The hyal and ventral branchial muscles in caecilian and salamander larvae: Homologies and evolution

Amphibians (Lissamphibia) are characterized by a bi‐phasic life‐cycle that comprises an aquatic larval stage and metamorphosis to the adult. The ancestral aquatic feeding behavior of amphibian larvae is suction feeding. The negative pressure that is needed for ingestion of prey is created by depression of the hyobranchial apparatus as a result of hyobranchial muscle action. Understanding the homologies of hyobranchial muscles in amphibian larvae is a crucial step in understanding the evolution of this important character complex. However, the literature mostly focuses on the adult musculature and terms used for hyal and ventral branchial muscles in different amphibians often do not reflect homologies across lissamphibian orders. Here we describe the hyal and ventral branchial musculature in larvae of caecilians (Gymnophiona) and salamanders (Caudata), including juveniles of two permanently aquatic salamander species. Based on previous alternative terminology schemes, we propose a terminology for the hyal and ventral branchial muscles that reflects the homologies of muscles and that is suited for studies on hyobranchial muscle evolution in amphibians. We present a discussion of the hyal and ventral branchial muscles in larvae of the most recent common ancestor of amphibians (i.e. the ground plan of Lissamphibia). Based on our terminology, the hyal and ventral branchial musculature of caecilians and salamanders comprises the following muscles: m. depressor mandibulae, m. depressor mandibulae posterior, m. hyomandibularis, m. branchiohyoideus externus, m. interhyoideus, m. interhyoideus posterior, m. subarcualis rectus I, m. subarcualis obliquus II, m. subarcualis obliquus III, m. subarcualis rectus II‐IV, and m. transversus ventralis IV. Except for the m. branchiohyoideus externus, all muscles considered herein can be assigned to the ground plan of the Lissamphibia with certainty. The m. branchiohyoideus externus is either apomorphic for the Batrachia (frogs + salamanders) or salamander larvae depending on whether or not a homologous muscle is present in frog tadpoles. J. Morphol., 2011. © 2011 Wiley‐Liss, Inc.     

Metamorphosis and evolution of feeding behaviour in salamanders of the family Plethodontidae

Plethodontid salamanders capture prey with enhanced tongue protraction relative to other salamander taxa, yet metamorphosing plethodontids are hypothesized to be constrained relative to direct-developing plethodontids in their degree of tongue evolution (protraction length and velocity) by the presence of a larval stage in development. In this biphasic life history the hyobranchial apparatus serves the conflicting functions of larval suction feeding and adult tongue protraction. The deletion of the larval stage removes one of the conflicting functions and has thus permitted direct-developing plethodontids to circumvent this constraint and evolve extremely long tongues, which in some species can be projected to 80% of body length. To evaluate this constraint hypothesis and explore taxonomic diversity of feeding behaviours, we studied feeding in larvae, adults and metamorphosing individuals of seven species of metamorphosing plethodontids from the basal taxa Desmognathinae and Hemidactyliini using direct observations, high-speed videography and kinematic analysis. We found that larval plethodontids suction feed, but feeding is suspended entirely during metamorphosis, and aquatic adults do not suction feed. Adults have exapted the terrestrial modes of tongue and jaw prehension for aquatic prey capture. These findings substantiate the premise that suction feeding and tongue protraction are conflicting functions, and thus our results support the constraint hypothesis. Plethodontid adults have evolved their extreme tongue protraction ability at the expense of adult suction feeding. The rapid metamorphosis that characterizes plethodontids may be an adaptation that minimizes the non-feeding period imposed by the evolution of derived tongue protraction in adults. © 2002 The Linnean Society of London, Zoological Journal of the Linnean Society , 2002, 134 , 375–400.     

Linkage mechanisms in the vertebrate skull: Structure and function of three‐dimensional,parallel transmission systems

Many musculoskeletal systems, including the skulls of birds, fishes, and some lizards consist of interconnected chains of mobile skeletal elements, analogous to linkage mechanisms used in engineering. Biomechanical studies have applied linkage models to a diversity of musculoskeletal systems, with previous applications primarily focusing on two‐dimensional linkage geometries, bilaterally symmetrical pairs of planar linkages, or single four‐bar linkages. Here, we present new, three‐dimensional (3D), parallel linkage models of the skulls of birds and fishes and use these models (available as free kinematic simulation software), to investigate structure–function relationships in these systems. This new computational framework provides an accessible and integrated workflow for exploring the evolution of structure and function in complex musculoskeletal systems. Linkage simulations show that kinematic transmission, although a suitable functional metric for linkages with single rotating input and output links, can give misleading results when applied to linkages with substantial translational components or multiple output links. To take into account both linear and rotational displacement we define force mechanical advantage for a linkage (analogous to lever mechanical advantage) and apply this metric to measure transmission efficiency in the bird cranial mechanism. For linkages with multiple, expanding output points we propose a new functional metric, expansion advantage, to measure expansion amplification and apply this metric to the buccal expansion mechanism in fishes. Using the bird cranial linkage model, we quantify the inaccuracies that result from simplifying a 3D geometry into two dimensions. We also show that by combining single‐chain linkages into parallel linkages, more links can be simulated while decreasing or maintaining the same number of input parameters. This generalized framework for linkage simulation and analysis can accommodate linkages of differing geometries and configurations, enabling novel interpretations of the mechanics of force transmission across a diversity of vertebrate feeding mechanisms and enhancing our understanding of musculoskeletal function and evolution. J. Morphol. 277:1570–1583, 2016. © 2016 Wiley Periodicals, Inc.     

Comments on the evolution of the jaw adductor musculature of snakes

The aim of this study is to provide a general view of the adductor musculature of the alethinophidian snakes. The aponeurotic system present in anilioid snakes is here described as being also present in colubroid and booid snakes. Although modified in various groups, this aponeurotic system retains the same topographical pattern in the anilioids, booids and colubroids, and is thus hypothesized to be homologous. An analysis of the aponeurotic system and related muscular bundles within the alethinophidian snakes is given. A new terminology is proposed for the jaw adductor muscles where the muscles levator anguli oris and adductor mandibulae externus superficialis (proper) of snakes (sensu Lakjer, 1926; Haas, 1962) retain these names even if this fails to reflect the presumed homologies with the bundles of the same name in lizards (see Rieppel, 1988b); the fibres originating from the temporal tendon in the Anilioidea, and presumed to form a bundle of composite nature (Rieppel, 1980b), are named the M. adductor mandibulae externus temporalis (lost by the Macrostomata); the M. adductor mandibulae externus medialis is a composite muscle in the Anilioidea (Rieppel, 1980b) which give rise to two different muscles in the ‘booids’, the M. adductor mandibulae externus medialis, pars anterior and the M. adductor mandibulae externus profundus, the former being secondarily lost by the Caenophidia which retains only fibres homologues of the 3b and 3c heads of the profundus layer of lizards; the so-called M. adductor mandibular externus profundus of snakes (sensu Lackjer, 1926; Haas, 1962) is also a composite muscle in the Anilioidea (Rieppel, 1980b), in the alethinophidians it is essentially made of fibres homologous with the posterior pinnate part of the medialis layer of lizards, and is here named the M. adductor mandibulae externus medialis, pars posterior. As a result from this analysis it follows that: (1) the Macrostomata are characterized by the downward extension of the fibres forming the M. adductor mandibulae externus medialis, pars anterior and the loss of the M. adductor mandibulae externus temporalis: (2) the Xenopeltidae are set apart from the remaining macrostomatan snakes by the retention of the M. levator anguli oris and of a well developed lateral sheet of the quadrate aponeurosis; (3) the ‘booids’ form a monophyletic group comprising only the Boidae and Bolyeriidae (with the exclusion of the Xenopeltidae and Tropidophiidae) which is characterized by a differentiated M. adductor mandibulae externus medialis, pars anterior inserting on the lateral surface of the compound bone via its own aponeurosis; (4) the Tropidophiidae are set apart from all other snakes by the peculiar course of their lateral head vein; however, they belong to the Caenophidia as they show a facial carotid artery which passes dorsally to the mandibular and maxillary branches of the trigeminus; (5) a possible additional character in favour of an Acrochordoidea + Colubroidea monophyletic unit may be given by the pattern of innervation of the jaw adductor muscles in these two taxa; (6) a new interpretation of the compressor glandulae muscular complex of Atractaspis resulted in a morphologically similar pattern to that of the viperids; the phylogenetic implications of such similarity are discussed in detail.     

Functional morphology of the feeding apparatus,feeding constraints,and suction performance in the nurse shark Ginglymostoma cirratum

The nurse shark, Ginglymostoma cirratum, is an obligate suction feeder that preys on benthic invertebrates and fish. Its cranial morphology exhibits a suite of structural and functional modifications that facilitate this mode of prey capture. During suction‐feeding, subambient pressure is generated by the ventral expansion of the hyoid apparatus and the floor of its buccopharyngeal cavity. As in suction‐feeding bony fishes, the nurse shark exhibits expansive, compressive, and recovery kinematic phases that produce posterior‐directed water flow through the buccopharyngeal cavity. However, there is generally neither a preparatory phase nor cranial elevation. Suction is generated by the rapid depression of the buccopharyngeal floor by the coracoarcualis, coracohyoideus, and coracobranchiales muscles. Because the hyoid arch of G. cirratum is loosely connected to the mandible, contraction of the rectus cervicis muscle group can greatly depress the floor of the buccopharyngeal cavity below the depressed mandible, resulting in large volumetric expansion. Suction pressures in the nurse shark vary greatly, but include the greatest subambient pressures reported for an aquatic‐feeding vertebrate. Maximum suction pressure does not appear to be related to shark size, but is correlated with the rate of buccopharyngeal expansion. As in suction‐feeding bony fishes, suction in the nurse shark is only effective within approximately 3 cm in front of the mouth. The foraging behavior of this shark is most likely constrained to ambushing or stalking due to the exponential decay of effective suction in front of the mouth. Prey capture may be facilitated by foraging within reef confines and close to the substrate, which can enhance the effective suction distance, or by foraging at night when it can more closely approach prey. J. Morphol., 2008. © 2008 Wiley‐Liss, Inc.     

Cranial muscles of the anurans leiopelma hochstetteri and ascaphus truei and the homologies of the mandibular adductors in lissamphibia and other gnathostomes

The frogs Ascaphus truei and Leiopelma hochstetteri are members of the most basal lineages of extant anurans. Their cranial muscles have not been previously described in full and are investigated here by dissection. Comparison of these taxa is used to review a controversy regarding the homologies of the jaw adductor muscles in Lissamphibia, to place these homologies in a wider gnathostome context, and to define features that may be useful for cladistic analysis of Anura. A new muscle is defined in Ascaphus and is designated m. levator anguli oris. The differences noted between Ascaphus and Leiopelma are in the penetration of the jaw adductor muscles by the mandibular nerve (V³). In the traditional view of this anatomy, the paths of the trigeminal nerve branches define homologous muscles. This scheme results in major differences among frogs, salamanders, and caecilians. The alternative view is that the topology of origins, insertions, and fiber directions are defining features, and the nerves penetrate the muscle mass in a variable way. The results given here support the latter view. A new model is proposed for Lissamphibia, whereby the adductor posterior (levator articularis) is a separate entity, and the rest of the adductor mass is configured around it as a folded sheet. This hypothesis is examined in other gnathostomes, including coelacanth and lungfish, and a possible sequence for the evolution of the jaw muscles is demonstrated. In this system, the main jaw adductor in teleost fish is not considered homologous with that of tetrapods. This hypothesis is consistent with available data on the domain of expression of the homeobox gene engrailed 2, which has previously not been considered indicative of homology. Terminology is discussed, and “adductor mandibulae” is preferred to “levator mandibulae” to align with usage in other gnathostomes. J. Morphol., 2011. © 2011 Wiley‐Liss, Inc.     

The plagiosaurid temnospondyl Plagiosuchus pustuliferus (Amphibia: Temnospondyli) from the Middle Triassic of Germany: anatomy and functional morphology of the skull

The cranial anatomy of the plagiosaurid temnospondyl Plagiosuchus pustuliferus, from the Middle Triassic of Germany, is described in detail on the basis of a newly discovered skull and mandibular material. The highly derived skull is characterized by huge orbitotemporal fenestrae, a reduction of the circumorbital bones – the prefrontal, postfrontal and (probably) postorbital are lost – and the expansion of the jugal to occupy most of the lateral skull margin. Ventrally the extremely long subtemporal vacuities correlate with the elongate adductor fossa of the mandible. The dentition is feebly developed on both skull and mandible. Ossified ?ceratobranchials and ‘branchial denticles’ indicate the presence of open gills clefts in life. The remarkably divergent cranial morphology of P. pustuliferus highlights the extraordinary cranial diversity within the Plagiosauridae, probably unsurpassed within the Temnospondyli. Specific structural aspects of the skull – including an extremely short marginal tooth row, feeble dentition and an elongated chamber for adductor musculature – together with evidence for a hyobranchial skeleton, suggests that P. pustuliferus utilized directed suction feeding for prey capture. © 2009 The Linnean Society of London, Zoological Journal of the Linnean Society, 2009, 155 , 348–373.     

Functional morphology of extreme jaw protrusion in Neotropical cichlids

The New World cichlids Petenia splendida and Caquetaia spp. possess extraordinarily protrusible jaws. We investigated the feeding behavior of extreme (here defined as greater than 30% head length) and modest jaw-protruding Neotropical cichlids by comparing feeding kinematics, cranial morphology, and feeding performance. Digital high-speed video (500 fps) of P. splendida, C. spectabile, and Astronotus ocellatus feeding on live guppy prey was analyzed to generate kinematic and performance variables. All three cichlid taxa utilized cranial elevation, lower jaw depression, and rotation of the suspensorium to protrude the jaws during feeding experiments. Extreme anterior jaw protrusion in P. splendida and C. spectabile resulted from augmented lower jaw depression and anterior rotation of the suspensorium. Morphological comparisons among eight cichlid species revealed novel anterior and posterior points of flexion within the suspensorium of P. splendida and Caquetaia spp. The combination of anterior and posterior loosening within the suspensorium in P. splendida and Caquetaia spp. permitted considerable anterior rotation of the suspensorium and contributed to protrusion of the jaws. Petenia splendida and C. spectabile exhibited greater ram distance and higher ram velocities than did A. ocellatus, resulting primarily from increased jaw protrusion. Petenia splendida and C. spectabile exhibited lower suction feeding performance than A. ocellatus, as indicated by lower suction-induced prey movements and velocities. Thus, extreme jaw protrusion in these cichlids may represent an adaptation for capturing elusive prey by enhancing the ram velocity of the predator but does not enhance suction feeding performance.     

Getting a grip on tetrapod grasping: form,function, and evolution

Human beings have been credited with unparalleled capabilities for digital prehension grasping. However, grasping behaviour is widespread among tetrapods. The propensity to grasp, and the anatomical characteristics that underlie it, appear in all of the major groups of tetrapods with the possible exception of terrestrial turtles. Although some features are synapomorphic to the tetrapod clade, such as well‐defined digits and digital musculature, other features, such as opposable digits and tendon configurations, appear to have evolved independently in many lineages. Here we examine the incidence, functional morphology, and evolution of grasping across four major tetrapod clades. Our review suggests that the ability to grasp with the manus and pes is considerably more widespread, and ecologically and evolutionarily important, than previously thought. The morphological bases and ecological factors that govern grasping abilities may differ among tetrapods, yet the selective forces shaping them are likely similar. We suggest that further investigation into grasping form and function within and among these clades may expose a greater role for grasping ability in the evolutionary success of many tetrapod lineages.     

Hyobranchial skeleton and hypobranchial muscles of rhipidistians

The hyobranchial skeleton of the porolepiform rhipidistian Laccognathus panderi Gross is described. The double composition of the ceratohyal in crossopterygians is proposed. The urohyal of porolepiforms, like that of Latimeria, consists of cartilaginous axial and membranous peripheral portions. The differences between porolepiforms and osteolepiforms in the structure of the hyobranchial skeleton, particularly, in the shape of the urohyal are attributable to different arrangements of the hypobranchial muscles. Porolepiforms and coelacanths have retained the coracomandibularis muscle inherited from early gnathostomes, whereas the same muscle of osteolepiforms was transformed into the geniohyoideus muscle. This transformation is accounted for by functional changes in the hyobranchial apparatus.     

Biomechanical trade-offs bias rates of evolution in the feeding apparatus of fishes

Morphological diversification does not proceed evenly across the organism. Some body parts tend to evolve at higher rates than others, and these rate biases are often attributed to sexual and natural selection or to genetic constraints. We hypothesized that variation in the rates of morphological evolution among body parts could also be related to the performance consequences of the functional systems that make up the body. Specifically, we tested the widely held expectation that the rate of evolution for a trait is negatively correlated with the strength of biomechanical trade-offs to which it is exposed. We quantified the magnitude of trade-offs acting on the morphological components of three feeding-related functional systems in four radiations of teleost fishes. After accounting for differences in the rates of morphological evolution between radiations, we found that traits that contribute more to performance trade-offs tend to evolve more rapidly, contrary to the prediction. While ecological and genetic factors are known to have strong effects on rates of phenotypic evolution, this study highlights the role of the biomechanical architecture of functional systems in biasing the rates and direction of trait evolution.     

Cranial shape in fruit,nectar, and exudate feeders: Implications for interpreting the fossil record

At least 29 species of fossil primates have been referred to fruit, nectar, and/or exudate feeding dietary niches. Many studies have detailed the morphological correlates of fruit feeding in comparison to insectivory and folivory. In contrast, few studies have sought to differentiate the morphological correlates of fruit feeding from those of nectar and exudate feeding. This study investigates the differences between fruit, nectar, and exudate feeders using 22 cranial and dentary shape variables representing 28 species of living marsupials, bats, and primates. Discriminant function analysis is used to investigate the differences between these dietary categories using both the complete data set and a reduced data set composed of variables that might reasonably be available from fragmentary fossil material. The success rates of post-hoc classifications are 94 and 88%, respectively. These results demonstrate that it is possible to discriminate among fruit, nectar, and exudate feeders among fossil taxa with a reasonable degree of certainty using the data and techniques outlined here. Nectar feeders exhibit a unique combination of features that are associated with reduced masticatory strength and their role as pollination agents. Exudate feeder skulls and dentaries exhibit a combination of features that reflect the high stresses encountered by the anterior dentition through bark gouging behavior. Fruit feeders are morphologically diverse, exhibiting cranial and mandibular shape values that overlap with both nectar and exudate feeders. It is suggested that this diversity reflects the variety of physical properties represented among fruits, and the tendency for individual frugivore species to specialize on particular fruits. Am J Phys Anthropol 102:187–202, 1997 © 1997 Wiley-Liss, Inc.     

Dry versus wet and gross: Comparisons between the dry skull method and gross dissection in estimations of jaw muscle cross-sectional area and bite forces in sea otters

Bite force is a measure of feeding performance used to elucidate links between animal morphology, ecology, and fitness. Obtaining live individuals for in vivo bite-force measurements or freshly deceased specimens for bite force modeling is challenging for many species. Thomason's dry skull method for mammals relies solely on osteological specimens and, therefore, presents an advantageous approach that enables researchers to estimate and compare bite forces across extant and even extinct species. However, how accurately the dry skull method estimates physiological cross-sectional area (PCSA) of the jaw adductor muscles and theoretical bite force has rarely been tested. Here, we use an ontogenetic series of southern sea otters (Enhydra lutris nereis) to test the hypothesis that skeletomuscular traits estimated from the dry skull method accurately predicts test traits derived from dissection-based biomechanical modeling. Although variables from these two methods exhibited strong positive relationships across ontogeny, we found that the dry skull method overestimates PCSA of the masseter and underestimates PCSA of the temporalis. Jaw adductor in-levers for both jaw muscles and overall bite force are overestimated. Surprisingly, we reveal that sexual dimorphism in craniomandibular shape affects temporalis PCSA estimations; the dry skull method predicted female temporalis PCSA well but underestimates male temporalis PCSA across ontogeny. These results highlight the importance of accounting for sexual dimorphism and other intraspecific variation when using the dry skull method. Together, we found the dry skull method provides an underestimation of bite force over ontogeny and that the underlying anatomical components driving bite force may be misrepresented.     

Molecular and cellular changes associated with the evolution of novel jaw muscles in parrots

Vertebrates have achieved great evolutionary success due in large part to the anatomical diversification of their jaw complex, which allows them to inhabit almost every ecological niche. While many studies have focused on mechanisms that pattern the jaw skeleton, much remains to be understood about the origins of novelty and diversity in the closely associated musculature. To address this issue, we focused on parrots, which have acquired two anatomically unique jaw muscles: the ethmomandibular and the pseudomasseter. In parrot embryos, we observe distinct and highly derived expression patterns for Scx, Bmp4, Tgfβ2 and Six2 in neural crest-derived mesenchyme destined to form jaw muscle connective tissues. Furthermore, immunohistochemical analysis reveals that cell proliferation is more active in the cells within the jaw muscle than in surrounding connective tissue cells. This biased and differentially regulated mode of cell proliferation in cranial musculoskeletal tissues may allow these unusual jaw muscles to extend towards their new attachment sites. We conclude that the alteration of neural crest-derived connective tissue distribution during development may underlie the spatial changes in jaw musculoskeletal architecture found only in parrots. Thus, parrots provide valuable insights into molecular and cellular mechanisms that may generate evolutionary novelties with functionally adaptive significance.     

Restoration of the superficial facial musculature in nimravids

The size, shape and position of the mouth and sensory features such as the nose, eyes and ears in the nimravid barbourofelins are reconstructed. The earliest barbourofelin studied, Sansanosmilus , is the most similar to both pantherine and nonpantherine felids in sensory structure morphology. Through time, the species of the barbourofelin lineage diverge from felids with regard to facial features. Evolutionary trends include expansion of the nasal region, a more lateral orientation of the eyes and lowered position for the ears. Increased width and shortening of the muzzle accommodates the large oral opening required by the large gape of Barboumfelis , culminating in B. fricki with eyes oriented laterally enough to reduce stereoscopic vision. The ear position in this species is also significantly lower than in the earlier barbourofelins or felids. This may reflect an anatomic compromise between sensory structure position and changes in relative size, shape and position of other skull features. These changes correlate with increased canine tooth length and increased gape, exemplified by Barboumfelis fricki. Cranial soft tissue structures are restored through scars of attachment on the fossil bones, and the proportions of these features to one another determined. New observations provide criteria for reconstructing soft tissue anatomy in extinct animals.     

Odontocete suction feeding: Experimental analysis of water flow and head shape

The role of cranial morphology in the generation of intraoral and oropharyngeal suction pressures in odontocetes was investigated by manipulating the jaw and hyolingual apparatus of submerged heads of three species presenting varied shapes. Hyoid and gular muscles were manually employed to depress and retract the tongue. Pressures were recorded at three locations in the oral cavity, as gape and site, speed, and force of pull were varied. A biomechanical model was also developed to evaluate pressure data. The species with the shortest, bluntest head and smallest mouth opening generated greater negative pressures. Suction generation diminished sharply as gape increased. Greatest negative pressures attained were around -45 mmHg (-6,000 Pa), a magnitude deemed suitable for capture of small live prey. Odontocetes utilizing this bidirectional flow system should profit by evolution of a rounder mouth opening through progressive shortening and widening of the rostrum and jaws, a trend evident in cranial measurements from fossil and recent odontocetes. Blunt heads correlate with anatomical, ecological, and behavioral traits associated with suction feeding. Small-gape suction (with minimally opened jaws) could be used by odontocetes of all head and oral shapes to draw prey sufficiently close to the mouth for suction ingestion or grasping via dentition. Principal limitations of the experimental and mathematical simulations include assumption of a stationary odontocete with static (open or closed) jaws and potential scaling issues with differently sized heads and gapes.     

Loss of traditional mucous glands and presence of a novel mucus-producing granular gland in the plethodontid salamander Ensatina eschscholtzii

All amphibians are thought to possess two distinct types of dermal gland: mucous and granular. Mucous glands typically produce an acidic, flocculent and carbohydrate positive secretion that keeps skin moist and helps maintain homeostasis. In this study, we use a battery of histochemical tests to identify these traditional mucous glands in the dermis of the plethodontid salamander Ensatina eschscholtzii . The periodic acid-Schiff, alcian blue, toluidine blue and carmine reactions reveal the absence of both acidic glycosaminoglycans and traditional mucous glands. Instead, E. eschscholtzii produces neutral mucus in specialized granular glands, which we name mucus-producing granular glands (MPGGs). The loss of traditional mucous glands is a derived trait for Ensatina .  © 2006 The Linnean Society of London, Biological Journal of the Linnean Society , 2006, 87 , 469–477.     

Functional design and evolution of the pharyngeal jaw apparatus in euteleostean fishes

Functional and structural patterns in the pharyngeal jaw apparatus of euteleostean fishes are described and analysed as a case study of the transformation of a complex biological design. The sequential acquisition of structural novelties in the pharyngeal apparatus is considered in relation to both current hypotheses of euteleostean phylogeny and patterns of pharyngeal jaw function. Several euteleostean clades are corroborated as being monophyletic, and morphologically conservative features of the pharyngeal jaw apparatus are recognized. Functional analysis, using cinematography and electromyography, reveals four distinct patterns of muscle activity during feeding in primitive euteleosts (Esox) and in derived euteleostean fishes(Perca, Micropterus, Ambloplites, Pomoxis). The initial strike, buccal manipulation, pharyngeal manipulation, and the pharyngeal transport of prey into the oesophagus all involve unique muscle activity patterns that must be distinguished in analyses of pharyngeal jaw function. During pharyngeal transport, the upper and lower pharyngeal jaws are simultaneously protracted and retracted by the action of dorsal and ventral musculoskeletal gill arch couplings. The levator externus four and retractor dorsalis muscles, anatomical antagonists, overlap for 70–90°of their activity period. Levatores externi one and two are the main protractors of the upper pharyngeal jaws in the acanthopterygian fishes studied. The major features of pharyngeal jaw movement in primitive euteleosts are retained in many derived clades in spite of a dramatic structural reorganization of the pharyngeal region. Homologous muscles have radically changed their relative activity periods while pharyngeal jaw kinematics have been modified relatively little. Patterns of transformation of activity may thus bear little direct relationship to the sequence of structural modification in the evolution of complex designs. Overall function of a structural system may be maintained, however, through co-ordinated modifications of the timing of muscle activity and anatomical reorientation of the musculoskeletal system. Deeper understanding of the principles underlying the origin and transformation of functional design in vertebrates awaits further information on the acquisition of both structural and functional novelties at successive hierarchical levels within monophyietic clades. This is suggested as a key goal of future research in functional and evolutionary morphology.