Amia basiloides, Brownstein & Near, 2024

Brownstein, Chase Doran & Near, Tomas J., 2024, A giant bowfin from a Paleocene hothouse ecosystem in North America, Zoological Journal of the Linnean Society 202 (4), pp. 1-20 : 3-14

† Amia basiloides sp. nov.

( Figs 1–14 )

ZooBank LSID:

Holotype: YPM VPPU 16236 ,

nearly complete, threedimensional skull ( Figs 1–14 ) and partial skeleton including 35 vertebrae and numerous fin rays. The fossil was recovered articulated ( Boreske 1974) but subsequently mechanically disarticulated during preparation. All bones are identical in colour and preservation, none are duplicated, and some are still articulated (i.e. the skull roof). At least 35 complete and several fragmented vertebrae were found together with the mostly complete skull; five of these are in semi-articulation in a block and two are fused to the neurocranium. Numerous fin rays, as well as possible portions of the pelvic girdle, are also included in the holotype.

Locality and horizon: The holotype and referred material of † A. basiloides described in this contribution come from the Fort Union Formation in Montana, USA ( Fig. 1A ). The holotype YPM VPPU 0.16236 was recovered from the Burns Mine locality in the Bear Creek-Washoe area of Carbon County, Montana, by Robbert V. Witter, John Dyer, and the 1948 Princeton Expedition team. This locality produces fossils, including large mammals, characteristic of the terrestrial Bear Creek local fauna and is Late Paleocene in age (Tiffanian to Clarkforkian North American land mammal stages, 61.2–56.6 Mya; Boreske 1974, Barnosky et al. 2014).

The referred vertebrae YPM VPPU 17064 (from an individual ~20–30% larger than the holotype) was recovered from carbonaceous clay just above a coal layer dubbed layer # 3 in the Eagle Mine near Bear Creek , Carbon County, Montana. This layer also corresponds to the Fort Union Formation , implying a Tiffanian–Clarkforkian age. In addition to the referred vertebrae, this locality has produced an isolated dermal scute of a very large acipenserid (likely Acipenser ) (Brownstein 2023), and records of large choristoderes previously referred to the European genus † Simoedosaurus , but potentially also referable to † Kosmodraco ( Brownstein 2022b) .

Etymology: The species name is from the Ancient Greek βασιλεύς, meaning king, referencing the massive size of this species. A new species name for this specimen, previously referred to † Amia ‘ uintaensis ’ by Boreske (1974) without comment, is needed since †‘ Amia uintaensis ’ is a nomen vanum ( Grande and Bemis 1998). The other large Paleocene amiid, † Amia ‘ robusta ’, is based on a taxonomically uninformative holotype and therefore a nomen dubium ( Grande and Bemis 1998).

Diagnosis: † Amia basiloides ( Figs 1–14 ) is distinguished from other amiines by the following combination of features: teeth include numerous large to small teeth on maxilla ( Fig. 2A–C ), enlarged fang-like teeth on the anterior ectopterygoid reach the size of largest teeth in the main maxillary and dentary rows ( Fig. 2D–G ), and strongly recumbent anterior vomerine teeth ( Fig. 5 ); endopterygoid and ectopterygoid fused, with sutures not distinguishable ( Fig. 2D–G ); robust ceratohyals are heavily arched medially to contact hypohyals ( Fig. 12 ); parasphenoid tooth patch extends anteriorly and terminates close to the vomers ( Fig. 5 ). † Amia basiloides and † A. patersoni , which are resolved as sister species in parsimony-based phylogenetic analysis, share the following features to the exclusion of all other amiids: proportionately elongated skull roof ( Figs 3 , 4 ), subtriangular first postinfraorbital reduced in size relative to second postinfraorbital ( Fig. 6 ), extremely narrow gular ( Fig. 11 ) ( Grande and Bemis 1998, Grande et al. 2000). † Amia basiloides differs from † A. patersoni in the following features: absence of a sigmoid posterior border of the opercle ( Fig. 10 ); heavily ornamented opercle ( Fig. 10 ); preopercle diaphysis is strongly posteriorly flexed ( Fig. 10 ).

Ontogenetic assessment: Following Griffin et al. (2021), we provide an ontogenetic assessment of the holotype of † A. basiloides . The holotype is clearly from a large, osteologically mature adult based on the well-ossified nature of all cranial and postcranial elements ( Fig. 2 ; these are unfused in juvenile A. calva and A. ocellicauda ; Grande and Bemis 1998, Brownstein et al. 2022: supplement), large body size, and the extensive ornamentation of the cranial bones ( Figs 2–4 ). Observation of a collection of computed tomography (CT)-scanned skulls from both living Amia species (Brownstein et al. 2022) also confirms that the proportions of the skull in the holotype of † A. basiloides generally match those of adult bowfins ( Fig. 2 ).

General description: The skull and skeleton of † A. basiloides is large and robust relative to the skulls of most other amiids, including A. calva , † Amia hesperia , † Amia scutata , and † Amia sp. from the Kishenehn Formation ( Grande and Bemis 1998, Gardner and Wilson 2022). Most external skull bones are heavily ornamented, with deeply incised reticulated texture across the entirety of the nasals, lacrimals, frontals, parietals, dermopterotics, dermosphenotics, lacrimals, postinfraorbitals, opercular series, supramaxillae, and maxillae. This ornamentation more closely compares with the condition in † Cyclurus and other stemward forms compared to A. calva , † A. scutata , † A. godai , or † A. patersoni . † Amia basiloides is assignable to the genus Amia based on the following features: presence of an elongated, narrow parasphenoid tooth patch vs. shortened and cardioid in † Cyclurus ; conical coronoid teeth vs. styliform teeth in † Cyclurus ( Grande and Bemis 1998, Grande et al. 2000).

Skull roof: The skull roof of † Amia basiloides is closely comparable to other species of Amia ( Figs 3–4 ). Posteriorly, the skull roof is composed of the extrascapulars ( Fig. 3 ). These lack the posterior extension found in some specimens of A. calva . The extrascapulars also lack ornamented surfaces, unlike the rest of the skull roof, and are elliptical in shape. The dermopterotics and parietals are heavily ornamented rectangular bones that composed slightly over one-third of the dermal skull roof. These bones are indistinguishable from the same elements in other Amia species, although the elongated proportions of the parietals are particularly comparable to those of † A. patersoni ( Grande and Bemis 1998) .

The frontals, parietals, dermopterotics, and dermosphenotics are tightly sutured together ( Fig. 4 ). As with the parietals, the frontals are elongated as in the Eocene species † Amia patersoni ( Grande and Bemis 1998) . Ornamentation on these bones consists of rugosities radiating from ossification centres. The ornamented surfaces on the skull roof of † A. basiloides are more developed than in any other amiid that we have examined, although this may be related to the body size of the holotype ( Grande and Bemis 1998). Sutural connections between opposing elements in the skull roof, including between the paired parietals, paired frontals, and adjacent parietals and dermopterotics, are linear, whereas those between the frontals and parietals, and frontals and nasals, are interdigitating ( Grande and Bemis 1998). One peculiar feature of the posterior and middle skull roof of † A. basiloides is a pronounced depression along the midline of the posterior skull roof extending out to the lateral borders of the parietals. The preserved ventral surface of the skull roof indicates this is not due to postmortem compression and may be a genuine feature of this taxon.

Anterior to the frontals are the nasals ( Figs 4 , 6A ), represented by the complete left and partial right bones. The nasal is loose from the rest of the skull roof and articulated with the left frontal in an interdigitating lap suture. Like the rest of the skull roof, the shape and level of ornamentation of the nasal is highly variable in A. calva , † A. scutata , and † A. patersoni , the three pan- Amia species for which enough skeletons are known to investigate intraspecific variation in these features ( Grande and Bemis 1998).

Infaorbital, suborbital, and supraorbital bones: This region of the skull is represented in the holotype of † A. basiloides by the lacrimals and postinfraorbitals ( Fig. 6 ). The shape of the left lacrimal indicates it may represent a fusion of one of the suborbitals and the lacrimal ossification. The lacrimals are large, heavily ornamented bones that represent the anteriormost infraorbitals, as in other species of amiid ( Grande and Bemis 1998). The lacrimal is approximately as tall as long, unlike the condition in A. calva or † A. scutata but similar to † A. patersoni and † Cyclurus ( Grande and Bemis 1998) . Ornamentation consists of ridges radiating from the centrally-placed centre of ossification, as well as irregular pits. There are two postinfraorbitals. † Amia basiloides displays a small, triangular first postinfraorbital, somewhat similar to the condition in † A. patersoni among amiids ( Grande and Bemis 1998). Unlike † A. patersoni ( Grande and Bemis 1998) , the first postinfraorbital is both anteroposteriorly and dorsoventrally smaller than the second postinfraorbital.

Braincase and ventral ethmoid region: Because the skull of the holotype of † A. basiloides was partially disarticulated during preparation, there is easy access to the three-dimensionally preserved internal cranium, including the braincase, parasphenoid, vomers, and premaxillae ( Fig. 5 ). As in other amiids, the basioccipital fuses with the two anteriormost vertebral centra to form the occipital condyle ( Grande and Bemis 1998). The elongated parasphenoid forms a large bar that runs across the length of the skull. This bone is posteriorly developed into two flanges that underly the basioccipital, as in other species of pan- Amia ( Grande and Bemis 1998) . The contact surfaces between the basioccipital and parasphenoid are partially ablated due to fusion of these bones, which may be related to the size and ontogenetic status of the holotype specimen ( Grande and Bemis 1998). The parasphenoid is elongated relative to A. calva , † A. scutata , † A. hesperia , and † Cyclurus , instead most closely resembling the condition in † A. patersoni ( Grande and Bemis 1998) . Midway along the anteroposterior axis of the parasphenoid are the ascending rami of the parasphenoid, which point anterodorsally ( Grande and Bemis 1998). Anteriorly, the sutural contacts between the vomers and parasphenoid are almost entirely ablated. Ventrally, the parasphenoid bears a long patch of small, sharp, conical teeth termed a shagreen by Grande and Bemis (1998). The shape of the parasphenoid tooth patch is highly intraspecifically variable in A. calva ( Grande and Bemis 1998) . However, the patch in the holotype of † A. basiloides is longer than in most A. calva or other extinct species of amiid with the exceptions of † A. patersoni , † Solnhofenamia elongata , † Calamopleurus cylindricus , and several species in the genus † Amiopsis ( Grande and Bemis 1998) .

In contrast to the parasphenoid tooth patch, the teeth on the vomers are large and approximate the size of dentary, premaxillary, and maxillary tooth crowns ( Fig. 5 ). The shape of the vomerine crowns in † A. basiloides differs from those in all other species of Amia and † Cyclurus . Unlike the crushing dentition of † Cyclurus or the large conical crowns found in A. calva , † A. scutata ,† A.hesperia ,and † A. patersoni ( Grande and Bemis1998) , the vomerine teeth of † A. basiloides appear as large, strongly posteriorly recurved crowns that project posteromedially. These crowns are proportionally much larger than the vomerine teeth of † A. patersoni ( Grande and Bemis 1998) . The vomerine tooth patches extend to the posterior borders of both elements, contrasting with the condition in A. calva , † A. hesperia , † A. scutata , and † A. patersoni where they are localized within a subcircular area on the vomer.

Jaws, palatal bones, suspensorium, and jaw articulation: The tooth-bearing bones of the jaws in † A. basiloides include the premaxillae, vomers, maxillae, dermopalatines, parasphenoid, coronoids, and mandibles ( Figs 2 , 5 , 7–9 ). The premaxillae are elongated bones fused to the rest of the basicranium and vomers ( Fig. 5 ). These bones are broadened with a crescentic anterior margin. Posteriorly, the premaxillae bear large nasal processes. Nine to ten tooth positions are present on either premaxilla.

Posterior to the premaxillae in the dental arcade are the maxillae ( Figs 2 , 7 ), which are large, ornamented bones that are proportionally deeper than in other species in pan- Amia ( Grande and Bemis 1998) . Anteriorly, the maxillae possess large, upturned premaxillary processes, as in A. calva but unlike other extinct species in the genus. Laterally, the posterior half of the maxilla is ornamented with numerous ridges; ornamentation is sparse on the anterior surface of this element. The supramaxillary notch appears larger in † A. basiloides than in any other species of Amia or † Cyclurus ( Grande and Bemis 1998, Grande et al. 2000), separating the large, ornamented supramaxilla from the maxilla for at least 40% of the anteroposterior length of the former bone. Maxillary teeth are considerably smaller than the premaxillary teeth, as in other species of Amia and † Cyclurus but unlike some members of the amiid clade Vidalamiinae ( Forey and Grande 1998, Grande and Bemis 1998).

The anteriormost element in the lower jaw is the dentary ( Fig. 8 ), which is deeper than in † Amia patersoni and approximates the condition in A. calva , † A. scutata , and † A. hesperia ( Grande and Bemis 1998) . The dentaries of † A. basiloides and † A. patersoni are similar in the reduction of medial curvature expressed along the anterior portion of this bone, as well as in the degree to which the anterior dentary is ventrally downturned in lateral view. Along with the proportionately elongated gulars in both these species, the morphology of the dentaries indicates the skulls of † A. basiloides and † A. patersoni were mediolaterally compressed relative to other species of amiid and likely also had a larger gape ( Grande and Bemis 1998). The dentary tooth row consists of large, straight to slightly recurved conical crowns as in other species of Amia ( Grande and Bemis 1998, Grande et al. 2000). In contrast to A. calva , † A. scutata , and † A. hesperia ( Grande and Bemis 1998) , the lateral surface of the dentary of † A. basiloides is heavily ornamented with irregular ridges running around the external dentary neurovascular foramina.

Posteriorly, the dentary is dorsally expanded to form a large ascending process that articulates with the supraangular dorsolaterally. Medially, the dentary articulates with the coronoids, which bear smaller rows of teeth, and the large, triangular retroarticular ( Grande and Bemis 1998). Like other pan- Amia species, the retroarticular laterally bears a tooth patch consisting of extremely small denticles ( Grande and Bemis 1998). The first articular element is not preserved. A small bone may be the second articular element of Grande and Bemis (1998) based on its similarity to this bone in other amiids. Both angulars are well preserved and are heavily ornamented as in other members of pan- Amia . The anterior half of the angular is proportionally elongated in † A. basiloides and † A. patersoni relative to other members of the genus ( Grande and Bemis 1998). The supraangulars are also partially preserved for both sides of the mandible and are generally comparable to this bone in other amiid species in shape.

Because the skull of the holotype of † A. basiloides was partially disarticulated during preparation, the three-dimensionally preserved palatal bones are all easily observable ( Fig. 9 ). The palate is extremely poorly known for many fossil amiid species, making the holotype of † A. basiloides particularly important for understanding the evolutionary history of this cranial region in the bowfin total clade. Although the palate of † A. basiloides generally resembles that of other species of Amia , it differs in showing the near complete fusion of the ectopterygoid with the endopterygoid, an enlarged, shallower foramen on the hyomandibular, and the presence of strongly recumbent tooth rows on the vomers. The first and second dermopalatines bear slightly recurved teeth that greatly exceed the strongly recumbent ectopterygoid detention in size. The dermopalatine teeth reach the size of crowns in the main maxillary and dentary tooth rows. All dermal palate bones are ornamented with ridges and grooves. There are no traces of tooth plates on the hyomandibular and metapterygoid as in extant A. calva or other amiids ( Grande and Bemis 1998, Grande et al. 2000). The absence of this feature in † A. basiloides is almost certainly not due to erosion, as all bone surfaces in the holotype are excellently preserved.

The hyomandibular of † A. basiloides is proportionately smaller and less rectangular than the hyomandibular in † A. patersoni ( Grande and Bemis 1998) . The hyomandibular foramen incompletely pierces the lateral surface of the bone in † A. basiloides , as in some individuals of A. calva ( Grande and Bemis 1998) . As in other amiids, the quadrate is a small, subrectangular bone with a posteroventrally-placed condyle for the lower jaw. The symplectic is not known for † A. basiloides , but we infer dual articulation between the upper and lower jaws was present in this taxon as in other halecomorphs.

Opercular series, branchiostegal rays, and gular: The opercular series ( Fig. 10 ) is represented in † A. basiloides by both preopercles, the left opercle, a fragment of what might be the left subopercle, the right interopercle, and both branchiopercles (the last branchiostegal ray; Grande and Bemis 1998). The opercle is the largest bone in the opercular series and is approximately square as in pan- Amia , † Cyclurus , and other amiids ( Grande and Bemis 1998). The opercle of † A. basiloides lacks the sigmoidal posterior border found in the opercle of † A. patersoni . Although Grande and Bemis (1998) hypothesized that the formation of a sigmoidal posterior border of the opercle was related to size in pan- Amia , the absence of this feature in the holotype of † A. basiloides suggests it should be considered an apomorphy of † A. patersoni and potentially †‘ A. robusta ’ ( Grande and Bemis 1998) . Ornamentation on the opercle of † A. basiloides also markedly contrasts with the conditions in all other pan- Amia with the exception of † A. hesperia ; both † A. basiloides and † A. hesperia share an extremely heavily ornamented opercle characterized by the presence of deep incisions between numerous, slightly irregular ridges radiating from the centre of ossification. The possible subopercle is too fragmentary for description. Ventrally, this bone would have articulated with the ornamented interopercle, which is relatively short compared to † A. patersoni and more closely resembles the bone in A. calva , † A. scutata , and † A. hesperia . Like the largest known individuals of † A. patersoni , the branchiopercle of † A. basiloides is heavily ornamented but otherwise agrees with the morphology of this bone in other amiids ( Grande and Bemis 1998). The other branchiostegal rays show a lower degree of ornamentation. Ornamentation on the branchiostegal series is ontogenetically variable in amiids ( Grande and Bemis 1998). The preopercle is the anteriormost bone in the opercular series. This bone is ornamented in † A. basiloides and strongly posteriorly flexed along its dorsoventral axis. In † A. patersoni , the preopercle is straightened ( Grande and Bemis 1998).

The gular of † A. basiloides is extremely elongated ( Fig. 11 ), contrasting with the mediolaterally widened elements in all amiines except for † A. patersoni ( Grande and Bemis 1998) . The gular anteriorly projects to a discrete apex and is posteriorly squared off. This bone is weakly ornamented relative to most other external skull bones; this ornamentation consists of radiating ridges.

Gill arches: The gill arches are represented in the holotype skull of † A. basiloides by both ceratohyals ( Fig. 12 ). The ceratohyals of † A. basiloides are far more robustly constructed than in any other species of pan- Amia , and each strongly curves medially to contact each hypohyal. The high degree of curvature of the ceratohyals in † A. basiloides is not seen in other members of Amia ( Grande and Bemis 1998) . The right ceratohyal shows pathological growth along its anterior border, which may represent fusion between this bone and one of the branchiostegal rays in response to an injury.

Vertebral column: At least 35 complete and numerous partial vertebral centra are included in the holotype specimen of † A. basiloides . These ( Fig. 13 ) are closely comparable to those of other amiines ( Grande and Bemis 1998). Unfortunately, much of the vertebral column was not preserved in articulation and lacks the various vertebral processes found in amiids. The vertebral centra included in the holotype of the new species constitute the largest known partial amiid spine and are comparable in size to the largest reported amiid vertebrae ( Grande and Bemis 1998). An isolated Amia vertebra recovered from nearby the holotype locality ( Fig. 13F–G ) is from a fish approximately 30% larger than the holotype.

Pectoral girdle and fin: The pectoral girdle of † A. basiloides is represented by both cleithra, one supracleithrum, and numerous fin rays ( Fig. 14 ). The massive cleithra are similar to those of other amiines and show a high degree of ornamentation ( Grande and Bemis 1998). Several branchiostegal rays are still tucked within the medial surface of the cleithrum. The supracleithrum is a poorly ornamented plate-like bone that overlies the lateral surface of the dorsal third of the cleithrum, as in other amiids. This bone more closely resembles the supracleithra of † A. hesperia and † A. patersoni in its strongly rectangular shape.

Phylogenetic results

Parsimony analysis ( Fig. 15 ) of an updated matrix that includes 41 species of Halecomorphi with wildcard species secondarily removed (Material and methods) found three most-parsimonious trees of 132 steps (CI = 0.606, RI = 0.881), all of which resolved † A. basiloides and † A. patersoni as a monophyletic group that is the sister lineage of a clade containing † A. scutata , A. calva , and A. ocellicauda . The clade containing † A. basiloides and † A. patersoni is supported by a single synapomorphy: an elongated, narrow gular (character 30:2). The Amia total clade was supported with a low bootstrap score, the clade containing † A. basiloides and † A. patersoni was supported by a moderate bootstrap score of 0.64 (Supplementary Text). The rest of the phylogeny closely agrees with the topology recovered in Grande and Bemis (1998) but disagrees with a recent study that united Amiidae and † Sinamiidae on the basis of one unambiguous and two homoplastic characters ( Deesri et al. 2023). We favor the traditionally recovered position of †Vidalamiinae within Amiidae , as Deesri et al. (2023) include a much smaller sample of amiines, vidalamiines, and basal amiids, and their morphological dataset is based on a character list employed to test the relationships of early halecomorphs rather than Amiidae proper.

A Bayesian fossilized birth-death analysis of the morphological dataset also resolved a monophyletic pan- Amia with † A. basiloides and † A. patersoni forming an early-diverging clade one node closer to the crown than † A. hesperia ; this clade was supported by a posterior probability of 0.84 and pan- Amia was supported by a moderate posterior of 0.68 ( Fig. 16 ). pan-Amia was strongly supported as the sister taxon to † Cyclurus in Amiinae, and this clade was weakly supported as the sister taxon to the †Vidalamiinae, which went extinct at the end of the Cretaceous. Ingroup relationships among species of † Cyclurus 14 • Brownstein and Near and pan- Amia are poorly supported, indicating a high degree of uncertainty underlies the phylogeny of Cenozoic bowfins. The tip-dated maximum clade credibility tree generated from Bayesian analysis of the morphological dataset ( Fig. 16 ) inferred a mid-Cretaceous age for the divergence of pan- Amia and † Cyclurus [most recent common ancestry MRCA median age: 95.47 Mya (95% highest posterior density, HPD: 126.29, 73.04 Mya)] and a latest Cretaceous origin for the divergence at the base of pan- Amia [MRCA median: 77 Mya (95% HPD: 59.49, 100.72 Mya)]. The large-bodied pan- Amia clade formed by † A. patersoni and † A. basiloides was found to diverge from other pan- Amia around the Cretaceous–Paleogene boundary 69.87 Mya (95% HPD: 57.37, 90.3 Mya).

Body size evolution

Standard length estimates for the holotype of † A. basiloides based on mandibular, gular, and frontal length yielded values of 1.42 m, 1.24 m, and 1.32 m, respectively, for the standard length of the holotype of the new species, corresponding to a specimen that exhibited a total length in the range of 1.4 to 1.6 m assuming the length of the caudal fin was ~10% of the length of the fish as in other Amia ( Grande and Bemis 1998) . This exceeds the total length of other known species of Halecomorphi known from non-fragmentary skeletal remains ( Bryant 1988, Grande and Bemis 1998, Sullivan et al. 2023) and is comparable to the largest gar species ( Grande 2010) and large living freshwater ray-finned fishes ( Grande and Bemis 1991, Grande 2010, Hilton et al. 2011, Zhang et al. 2019). Precaudal vertebrae (maximum width = 55 mm) from the Fort Union Formation in the Bear Creek region exceeded the width of the largest centra in the holotype of † A. basiloides by 30%, implying individuals that exceeded 2.0 m in length. Analyses of the evolution of body size in amiids demonstrated two instances of major size increases in the clade, a hypothesis that remains robust to the type of analysis used to infer the phylogeny of Amiidae ( Fig. 17 ). One rapid episode of body size evolution takes place in the latest Cretaceous North American † Melvius , which is a lineage in the clade †Vidalamiinae ( Bryant 1988, Grande and Bemis 1998). The second episode occurs directly after the K–Pg boundary near the base of pan- Amia and corresponds to the massive sizes attained by † A. basiloides and † A. patersoni .

Biogeography

Ancestral biogeographic reconstruction using BioGeoBears on the Bayesian-inferred maximum clade credibility phylogeny ( Fig. 16 ) favoured the Dispersal-Vicariance-like (DIVALIKE) model as implemented in BioGeoBears without the jump parameter ( Matzke 2013); however, all biogeographic models produced similar hypotheses (Supplementary Data). Although most of the biogeographic history of Amiiformes is reconstructed to take place in Eurasia, † A. basiloides is part of the North American diversification of Amiinae, which secondarily appears to have dispersed to Eurasia ( Grande and Bemis 1998) in a pattern following those seen in ecologically similar and phylogenetically proximate clades like gars ( Brownstein et al. 2023). In contrast, the †Vidalamiinae appears in Eurasia and secondarily disperses to North America and Gondwana. As with gars ( Grande 2010, Brownstein et al. 2023), amiids are restricted to eastern North America by the Neogene, implying that this region has served as a refugium of ancient ray-finned fish biodiversity for at least 20 Myr.

Figures

Figure 1. Locality data of †Amia basiloides. a, Locality and stratigraphic context off the type locality of †A. basiloides, with Carbon County, Montana, highlighted in red.The stratigraphic column was generated by the USGS (cite: Flores R.M., Bader L.R. Fort Union coal in the Powder River Basin, Wyoming and Montana: a synthesis Fort Union Coal Assessment Team, 1999 Resource Assessment of Selected Tertiary Coal Beds and Zones in the Northern Rocky Mountains and Great Plains Region.U.S. Geological Survey Professional Paper 1625-A, Chapter PS, Disc 1, Version 1.0 (1999), pp. PS-1–PS-71; Trippi, M.H., Stricker, G.D., Flores, R.M., Stanton, R.W., Chiehowsky, L.A., and Moore, T.A., 2010, Megascopic lithologic studies of coals in the Powder River basin in Wyoming and in adjacent basins in Wyoming and North Dakota: U.S. Geological Survey Open-File Report 2010–1114, 197 p., available at https://pubs.usgs.gov/of/2010/1114/.) b, Partial reconstruction of skull of †A. basiloides based on the holotype specimen compared with (c) A.calva.

Figure 2. Computed tomography shows diagnostic features of the feeding apparatus in †Amia basiloides. Holotype bones of †A. basiloides: right maxilla-supramaxilla complex in (a) lateral, (b) medial, and (c) ventral views, left ectopterygoid-endopterygoid-dermopalatine complex in (d) lateral, (e) medial, and (f) ventral views, with (g) a close-up of the dermopalatine teeth, median gular in (h) ventral and (i) dorsal views, and left mandible in (j) lateral, (k) ventral, (l) medial, and (m) dorsal views, with (n) a close-up of the dentary anterior tooth row.

Figure 3. Skull roof of †Amia basiloides. Extrascapulars (a, b) in dorsal view.

Figure 4. Skull roof of †Amia basiloides. Main skull roof portion in (a) dorsal and (b) ventral views.

Figure 5. Braincase of †Amia basiloides. Braincase in (a) dorsal and (b) ventral views.

Figure 6. Suborbital, infraorbital, and nasal bones of †Amia basiloides. a, Right nasal, (b)?suborbitals, (c) left and right lacrimals, and right (d) and left (e) postinfraorbitals in lateral view.

Figure 7. Maxillae of †Amia basiloides. Right maxilla and supramaxilla in (a) lateral and (b) medial views. Left maxilla in (a) lateral and (b) medial views.

Figure 8. Mandibles of †Amia basiloides. Partial right mandible in (a) lateral and (b) medial views. Left mandible in (c) medial and (d) lateral views.

Figure 9. Palate of †Amia basiloides. Partial right palate in (a) medial and (b) lateral views.Partial left palate in (c) lateral and (d) medial views.

Figure 10. Opercular series of †Amia basiloides. Partial right opercular series in (a) lateral view.Partial left opercular series in (b) lateral view.

Figure 11. Gular of †Amia basiloides. Gular of †A. basiloides in (a) compared with the gulars of A. calva (b) and †A. scutata (c), all in ventral view.

Figure 12. Gill arches of †Amia basiloides. Ceratohyals in (a) dorsal and (b) ventral views.

Figure 13. Vertebral column of †Amia basiloides. Partially articulated block of vertebrae of holotype YPM VPPU 16236 in (a) left lateral and (b) right lateral views.Several precaudal vertebrae of holotype YPM VPPU 16236 in anterior (c, d) and posterior (e) views. Referred specimen YPM VPPU 17064 in anterior (f) and posterior (g) views.

Figure 14. Shoulder girdle of †Amia basiloides. Right cleithrum (a) in lateral (top) and medial (bottom) views. Left cleithrum (b) in lateral (top) and medial (bottom) views.c, Left supracleithrum in lateral (top) and medial (bottom) views.

Figure 15. Phylogenetic position of †Amia basiloides. Amiidae as resolved in the strict consensus topology from the parsimony analysis of the morphological matrix using TNT v.1.5.

Figure 16. Phylogeny and biogeography of Amiiformes. BEAST maximum clade credibility tree generated from tip-dated phylogenetic analysis using the Fossilized Birth-Death model.Numbers at nodes are posterior support values. Grey bars are 95% highest posterior density intervals for divergence times.Overlain is the favoured biogeographic reconstruction generated using the DIVALIKE model in BioGeoBears. Photograph of Amia calva is by Z. Randall (used with permission).

Figure 17. Size of †A. basiloides and the evolution of gigantism in Amiiformes.Linear regressions of (a) mandibular and (b) frontal length against standard length for bowfins (Amia). Phenogram (c) of a proxy of body size (log-transformed standard length) along a reduced version of the Bayesian maximum clade credibility tree with only taxa for which standard length values were available for adults, showing the pronounced episodes of size increases at the base of Amia and †Melvius adjacent to either side of the Cretaceous–Paleogene boundary.