Definition: The last common ancestor of Actinopterygii (ray-finned fish) and Sarcopterygii (lobe-finned fish).
Osteichthyes weren't the first vertebrates with bone, but they developed and used it in novel ways that cause their skeletons to preserve much more evolutionary information: Major trends:
- Widespread endochondral bone in the braincase and postcranial skeleton.
- Emergence of recognizable dermal elements of the head and pectoral girdle.
- Emergence of a tendency for superficial dermal elements to interact with deeper endochondral ones.
Synapomorphies of Osteichthyes:
A thorough survey goes beyond the technical limitations of this course. If you really want to know, consult the technical literature. For us, a general review must do:
- Bone ossifies endochondrally: Many fossil vertebrates have extensive dermal bone. Osteostraci, Galeaspida, and placoderm-grade gnathostomes have in addition perichondrally ossified three-dimensional bone preformed in cartilage, however in Osteichthyes, there is extensive endochondral ossification as well, in the braincase, girdles, and vertebral elements.
Sensory canals in an Arthrodira and Osteichthyes
- Sensory canals: Present in vertebrates ancestrally, but only identifiable in vertebrates with large dermal plates. In Arthrodira we see
- The extension of the main lateral line onto the skull roof.
- Supraorbital canal
- Infraorbital canal
- Preopercular canal
- Mandibular canal
- Occipital commissure
- Jugal canal (in Sarcopterygii)
Neurocranium of Mimiapiscis toombsi
- Neurocranium articulates with palatoquadrate via basipterygoid process (Although the stem chondrichthyan Pucapampella seems also to have had this (Maisey and Anderson 2001).)
- Postorbital process forms the dorsal surface of a partition called the lateral commisure. the hyomandibula articulates with its posterior surface rather than with the otic process, as in Chondrichthyes.
- Spiracular groove passes between lateral commisure, basipterygoid process and opening of spiracle.
- Endolymphatic duct does not communicate with the external environment.
- The anteroventral extremity of the lateral occipital fissure widens into an unossified vestibular fontanelle adjacent to the sacculus of the inner ear.
- Dermal bony plates form dermal skull roof (right), including:
- Paired elements:
- Parietals (p)
- Postparietals (pp)
- Supratemporals (st)
- Intertemporals (it)
- Large cheek plate that houses the preopercular canal. (Named the preopercular (pop) in actinopterygians, and arguably homologous to the preopercular and squamosal in srcopterygians)
- Median elements:
- Rostral (r)
- Postrostral (por)
- Paired elements:
- Teeth are developed in the dermal bones forming the margin of the mouth (premaxilla, maxilla, dentary) as well as from bones covering the palatoquadrate and Meckel's cartilage.
- Dermal bones enclose Meckel's cartilage. Adductor muscles insert on Meckel's cartilage through an adductor fossa.
Synapomorphies of Mandibular arch:
- Dermal bones of the jaw:
- At least one infradentary beneath dentary on medial surface
- Prearticular medial to adductor fossa
- Coronoid series anterior to adductor fossa.
Palate of Mimiapiscis showing dermal elements.
- Dermal bone lines the palate:
- Vomers (v)
- Pterygoids (pt)
- A lateral arcade of palatal elements
Hyoid and branchial arches of Mimiapiscis.
- Dermohyal - Dermal ossification of the hyomandibula:
- Branchiostegal rays with at least two elements modified as operculars and suboperculars (in contrast to their status in acanthodians).
- Hyoid arch cartilages ossify as:
- Hyomandibula (eh) ("epihyal" of some authors) and interhyal (ih) dorsally
- Ceratohyal (ch) and hypohyal ventrally.
Synapomorphies of branchial arch:
- First two branchial arches articulate to a single basibranchial
- Cleithrum (ct) (which adheres to the lateral aspect of the endochondral scapulocoracoid.)
- Dorsal to the cleithra (in order).
- Anocleithra (aka postcleithra)
- Ventral to the cleithra
Lepidotrichia from dkimages.com
- Fins supported by lepidotrichia - dermal fin rays developed from modified rows of scales.
- Lungs - outpouchings of the esophagus serving as an accessory breathing mechanism. (Modified in some as a "swim-bladder" for hydrostatic control.)
- Primary fin supports make minor contribution to median fins outside body wall.
- Rhombic body scales with peg and socket articulation.
- Fulcral scales support leading edges of fins.
- Persistent lateral occipital fissure (lateral cranial fissure of some authors)between otic and occipital regions.
- Persistent Ventral cranial fissure in floor of braincase between otic (parachordal) and sphenoid (trabecular) regions.
- The post-cranial axial skeleton is simple:
- The notochord is not ossified
- Neural arches protect the nerve cord dorsal to the notochord
- Ribs may project from the notochord in the trunk region
- Haemal arches protect blood vessels ventral to the notochord
- Supraneurals may extend from the neural arches
- Occasionally, intercalary ossifications (aka "intercalaries" - not shown) form between neural or haemal arches.
- Fin ray radials support the median fins.
In most Paleozoic osteichthyes, heavy body scales take up much of the mechanical load that is born by the axial skeleton in later fish.
Latimeria menadoensis (Sarcopterygii) and Xyrauchen texanus (Actinopterygii)
Two major living groups:
- Actinopterygii (Devonian - Quaternary) - Living ray-finned fish and their fossil relatives.
- Sarcopterygii (Silurian - Quaternary) - living lobe-finned fish (including tetrapods) and their fossil relatives.
Moythomasia nitida (Actinpoterygii) and Strunius sp. (Sarcopterygii)
- Single dorsal fin
- Tail is heterocercal. Epichordal lobe lacks fin rays. Instead, it is lined by robust fulcral scales
- Two dorsal fins
- Tail is diphycercal in the most basal forms. Even those that evolve heterocercal tails retain epichordal fin rays.
Xiphias gladius (Actinpoterygii) and Sauripteris taylori (Sarcopterygii)
Paired limb skeleton:
- Parallel endochondral radials articulate with the endochondral scapulocoracoid
- Pre- and postaxial elements branch from a metapterygial axis.
- Thus, a single humerus articulates with the endochondral scapulocoracoid, followed by radius and ulna, etc.
Mimiapiscis toombsi (Actinpoterygii) and Eusthenopteron foordi (Sarcopterygii)
- Strongly integrated or coossified endochondral braincase.
- Hyomandibula articulates through a single articular surface dorsal to the jugular vein.
- Ethmosphenoid region and otico-occipital regions completely separated by ventral cranial fissure and meet at a mobile joint.
- Hyomandibula articulates through two articular surfaces between which the jugular vein passes.
Ganoid scale (Actinpoterygii) and cosmoid scale (Sarcopterygii)
Actinopterygii: Ganoid scales
- Scales form as concentric bony layers around a primordium, as in Acanthodii.
- However superficial layers are of dentine and deep layers of acellular bone, separated by by a vascularized layer.
- Superficial surfaces of upper layers are made of thick layers of ganoine - an enamel-like substance.
- Typically lack ganoine (although the most basal members have it.)
- The cosmine layer is, instead, covered by a thin lamina of enamel, forming the scale surface.
Sarcopterygii: Cosmoid scales
Dialipina markae from Adventure Canada
Actinopterygii and Sarcopterygii are total groups that include all members on their stems. Thus, every member of Osteichthyes is, by definition, a member of Actinopterygii and Sarcopterygii. Nevertheless, we should expect to find "stem osteichthyans" displaying some of the osteichthyan synapomorphies. Alas, the only candidates known are:
- Dialipina: (Early Devonian - right) Described as a basal actinopterygian based on its possession of ganoid scales. Zhu et al., 2013, however, place it in a polytomy at the base of Osteichthyes.
- Two Late Silurian taxa known from isolated bones and scales including jaw fragments (Botella et al., 2007).
- It was only recently found that ganoine occurs in both actinopterygians and basal sarcopterygians. Until the last decade, a fish with ganoid scales would have been assumed to be an actinopterygian.
- Prior to the era of cladistics, "stem-Osteichthyes" was not part of our search-image.
But the most significant landmark are among basal actinopterygians and sarcopterygians. Our most informative example:
Guiyu oneiros from Wikipedia
- The oldest known osteichthyan and the oldest well-known gnathostome.
- Our closest well-known approximation of the last common ancestor of Osteichthyes.
- Stout pectoral and dorsal fin spines
- Ganoine covering surface of dermal bones of skull
- Dermal elements of the pelvic girdle, a feature otherwise only known in the antiarch Parayunanolepis (Zhu et al., 2012).
- A prerostral on the midline of the palate - an element otherwise seen only in placoderm-grade gnathostomes.
An osteichthyan for sure, but it remembers its placoderm-grade origins.
Cheirolepis canadensis from Wikipedia
Actinopterygian evolutionary trends:
- From biting to sucking:
The jaws of the ancestral actinopterygians like Moythomasia relied on the scissor-like action of the upper and lower jaws to bite and hold prey. Derived actinopterygians employ a suction feeding mechanism, in which the the mouth extends to form a tube while the pharynx is expanded, sucking prey items in. Once inside, prey is dealt with by any of a variety of jaw, tongue, or pharyngeal teeth. To achieve this, the premaxilla and maxilla have become extremely mobile. (A modest example)
Basal sarcopterygian vertebral column
- Ossifying the vertebral column:
- The ancestral osteichthyan had ossified neural and haemal arches, but the notochord remained unossified.
- Among both actinopterygians and sarcopterygians, the vertebral column ossifies as proper bone (in a variety of patterns), yielding vertebrae that represent a close association of:
- Neural arches
- Haemal arches
- Making the tail symmetrical: During actinopterygian evolution, caudal vertebrae and their associated hameal arches shorten, curl up sharply, and fuse into a small number of triangular elements that support a superficially symmetrical - looking tail termed homocercal.
Kissing gourami Helostoma temminckii from FishNavy Films
Actinopterygian Homology Headache:
Newcomers to actinopterygian paleontology confront a frustrating amount of disagreement about the names of actinopterygian dermal head skeletal elements. This is because in literature published prior to the 1990s, such elements were named based on the similarity of their topographic position on the skull to the cranial elements of sarcopterygians, including tetrapods. Thus, the paired bones between the orbits were called frontals, because they occupied the same general position as the frontals of tetrapods, even though they enclosed the pineal foramen. This results in two problems:
- Many actinopterygian cranial element names suggest homologies for which there is no good evidence. In these notes, I place such names in "quotation marks."
- At least some have sarcopterygian homologs with different names. Since the 1990s, researchers like Hans-Peter Schultze have attempted to verify the homologies of actinopterygian skull bones and adjust their names accordingly (see Schultze, 1993).
Note, also, that in derived actinopterygians, the homologies of the elements shown at right can be obscured when:
- New elements such as suborbitals appear
- Elements of known homologies fuse (E.G. the dermopterotic = supratemporal + Intertemporal) or fragment.
Groups with living representatives include:
- Cladistia: Includes living Polypterus and Erpetoichthys, and fossil relatives.
- Acipenseridae: Sturgeons
- Polyodontidae: paddlefish
- Ginglymodi: Gars and fossil relatives.
- Halecomorphi: Bowfins and fossil relatives.
- Teleostei: The majority of living ray-fins, from goldfish to blue marlins.
Synapomorphies of Actinopterygii:
- CaCO3 otoliths: CaCO3 structures that fill the sacculus or utricle of the inner ear.
- Mandibular canal enclosed by dentary.
- Single dorsal fin (right).
- Anterior of dorsal and anal fins reinforced by narrow parallel radials (right).
- Caudal radials much shorter than haemal arches.
- Fenestration on anterior ramus of palatoquadrate
- Jugal sensory canal absent. (Link to Mimiapiscis.)
Although controversial, most phylogenies agree that the Middle - Late Devonian Cheirolepididae (monogeneric for Cheirolepis) is the basal branch followed by Cladistia. There follows a speciose and morphologically disparate paraphyletic assemblage, traditionally called "paleoniscids." They represent the major diversification of actinopterygians during the Late Devonian and Carboniferous, with many members surviving well into the Mesozoic. Alas we can only touch on them.
- Retains a small tuft of epichordal caudal fin rays.
- Pelvic fins still reflect their origin as elongate posteroventral fin folds.
- Extremely small scales.
From this point onward, we follow the result of Giles, et al., 2017, noting its differences with older hypothesis commonly cited in technical literature.
(Devonian - Cretaceous) A host of fossil forms are closer to living "crown Actinopterygii" than to Cheirolepis. Traditionally termed "Paleoniscoids" after Paleoniscum. Their derived features include:
- Palatal toothplates on the vomers.
- branching lepidotrichia
- Heterocercal caudal fin, completely lacking epichordal fin rays.
- Midline canal in posterior ventral braincase for the dorsal aorta. (Link to skull of Mimiapiscis in palatal view.)
- teeth capped with acrodin, a unique enameloid tissue.
- Parasphenoid with dorsal processes that interact with neurocranium.
- Propterygial canal: The propterygium - the anterior basal element of the pectoral fin houses a prominent vascular canal.
Polypterus bichir neurocranium after Allis, 1922.
- Comparatively short ethmoid region
- The maxilla extends far behind the orbit and the preopercular slopes anteriorly.
- A small number of large elements encircle the orbit.
- Heavy ganoid scales.
- Strongly heterocercal tail
Note that "paleoniscoids" also included deep-bodied forms, and forms with special feeding adaptations.
Saurichthys from Prehistoria Wikia
Body elongation: Within this group, Maxwell, et al., 2013 show a unique mode of elongation of the vertebral column. Whereas other vertebrates either:
- increase the length of vertebrae
- increase the overall number of myomeres
- a row of small post-spiracular bones
- a parasphenoid that extends the entire length of the braincase
- a dorsal fin divided into a series of finlets
- a diphycercal tail
Ecologically convergent on sarcopterygian lungfish in their ability to tolerate changing conditions in ephemeral pools and streams and their need to supplement gill-derived oxygen with air. Indeed:
- Their lungs show the ancestral condition retained by sarcopterygians - paired and communicating ventrally with the esophagus.
- Although they can remain out of water for considerable periods, they will drown if not given access to air.
- They are known to breathe air through their dorsally positioned spiracles. (Could this have been a common practice among other osteichthyans with similar spiracle configurations?)
Polypterus bichir after Allis, 1922.
- Serenoichthys kemkemensis: (Late Cretaceous) (Dutheil, 1999) Known from two headless specimens, a small, relatively stout polypteriform.
- Bawitius bartheli: (Late Cretaceous) (Grandstaff et al., 2012) Known from ectopterygoids (palatal bones). Extrapolating based on their size, the entire fish may have approached two meters in length.
Beishanichthys brevicaudalis from Xu and Gao, 2011.
(Triassic - Quaternary) The last common ancestor of all living actinopterygians excluding Polypteriformes. The crown of Actinopteri includes:
- Chondrostei: Includes living sturgeons and paddlefish, and their fossil relatives.
- Neopterygii: Gars, bowfins, teleosts, and their fossil relatives.
- Supraorbital bones present.
- Intertemporal fused with supratemporal to form dermopterotic
- Parasphenoid extends across ventral cranial fissure.
Prominent issues: Although speciose, Paleozoic actinopterygians tended to be small, and a minor component of faunas in which large placoderm-grade fish (prior to Devonian extinctions), chondrichthyans and sarcopterygians figured prominently. The cActinopteran radiation, in contrast:
- Was a Mesozoic radiation. That event seems to have released Actinopteri from ecological constraints.
- Included large fish as well as small ones.
Birgeria stensioei from Universität Zürich
- Large size.
- Reduction in body scales (which in Bergeria support the the tail.
- Chondrostei: Sturgeons, paddlefish, and their fossil relatives
- Neopterygii: Gars, bowfins, and teleosts, and their fossil relatives.
- Acipenseridae (sturgeons - Cretaceous - Quaternary) Large fish of Eurasian and North American rivers and lakes, these notable as the source of caviar. Predators taking prey from the bottom. Interesting morphological features:
- Body scales are lost but for five rows of heavy plates.
- The small jaws are hyostylic.
Polyodon spathula from Wikipedia
- Polyodontidae (paddlefish - Cretaceous - Quaternary) Large Suspension feeders of China and North American rivers and lakes. Interesting morphological features:
- A long paddle-shaped rostrum can be up to a third of the body length
- Ram suspension feeders, straining food from the water by gill-rakers projecting from the branchial arches.
Acipenser oxyrhynchus from Wikipedia
Acipenser fulvescens from Digimorph
- Opercular lost, leaving the subopercular to support the operculum
- Fewer than four branchiostegal rays
- Extensive rostrum supported by dorsal and ventral rostral bones.
- Ventral bones of the rostrum form a distinct keel.
Living chondrosteans significantly reduce their amount of endochondral ossification. Examining fossil chondrosteans, one sees that this is a derived feature.
- Chondrosteidae (Early Jurassic) Large (0.5 - 1 m) fish broadly similar to sturgeons or paddlefish but lacking specializations such as their long rostra, but showing significant loss of ossification of the skull roof that effectively disconnects the mandibular arch from the skull roof, and modification of the mandibular arch facilitating its protrusion.
Together, sturgeons, paddlefish, and chondrosteids make up Acipenseriformes. Synapomorphies include:
- Palatoquadrates meet in an anterior symphysis. (taking the place of stabilizing articulations to the neurocranium.)
- "Quadratojugal" connects maxilla to palatoquadrate posteriorly. (Allowing the maxila to move with the palatoquadrate.)
- "Preopercular" reduced to a series of ossicles housing the preopercular canal
- A tall triangular suborbital bone makes up the anterior margin of the cheek.
- Mandibular canal is short or absent
- Body scales reduced to tiny elements
This remaining actinopterygians belong to Neopterygii and comprise roughly half of vertebrate diversity. Living neopterygians include:
- Lepisosteiformes: (Jurassic - Quaternary) Gar fish and their immediate fossil relatives (Seven extant species).
- Amiidae: (Jurassic - Quaternary) Bowfins and their fossil relatives (One extant species).
- Teleostei: (Triassic - Quaternary) The vast diversity of derived actiniopterygians (~28,000 extant species).
This huge radiation seems to have resulted from signifcant morphological adaptations beginning with:
- Modification of the cranial skeleton to facilitate suction-feeding.
- Coordinated reduction of body scales and elaboration of the vertebral column
- A full-genome gene duplication event at the base of crown-group Teleostei that greatly increased the amount of genetic variation for natural selection to work with.
- The maxilla is long, expands onto the cheek behind the orbit, and is firmly attached to adjacent bones.
- The preopercular (and hyomandibula beneath it) slant posteroventrally from the neurocranium.
- Heavy ganoid scales
- Heterocercal tail
Schematic of Cheirolepis, a non-neopterygian and Amia, a neopterygian
- opens dorsally, allowing these muscles to originate from the side of the braincase as well
- expands laterally increasing the number of muscle fibers.
- The muscle mass came to insert on a coronoid process extending dorsally from the jaw.
- The jaw articulation shifted below the line of the tooth row.
- The hyomandibula and preopercular became roughly vertical, with the hyomandibula hinged to the neurocranium in such a way as to rotate laterally.
- The maxilla shortened and became detached from other bones of the skull roof, acquiring a new anterior peg-and-socket articulation with the palatoquadrate. As a result, it became able to rotate downward when the mouth opens. (It's structural role in the side of the face was taken over by an expanded series of infraorbital bones.)
- A new element, the supramaxilla (sm) was attached to it dorsally.
- A new opercular element, the interopercular (iop) served to close the gap between the opercular series and jaw.
- the tail remains heterocercal, but in most forms is superficially symmetrical
- Although the tail remains heterocercal, the caudal fin is supported above the notochord by epurals (ep. right) - specialized supraneurals of the tail.
- we begin to see a tendency toward the ossification of the notochord in the form of spool-shaped centra in some members. (Note: We have already seen the independent acquisition of centra in Polypteriformes.)
(Triassic - Quaternary) Living Lepisosteidae, gars (Cretaceous - Quaternary) and their fossil relatives. Gars are specialized ambush predators. Their skulls are highly derived in having long, drawn-out jaws. Posteriorly, they are more primitive, with heavy ganoid scales and heterocercal tails (albeit outwardly almost symmetrical.)
- There is no interopercular
- The tiny reduced maxilla has no supramaxilla
- The dermohyal is present
- The snout is elongate
- The jaw articulation is below the tooth row and well in front of the orbit
- The preopercular (?) is positioned at the ventral margin of the cheek and rotated 90 degrees.
- The cheek region is made up of a mosaic of small elements.
- The maxilla is greatly reduced.
- Notochord ossifies as a series of opistocoelous centra: I.e. centra that are convex anteriorly and concave posteriorly. This is a very unusual condition for fish vertebrae and makes even fragmentary fossil gars readily identifiable.
Isanichthys palustris from Cavin and Suteethorn, 2005
- a jaw articulation even with or anterior to the orbit
- a proliferation of infraorbital elements forming a mosaic in the cheek region.
- mobile maxilla
Semionotiformes are interesting in their own right, as one of the best documented fossil examples of species flocks - radiations of closely related species occupying adjacent ecological niches in the same general environment, and distinguishable by body shape, color, and scale pattern. A modern example is that of African rift valley lake cichlids. During the Late Triassic, similar flocks of semionotiformes occupied similar environments - the rift valley lakes of the Newark Supergroup.
Amia calva from Wikipedia
(Triassic - Quaternary) Living Amia calva and fossil Amiiformes, (Jurassic - Quaternary) and their other fossil relatives. Freshwater ambush predators. In them, the rest of the body begins to catch up, evolutionarily, with the head:
Amia calva caudal skeleton. Click for comparison with ancestral halecomorph.
- The notochord ossifies as spool-shaped amphicoelous (concave on both ends) centra. In the caudal region, two centra ossify for each myomere.
- Body scales are reduced in thickness, with the vascular layer and ganoine reduced.
symplectic, a new ossification directly linking the hyomandibula to the quadrate, the ossification of the palatoquadrate forming the jaw joint. Together, the hyomandibula, symplectic, and quadrate form the suspensorium - the primary load-bearing attachment of the jaws to the neurocranium.
Ionoscopus analibrevis from Grande and Bemis, 1998
Amiiformes (Jurassic - Quaternary) Members of Amiiformes extend back to the Jurassic and include both fresh water ambush predators similar to the living Amia calva, marine pursuit predators such as Ionoscopus (Late Jurassic - right). Note that Amia's long dorsal fin is derived within the group.
Synapomorphy of Halecomorphi:
- The jaw articulates both with the quadrate (ossification of the palatoquadrate) and the symplectic.
Neopterygian phylogeny headache:
Alas, there is no consensus on the phylogenetic pattern formed by Ginglymodi, Halecomorphi, and Teleostei (the derived actinopterygians). Two hypotheses compete:
- The Holostei hypothesis: Prior to the rise of phylogenetic systematics, non-teleosts with the derived neopterygian condition were lumped into this group of unknown monophyly. The first phylogenetic analyses found it to be paraphyletic, however during the last decade, this "ancient fish clade" has experienced a resurgence in phylogenetic analyses. Potential synapomorphies concern details of the articulation of the premaxilla, and ennervation of the snout.
- The Halecostomi hypothesis: would group Halecomorphi and Teleostei, based on characters such as the presence of the symplectic. Analyses of the 1980s and 90s that recovered this clade often lacked complete information on stem halecomorphs, ginglymodans, and teleosts, causing features like the presence of the interopercular or supramaxilla to look like a halecostome synapomophy when, in fact, they diagnosed neopterygii.
Proscinetes sp., a pycnodontiform
- Deep bodies
- Longish snouts full of teeth specialized for crushing, and front incisors for nipping
- Body scales reduced to long bars on the front half of the body.
- The maxilla is not mobile
- There is no interopercular or supramaxilla
- The caudal fin is outwardly symmetrical, as in halecomorphs and teleosts.
- A symplectic is present
What is Teleostei?: Wait for it.
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