Vallecillichthys multivertebratum Blanco and Cavin, 2003

Vallecillichthys multivertebratum Blanco and Cavin, 2003

Verraesichthys bloti Taverne, 2010

Xiphactinus audax Leidy, 1870

Xiphactinus gaultinus Newton, 1877

Xiphactinus mantelli Newton, 1877

Portlandian. Russia

Albian. Mexico

Turonian. Mexico

Aptian. Africa

Cenomanian-Campanian. North America

Albian-Cenomanian. England

Cenomanian-Campanian. Europe

Xiphactinus vetus ( Leidy, 1870) Campanian-Maastrichtian. USA part of the posterior preurals and urals; the pectoral and pelvic fins lack saber-like rays; and, if it is present, the ethmopalatine is not identifiable because a huge antorbital bone covers the base of the ethmoid skull region ( Hacker and Shimada, 2021, figures 3–5). Additionally, Bradackichthys shows multiple pores in the lateral surface of the vertebrae, which differ from the presence of two large oval longitudinal cavities separated by a ridge present in the “true” ichthyodectiforms.

The inclusion of Jinjuichthys and Mesoclupea within the order Ichthyodectiformes is also refutable. The case of Jinjuichthys is like that of Bardackichthys ; the available specimens show uroneurals in dorsal position, likely lack the ethmopalatine, have no saber-like rays in the pelvic and pectoral fins, and three or four longitudinal cavities ornament the lateral faces of the vertebrae ( Kim et al., 2014, figures 3–6). In contrast, the case of Mesoclupea differs notably. Ping and Yen (1933) erected this genus with two nominal species, thinking that they were primitive herrings attributable to the family Clupeidae View in CoL . Later, Chang (1963) synonymized these species and put Mesoclupea showshangensis into the family Chirocentridae View in CoL (in which the genus type is the wolf-herring Chirocentrus Forsskål, 1775 View in CoL ) together with other genera now grouped into the order Ichthyodectiformes (e.g., Thrissops and Ichthyodectes ). Finally, Yabumoto (1994) erected the family Chuhsiungichthiidae into the order Ichthyodectiformes , including Mesoclupea and three species of the genus Chuhsiungichthys Lew, 1974 ( C. tsanglingensis Lew, 1974 from China, plus his Japanese species C. yanagidai and C. japonicus ). At first glance, these Asian fishes share the general body shape with the short-bodied ichthyodectiforms (i.e., Chiromystus Cope, 1885 and Africathrissops Taverne, 2010), but a closer view of the osteological data and images published by these authors ( Ping and Yen, 1933; Chang, 1963; Yabumoto, 1994) do not allow their addition to Ichthyodectiformes . Against the “true” ichthyodectiforms, in chuhsiungichthiids, the uronerals cover only part of the dorsal surface of the ural and preural centra, the pelvic and pectoral rays also lack the saber-shaped rays, and the presence of the ethmopalatine is questionable. Chuhsiungichthiids also have the mouth open anteriorly, comparatively short jaw bones, and the premaxilla has an ascending anterior process, which are features not present in any “true” ichthyodectiforms.

Phylogenetic Results

Figure 15 View FIGURE 15 contrasts the two phylogenetic hypotheses of Ichthyodectiformes . As noted in the “Materials and Methods” section, analysis 1 includes all the taxa considered in the data matrix, and analysis 2 excludes Bardackichthys , Jinjuichthys , and Mesoclupea (see Table S 3 in Appendix 1). The tree obtained with all the taxa is a Strict Consensus Tree (SCT) generated from the 1,269 equally most parsimonious trees (MPT) in analysis 1, in which all characters are informative and have the following parameters: Tree length = 255 steps, Consistency Index (CI) = 0.4157, Homoplasy Index (HI) = 0.5843, CI Excluding Uninformative Characters (CIEUC) = 0.4016, HI excluding uninformative characters (HIEUC) = 0.5962, Retention Index (RI) = 0.5962, and Rescaled Consistency Index (RCI) = 0.2478. Otherwise, the resulting tree of the phylogenetic analysis 2 represents an SCT of 276 equal MPTs of 230 steps, in which the characters 21, 33, 47, 54, and 56 are uninformative, CI = 0.4565, HI = 0.5435, CIEUC = 0.4444, HIEUC = 0.5556, RI = 0.6082, and RCI = 0.2776.

Overall, the hypotheses 1 and 2 have similar topologies ( Figure 16 View FIGURE 16 ) and resemble those recently published by other authors (i.e., Berrell et al., 2014; Yabumoto et al., 2018; Cavin and Berrell, 2019; Baños-Rodríguez et al., 2020; Hacker and Shimada, 2021). In these, the Jurassic genera — Occithrissops Schaeffer, Patterson, 1984 ; Allothrissops Nybelin, 1964 ; and Thrissops — represent the basal taxa, and all Cretaceous ichthyodectiforms occupy a more derived position (node E) ( Jinjuichthys and Mesoclupea are part of these basal forms in analysis 1). Although phylogenetic analysis 2 does not obtain the clade traditionally known as Saurodontidae (Node L) and only shows a minimal increase in the IC value compared with analysis 1 (0.4157 versus 0.4565), its topology is more consistent because it includes eight of the 15 clades with Bremer and Bootstrap values of greater than two and 52 (versus five of 16 obtained in hypothesis 1).

Table S 3 in Appendix 1 shows a complete description of hypotheses 1 and 2. Only the unambiguous supporting characters of the nodes that include Amakusaichthys benammii sp. nov. are described below. According to both resulting hypotheses ( Figure 16 View FIGURE 16 , Node A), four unambiguous synapomorphies support the monophyly of the Ichthyodectiformes [teeth aligned in a single alveolar row in both jaws (ch. 21-1); the articular bone forms part of the facet for the quadrate (ch. 33-2); the coracoids are broad and meet to each other middle-ventrally (ch. 47-1); and the first anal and dorsal pterygiophores elongated, arranged in clusters that extend into the interhemal spaces (ch. 54- 1)].

In the present hypotheses, Allothrissops mesogaster ( Agassiz, 1833) represents the basal taxon and the sister of an unnamed group ( Figure 16 View FIGURE 16 , Node B). Two unambiguous synapomorphies support this group [the intercalar bone is large, forms part of the hyomandibular facet of the skull, and encloses a canal for the jugular vein (ch. 6-1); the mouth is directed upward (ch. 35-1)]. An additional ambiguous character is defined by the optimization acctran in both hypotheses [the epineurals are elongated (ch. 56-1)]. Both hypotheses share four unambiguous homoplasies supporting this unnamed “ Ichthyodectiformes minus Allothrissops ” clade [the parasphenoid is toothless (ch.9-1); the ethmopalatine is large and has no membranous outgrowth (ch. 13-2); the premaxilla is poorly attached to maxilla (ch. 18-1); and the parietal branch of the supraorbital sensory canal present as an anterior pit line only (ch. 41-1)]. The presence of a parasphenoid bone with a pronounced angle beneath the posterior margin of the orbit (ch.10-1) is an additional ambiguous supporting homoplasy of this group in hypothesis 1.

Next, hypothesis 2 shows a group with the species of Thrissops and all the Cretaceous ichthyodectiforms ( Figure 16 View FIGURE 16 , Node C). Three unambiguous characters support this clade, including a synapomorphy [(presence of epioccipital crests (ch. 1-1)] and two homoplasies [the supraoccipital crest is large (ch. 2-2), and the basal sclerotic bone with serrated margins is present (ch. 16)]. Also, only hypothesis 2 shows a group that includes Thrissops “Kimeridgian” ( Cavin et al., 2013) plus Cretaceous ichthyodectiforms ( Figure 16 View FIGURE 16 , Node D) that is supported weakly by a single unambiguous homoplasy [the presence of symphyseal teeth (ch. 25-1)].

Although Cretaceous ichthyodectiforms form a natural group in both hypotheses obtained in this work ( Figure 16 View FIGURE 16 , Node E), this group is supported weakly by three unambiguous homoplasies [the ethmopalatine bone is large and has membranous outgrowings with membranous outgrowths separating and suturing with the rostrodermethmoid and lateroethmoid (ch. 13-3); the premaxilla is attached firmly to maxilla (ch. 18-2); and the lateral surfaces of vertebrae have two deep longitudinal grooves separated by a middle ridge (ch. 59-1)]. In hypothesis 2, an additional unambiguous homoplasy supports this group [the mandible-inferior quadratus joint is on the anterior margin of the forward orbit (ch. 34-1)].

In hypothesis 2 of this work, the Cretaceous ichthyodectiforms are subdivided into three subgroups showing a comb-like order. The first excludes Verraesichthys Taverne, 2010 ( Figure 16 View FIGURE 16 , Node F). It is weakly defined by four unambiguous homoplasies [the supraoccipital crest becomes large and exceeds the occipital region (ch. 2-1), the maxilla is machete-shaped (ch. 20-0), dentary teeth have irregular sizes (ch. 26-1), the first three or four extend anteroventrally, covering the lateral surface of the urals and posterior preural centra (ch. 64-1)]. The second one excludes Verraesichthys and Ogunichthys ( Figure 16 View FIGURE 16 , Node G) and is supported weakly by a single unambiguous homoplasy [dorsally, the rostrodermethmoid has a slight constriction behind the initial extension (ch. 14-1)]. Finally, the third group is a polytomy of nine species representing six genera ( Chirocentrites Heckel, 1849 ; Dugaldia Cavin and Berrell, 2019 ; Eubiodectes Hay, 1903 ; Gillicus Hay, 1898 ; Saurocephalus , Saurodon Hays, 1830 ; Unamichthys Alvarado-Ortega, 2004 , and Vallecillichthys Blanco and Cavin, 2003 ) plus three clades ( Figure 16 View FIGURE 16 , Nodes I, O, and S).

In hypothesis 1, Amakusaichthys benammii sp. nov. is included in the already named Family Bardackichthyidae , here scarcely supported in two unambiguous homoplasies [the caudal fin has a ventral lobe at least 1.4 times longer than the dorsal lobe (ch. 58-1), and the hemal arches fuse to the respective caudal centra (ch. 60-1)]. In contrast, hypothesis 2 excludes Bardachichthys for the reasons already discussed in the previous section of this work.

In both hypotheses of this work, the species of Amakusaichthys and Heckelichthys form a natural group ( Figure 16 View FIGURE 16 , Nodes I) supported on two unambiguous homoplasies [the lower jaw-quadrate articulation is in front of the orbit and below the lateroethmoid (ch. 34-2), and the dorsal fin is shorter than the anal fin, its anterior end opposes the anterior lobe of the anal fin (both arise far in the back of the trunk) (ch. 53-4)]. Additionally, hypothesis 1 shows a single synapomorphy of this group [the length of the preopercular horizontal limb is larger than the height of the preopercular vertical limb (ch. 73-1)], and hypothesis 2 displays two other unambiguous homoplasies [the articular and alveolar parts of the maxilla are equally high, and its ventral and dorsal margins are parallel (ch. 20-1); and the anterior ceratohyal has no foramen (ch. 40-0)].

This research demonstrates the naturalness of Amakusaichthys ( Figure 16 View FIGURE 16 , Nodes J). Its supporting characters in both hypotheses include two common unambiguous homoplasy [presence of replacement tooth alveoli (ch. 28-1), and the number of total vertebrae ranges between 76 and 85 (ch. 69-1)]. Additional supporting characters of this clade differ between these hypotheses. On the one side, hypothesis 1 also shows three unambiguous synapomorphies [the hypural 2 is triangular and larger than hypural 1 (ch. 75-1), hypural 2 has a folded ridge in the anterior part of its dorsal edge (ch. 76-1), and the neural spines of preural centra has a saddle-shaped process (ch. 77-1)]. On the other side, hypothesis 2 shows another unambiguous homoplasy [in the pectoral fin, the first ray is 1.5 times the breadth of the second (ch. 48-1)].

Osteological Remarks

Recent paleontological studies have significantly expanded the knowledge of the taxonomic diversity of Ichthyodectiformes . Without excluding some questionable species, today, the order involves 75 species representing 44 genera ( Table 2); however, most of their relationships are still unclear. Undertaking a comparative osteological exercise in this scenario looks hard; nevertheless, specimens of the type series of Amakusaichthys benammii sp. nov. exhibit enough osteological features to recognize it as an undisputed member of the order Ichthyodectiformes and the suborder Ichthyodectoidei sensu Patterson and Rosen (1977) .

This new species has the non-exclusive body features of Ichthyodectiformes previously documented by now classical researchers of the order ( Bardack, 1965; Bardack and Sprinkle, 1969; Patterson and Rosen, 1977; Schaeffer and Patterson, 1984; Maisey, 1991), such as an elongated and uniformly high body with a relatively small head, jaws with a single tooth row, the deeply bifurcated caudal fin, and unpaired fins placed far back on the trunk and opposed to each other (i.e., Figures 2 View FIGURE 2 , 7 View FIGURE 7 ). In addition, the new species has the most outstanding diagnostic features of this fish order, a pair of ethmopalatine bones forming the floor of the nasal capsule and the broad or saber-like shape of the first rays in the pectoral and pelvic fins ( Figures 5–6 View FIGURE 5 View FIGURE 6 ).

The review of the description of the only specimen of Bardaichthys carteri so far known ( Hacker and Shimada, 2021) allows us to challenge its belonging to the order Ichthyodectiformes and point out the unnatural condition of the Family Bardackichthyidae (which also includes Heckelichthys and Amakusaichthys ). Despite such observations, the reconsideration or definitive expulsion of Bardaichthys, Jinjuichthys , and Mesoclupea , as part of this order, must still pass through the task of reassessing the osteological evidence exhibited by the available fossil materials. For now, and given that both phylogenetic hypotheses obtained in this work, one including and the other excluding these three genera ( Figure 16 View FIGURE 16 ), show the sister-group relationship of Heckelichthys and Amakusaichthys , this natural group is named family Heckelichthyidae . The subsequent possible reincorporation of Bardackichthys as a “true” ichthyodectiform would only cause the re-ranking of the clade Heckelichthys-Amakusaichthys to subfamily level.

Among ichthyodectiforms, Heckelichthys and Amakusaichthys share the following unique combination of features. The supraoccipital crest is large but shallow and does not project beyond the base of the skull ( Figures 5–7 View FIGURE 5 View FIGURE 6 View FIGURE 7 ); the teeth in both jaws are evenly small or absent ( Figure 8 View FIGURE 8 ); the origin of the unpaired fins oppose each other ( Figure 4 View FIGURE 4 ); the preopercle exhibits a short horizontal limb; the mandibular articulation is below and in front of the orbit ( Figure 9 View FIGURE 9 ); and the caudal fin has the ventral lobe at least 1.4 times longer than the dorsal lobe ( Figure 2 View FIGURE 2 ). On the contrary, in other ichthyodectiforms, the crest is small or large, noticeably higher, and can extend backward beyond the limit of the base of the skull; the dorsal fin arises behind the anterior lobe of the anal fin; the preopercle has a vertical limb longer than the horizontal limb one; the mandibular articulation is below or behind the posterior half of the orbit; and the length of the caudal ventral lobe is no more than 1.3 times the caudal dorsal lobe.

Since the fossil of Amakusaichthys goshouraensis only includes the impressions of incomplete specimens, the study of its osteology is limited. Fortunately, the type series of A. benammii sp. nov. includes well-preserved and complete specimens that allow access to fine osteological details after being prepared by different methods. Thus, the information from these Mexican fossils amends the generic diagnosis and reveals unusual features not previously observed in other ichthyodectiforms. Both Amakusaichthys species share the long-nose appearance of the head because the ethmoid region of the skull is noticeably longer than the orbit and its postorbital region. In addition, these have a caudal skeleton reinforced by the anterior elongation of the caudal rays; the hypural 2 is triangular, the more prominent in the series, and expanded posteriorly, with a dorsal folded corner ridge in its anterior two-thirds, pierced by three large pores aligned longitudinally and roofing a cavity that allocates the anterior tips of some rays of the ventral caudal fin lobe. Although not included in the original description of A. goshouraensis ( Yabumoto et al., 2018, figures 5–6), these species also share the presence of dorsoanterior saddle-shaped processes in the neural arches of preurals 2–7, the parhypural and hypurals 1–2 have enlarged articular heads expanded laterally, and the hypural 3 is shorter than the hypural 2 ( Figures 9 View FIGURE 9 , 12 View FIGURE 12 , 13 View FIGURE 13 ).

Noticeably, the Late Cretaceous Italian ichthyodectiforms Garganoichthys alfonsoi Taverne, 2009 , and Altamuraichthys meleleoi Taverne, 2016, share at least part of the Amakusaichthys features listed above; nevertheless, their possible inclusion into a family Heckelichthyidae must wait for an accurate analysis of the available specimens. Overall, the features of the body, head, and unpaired fins of the single specimen of Altamuraichthys resemble Amakusaichthys ; its imperfect caudal skeleton shows traces of the saddle-shaped processes in the neural arches of posterior preural centra. In the incomplete and single specimen available of Garganoichthys , the origins of both unpaired fins oppose each other, and the hypural 2 (described initially as hypural 1 by Taverne (2009, figure 5) has three small perforations resembling those of A. benammii .

Finally, the diagnosis of Amakusaichthys benammii sp. nov. exhibits numerous features that support its specific identity and fully differentiate it from A. goshouraensis . It is noticeable that some of these features have not been observed in other ichthyodectiforms, suggesting that this fish is highly specialized. Among these, the most outstanding features are the frontoparietal fontanelle, the intense ornamentation in the dorsal surface of the dorsal skull bones, the anterior position of the ethmopalatine ahead the nasal capsule and with a dorsal extension bordering the anterior margin of this capsule, the ventral processes with ending plates (the anterior hard-shaped and posterior arrow-shaped) the present behind the parasphenoid basipterygoid process (unfortunately the probable masticatory function of these plates are unsure because pharyngeal teeth have not yet observed in A. benammii ); the oval opercle exposing the hyomandibular facet at the middle height of its curved anterior edge; the dorsal palatine notch in the maxilla; the hypural 3 shortened and with a small unpierced folded corner ridge turned upward.

Both Amakusaichthys species differ in two features, which may be controversial in the specimens of A. goshouraensis . As observed in this work, A. benammii sp. nov. has autogenous hemal arches and dentary teeth homogeneously small ( Figures 7 View FIGURE 7 , 9 View FIGURE 9 , 12 View FIGURE 12 , 13 View FIGURE 13 ). In contrast, in A. goshouraensis , such arches are interpreted as fused to their respective preural centrum, and the dentary teeth are reported as irregularly sized ( Yabumoto et al., 2018, appendix 2).