclupeines and † Diplomystus are closely related (as members of the family † Paraclupeidae ), but their respective distribution patterns are fundamentally different and may be the result of quite unrelated biogeographic histories. In both cases the fossil record is undoubtedly incomplete, and the known distributions of these forms may be so strongly biased as to make them biogeographically unintelligible. We restrict our remarks to a few salient points that may be worthy of further investigation.

BIOGEOGRAPHIC OBSERVATIONS. All the genera included here within the † Paraclupeinae († Ellimmichthys , † Ellimma , † Paraclupea , and perhaps † Scutatospinosus) are Early Cretaceous in age. † Ellimmichthys and † Ellimma both occur in Brazil, but † Ellimmichthys is also known from Africa and Mexico. The two better known species of † Ellimmichthys are † E. longicostatus (Hauterivian–Barremian, Marfim Formation, Recôncavo Basin, Brazil) and † E. goodi (late Aptian–Albian, West Africa). The genus † Ellimma contains three nominal species, all from Brazil: † E. branneri and † E. riacensis (considered synonymous; Schaeffer, 1947; Grande, 1985a) from the late Aptian (Muribeca Formation, Sergipe Basin; Jordan, 1910); and † E. cruzi from the Aptian–early Albian (Cabo Formation, Cabo Basin; Silva Santos, 1990; previously identified as † Ellimmichthys by Costa et al., 1979). The undescribed Albian species of † Ellimmichthys from Mexico mentioned by Chang and Grande (1997) represents a western Tethyan (Caribbean) rather than Gondwanan occurrence, and is the only marine paraclupeine record. Another double­armored clupeomorph, † Scutatospinosus itapagipensis , occurs along with † Ellimma branneri in the Marfim Formation of Brazil ( Silva Santos and Correa, 1985).

Today there is little doubt that the margins of Brazil and West Africa were contiguous during the Early Cretaceous ( Pitman et al., 1993). The earliest record of † Ellimmichthys (and consequently the minimum postulated age of divergence between † Ellimma and † Ellimmichthys ) is Hauterivian–Barremian, which predates the formation of an equatorial seaway by a considerable margin, although † Ellimmichthys goodi from Africa and all records of † Ellimma are Aptian or younger (approximately contemporary with permanent emplacement of the seaway in the late Aptian; Maisey, 2000). All Gondwanan paraclupeines are from strata deposited within rift basins that were involved in the final tectonic separation of Africa and South America. At that time these basins were intracontinental, but subsequent rifting, crustal extension, and drifting transformed them into parts of the new continental margins. Their distribution pattern differs from those of some other Early Cretaceous fishes from western Gondwana, such as † Mawsonia and † Calamopleurus , which occur more widely across interior localities of Africa and Brazil (including nonrift settings), although these occurrences probably also represent former regions of endemism which were disrupted by formation of the Equatorial Atlantic seaway ( Maisey, 2000).

The habitats and history of Gondwanan paraclupeine fishes were undoubtedly affect­ ed by the early (predrift) phases of these momentous tectonic processes. The distribution of Gondwanan † Ellimmichthys (and † Scutatuspinosus ?) may reflect vicariant isolation of lacustrine populations along the tectonically active Recôncavo­Tucano­Jatobá and Gabon­ Sergipe­Alagoas (GSA) rift trends ( Maisey, 2000), and † Ellimma may be an endemic South American genus which perhaps evolved as a result of vicariant isolation within lakes of the GSA trend (Sergipe and Cabo basins). Albian occurrences of † Ellimmichthys are restricted to freshwater deposits within African marginal rift basins, but have not been documented from the marine strata overlying them. The genus has nevertheless been identified in marine sediments from Mexico, suggesting that a salt­tolerant form reached western (Caribbean) Tethys by the Albian–Cenomanian.

The distribution of † Paraclupea is a biogeographic conundrum (Chang and Chow, 1986; Chang and Chen, 2000). † Paraclupea is not known from any Gondwanan locality; † P. chetungensis is found in freshwater deposits near the western Pacific coast of China, but an Early Cretaceous freshwater paraclupeid from Japan (described as † Diplomystus ; Uyeno, 1979; Uyeno and Yabumoto, 1980; Yabumoto, 1994) is rather similar to the Chinese form and may extend the range of † Paraclupea . These discoveries considerably expand the distribution of paraclupeines beyond western Gondwana and nearby western Tethys, far into eastern Asia. At this stage we can only speculate about the overall paraclupeine distribution pattern in the Cretaceous.

There is little geologic evidence to support an Early Cretaceous nonmarine paleogeographic connection between the eastern Asiatic margin and western Gondwana that would account for the observed distribution of paraclupeine fishes. We could plead for more widespread distribution and earlier occurrences of freshwater paraclupeine than is presently realized, especially since the † Diplomystus + † Armigatus clade represents the sister group to paraclupeines and is presumably of equal antiquity (in which case there should be relatives of that clade in the Barremian–Hauterivian). Another possibility is that paraclupeines may have marine origins and their distribution may reflect a largely unknown marine history. Unfortunately, both scenarios involve sweeping ad hoc assumptions, for example that the remains of a widespread fauna are still to be found elsewhere, or that their traces in other areas have been obliterated by later geological events.

† Diplomystus shengliensis from Bohai Gulf and † D. dentatus from Green River are morphologically so similar that there is little doubt they represent closely related sister species. Interestingly, besides † Diplomystus , four other genera of fishes († Eohiodon , † Knightia, † Amyzon , and Esox ) supposedly are shared by the Green River and Bohai Gulf faunas (Chang and Chow, 1986; Chang and Zhou, 1993, 2002; Chang et al., 2001), and several other lineages from the Bohai Gulf deposits are also represented in the Green River ichthyofaunas (e.g., ‘‘Dasyatidae’’, Amiidae ). As pointed out by Grande and Bemis (1998: 337), the Chinese amiid is still inadequately known and cannot yet be assigned either to Amia or † Cyclurus , although it is almost certainly an amiine. † Diplomystus , † Eohiodon , † Knightia, † Amyzon and Esox have an Eocene transpacific distribution pattern, but amiines and dasyatids are more broadly distributed.

Interestingly, similar faunal distribution patterns have been noticed among Eocene terrestrial vertebrates of Asia and North America. These have been used to suggest that a land bridge existed between these areas in the vicinity of the Bering Straits, permitting mammals and other tetrapods to pass freely between these regions ( Russell and Zhai, 1987). McKenna (1975, 1980, 1983, 1984) has also proposed that arctic connections between the continents have played a significant role in such dispersals.

Grande (1985b, 1989, 1994) noted this repeating pattern of area relationship in the early Tertiary between western North America and east Asia, based on the phylogenetic relationships of teleosts from Green River and the west Pacific region (including China, Indonesia, and Australia). He characterized this distribution as a ‘‘transpacific’’ pattern that could be explained by the Pacifica hypothesis ( Grande, 1994). While we do not question these observations, note that much of the similarity in ichthyofaunas is restricted to the Eocene fossil assemblages of Bohai Gulf and Green River, whereas other areas on the western side of the Pacific share far fewer taxa with Green River (known examples include osteoglossids of Australia and Indonesia; osteoglossids and catostomids of South China; amiines and catostomids of inland North China; amiines, hiodontids and catostomids of East Khazakstan).

More recent geophysical and geochemical studies (some still in progress) have raised a startling and radically different alternative to the Pacifica hypothesis. There is mounting evidence that, for a brief period (approximately 2 million years, during the late Paleocene and early Eocene), the Arctic Ocean was almost completely landlocked, with a terrestrial corridor between North America, Iceland, and Europe and another through Beringia, and a single outlet to the sea via the Turgai Strait ( McKenna, 1998). With several major river systems draining northward into the Arctic, net runoff/evaporation may have caused dramatic lowering in salinity, sufficient perhaps to allow freshwater fishes to move unimpeded by a saltwater barrier between Asia and North America.

Such a rapid dispersal event could certainly account for the sister­group relationship between † Diplomystus from China and North America. It would also offer a partial explanation for some Eocene fish distribution patterns, and perhaps even the holarctic distribution of some modern fishes (e.g., paddlefishes). † Eohiodon falcatus occurs in the Green River Formation, and hiodontids are represented in eastern China and East Khazakstan (Central Asia). Eocene catostomids also occur in both these areas and are widely spread in the vast plains area of East Asia, from Mongolia in the north to Guangdong Province at the very south of China ( Chang et al., 2001). The osteoglossid † Phareodus occurs in the Green River Formation, as well as in Australia († Phareodus queenslandicus ; Hills, 1934; Li, 1994). Eocene osteoglossids are found in Sichuan Province († Sinoglossus, Su, 1986 ) and Hubei provinces, South China (Song, in prep.) and Eocene or Oligocene of Indonesia († Musperia radiata ; Sanders, 1934). Eocene hiodontids, catostomids, and osteoglossids are also reported from British Columbia ( Wilson, 1977). The wide boreal distribution of amiines during the Eocene (including North America, northern China, East Khazakstan, and Europe) is remarkably similar to the pan­Arctic distribution of many riparian and terrestrial vertebrates during the Late Cretaceous and early Cenozoic ( Estes and Hutchison, 1980). Freshwater Arctic connections between the northern continents may therefore have played a significant role in amiine dispersal.

On the other hand, in the vast area between the Bohai Gulf and Green River occurrences there are several other Eocene freshwater deposits with abundant fossil fishes, yet these ichthyofaunas do not yet include dasyatids, † Diplomystus , or clupeids (e.g., middle Eocene of Washington State, U.S., and British Columbia, Canada; inland areas of South China and East Khazakstan; Wilson, 1977, 1978; Tang, 1959; Liu et al., 1962; Wang et al., 1981; Sytchevskaya, 1986). In addition, some taxa occur in one region but not the other (e.g., Eocene lepisosteids in Green River and western Canada but not Asia; Eocene cyprinids in Bohai Gulf, South China, and East Khazakstan but not in North America or Europe). Such absences from the fossil record may reflect a preservational bias against certain taxa in one area or the other, or it could indicate a real absence, suggesting that some taxa did not participate in any holarctic dispersal event during the late Paleocene and early Eocene.

Quite probably, there is no single factor responsible for the Eocene ‘‘transpacific’’ distribution pattern. A broad connection between Asia and North America in the Bering Strait area could have played a role, as could temporary desalination of the Arctic Ocean. Both hypotheses provide equally plausible biogeographic alternatives to the Pacifica model and are certainly more in accordance with geological data than the latter. At present the only common link between the Early Cretaceous distribution of paraclupeines and the Eocene distribution of † Diplomystus is China, for only there have both taxa been discovered.