The postorbital palatoquadrate articulation in elasmobranchs

Although modern hexanchiforms are the only extant elasmobranchs with a postorbital articulation, according to most morphological and molecular cladistic analyses they are not basal, suggesting that Huxley ( 1876 Proc Zool Soc 1876;24–59) correctly identified this articulation as “an altogether secondary connection.” A postorbital articulation is present in many Paleozoic sharks, but differs from that found in hexanchiforms in its morphology, topographic position on the braincase, and inferred ontogenetic origins. Furthermore, a postorbital articulation is absent in hybodonts (the putative extinct sister group to neoselachians). It is proposed that the term amphistylic should be restricted to the modern hexanchiform condition, where the articular facet is located on the primary postorbital process. An identical articulation probably existed in some extinct galeomorphs (e.g., ?Synechodus dubrisiensis, ?Paraorthacodus), but is not widespread within elasmobranchs generally. The term archaeostylic (“ancient pillar”) is proposed here for the suspensorial arrangement in Paleozic sharks with a postorbital articulation on the ventrolateral part of the lateral commissure. Such an articulation is not known in other gnathostomes and may represent a basal chondrichthyan synapomophy (especially if ?Pucapampella is a stem chondrichthyan), suggesting that the autodiastylic pattern is not primitive for chondrichthyans and that holocephalans have secondarily lost a postorbital articulation. The amphistylic condition may have arisen from the archaeostylic, or it could have been acquired independently within neoselachians, but in either case it is most parsimoniously viewed as apomorphic. J. Morphol., 2008. © 2008 Wiley‐Liss, Inc.     

Functional morphology of the palato‐maxillary apparatus in “Palatine dragging” snakes (Serpentes: Elapidae: Acanthophis,Oxyuranus)

Elapid snakes have previously been divided into two groups (palatine erectors and palatine draggers) based on the morphology and inferred movements of their palatine bone during prey transport (swallowing). We investigated the morphology and the functioning of the feeding apparatus of several palatine draggers (Acanthophis antarcticus, Oxyuranus scutellatus, Pseudechis australis) and compared them to published records of palatine erectors. We found that the palatine in draggers does not move as a straight extension of the pterygoid as originally proposed. The dragger palato‐pterygoid joint flexes laterally with maxillary rotation when the mouth opens and the jaw apparatus is protracted and slightly ventrally during mouth closing. In contrast, in palatine erectors, the palato‐pterygoid joint flexes ventrally during upper jaw protraction. In draggers, the anterior end of the palatine also projects rostrally during protraction, unlike the stability of the anterior end seen in erectors. Palatine draggers differ from palatine erectors in four structural features of the palatine and its relationships to surrounding elements. The function of the palato‐pterygoid bar in both draggers and erectors can be explained by a typical colubroid muscle contraction pattern, which acts on a set of core characters shared among all derived snakes. Although palatine dragging elapids share a fundamental design of the palato‐maxillary apparatus with all higher snakes, they provide yet another demonstration of minor structural modifications producing functional variants. J. Morphol. 2010. © 2009 Wiley‐Liss, Inc.     

Plasticity of ontogenetic trajectories in cyprinids: a source of evolutionary novelties

Research in evolutionary developmental (evo‐devo) biology is making an increasingly important contribution to our understanding of the molecular mechanisms underlying the establishment of complex morphological traits. Deciphering the ontogenetic trajectories leading to the differentiation of sister species (and the existence of hybrids) is a new challenge in our understanding of speciation processes. In the present study, we characterized the ontogenetic trajectory of lower lip morphology in two cyprinid species and their hybrids. Chondrostoma toxostoma has an arched lower lip and a generalist diet. Chondrostoma nasus has a straight lower lip and a specialist diet. An analysis of 99 C. toxostoma, 99 C. nasus and 25 first‐generation (F₁) hybrid individuals demonstrated that the difference between arched and straight lip morphology was found to depend strongly on the height/width ratio of the lower lip. A comparison of the ontogenetic trajectories of these morphometric traits showed that the height of the lower lip was isometric to body length in both species, whereas developmental changes involving an acceleration and a hypermorphosis of the widening of the lower lip led to a straight lip morphology in C. nasus. F₁ hybrids were characterized by an extreme phenotype resulting from a rate of lower lip widening slower than that in the two parent species. Therefore, we rejected a codominance hypothesis and concluded that the first stage of hybridization provides the foundations of evolutionary novelty. These results have important evolutionary implications because lower lip shape is linked to dietary behaviour in many fish species. © 2012 The Linnean Society of London, Biological Journal of the Linnean Society, 2012, 106 , 342–355.     

Allometry and performance: the evolution of skull form and function in felids

Allometric patterns of skull‐shape variation can have significant impacts on cranial mechanics and feeding performance, but have received little attention in previous studies. Here, we examine the impacts of allometric skull‐shape variation on feeding capabilities in the cat family (Felidae) with linear morphometrics and finite element analysis. Our results reveal that relative bite force diminishes slightly with increasing skull size, and that the skulls of the smallest species undergo the least strain during biting. However, larger felids are able to produce greater gapes for a given angle of jaw opening, and they have overall stronger skulls. The two large felids in this study achieved increased cranial strength by increasing skull bone volume relative to surface area. Allometry of skull geometry in large felids reflects a trade‐off between the need to increase gape to access larger prey while maintaining the ability to resist unpredictable loading when taking large, struggling prey.     

The structure of the skull and jaw adductor musculature in the Gekkota, with comments on the phylogenetic relationships of the Xantusiidae (Reptilia: Lacertilia)

The skull and trigeminal jaw adductor musculature of the lizard families Gekkonidae, Pygopodidae and Xantusiidae are described. The external jaw adductor shows a different structure in the Gekkonidae and Pygopodidae than is observed in other lizards, approached only by the Xantusiidae and Feyliniidae. Paedomorphosis seems to be involved in the differentiation of the jaw adductor musculature in the Gekkonidae. The Gekkonidae and Pygopodidae may be hypothesized to form a monophyletic group, the Gekkota, on the basis of numerous synapomorphies. Within the Gekkota, the Pygopodidae are the sister-group of the Gekkonidae and retain some plesiomorphous features which are absent in the latter. The Xantusiidae share few synapomorphies with the Gekkota on the one hand, and some with scincomorph lizards on the other, especially with the Lacertidae.     

The distribution and function of mucous cells and their secretions in the alimentary tract of Arrhamphus sclerolepis krefftii

The mucosa of the mouth, pharynx, oesophagus and rectum of Arrhamphus sclerolepis krefftii contain saccular mucous cells and the lining of the intestinal mucosa contains goblet mucous cells. Saccular mucous cells in the buccal epithelium are present in relatively low densities and contain acidic and neutral glycoprotein-secreting cells in an approximately 1:1 ratio. The saccular mucous cells in the mucosa of the pharynx, oesophagus and rectum are abundant and contain acidic glycoprotein which consists principally of sialomucin with traces of sulphomucin distributed around the periphery of the mucous vacuoles. Goblet cells in the intestinal mucosa contain neutral glycoprotein. Mechanically digested plant material within the lumen of the gut is bound by a sheath of acidic glycoprotein which is in contact with the intestinal mucosa. From these observations and with information on the known properties of acidic glycoproteins, a novel mechanism for the involvement of mucus in the extraction of nutrients from plant material mechanically digested by fish is proposed.     

Shaping of the lower jaw bone during growth of Nile tilapia Oreochromis niloticus and a Lake Victoria cichlid Haplochromis chilotes: a geometric morphometric approach

East African cichlids have evolved feeding apparatus morphologies to adapt to diverse feeding environments. However, little is known about how the morphologies are formed during development. Here, we assessed the shape changes of the lower jaw bone during growth of the Nile tilapia Oreochromis niloticus and a Lake Victoria cichlid Haplochromis chilotes using geometric morphometric methods. 'Early Juvenile to Late Juvenile' and 'Late Juvenile to Adult' transitions of the shape change during growth of both O. niloticus and H. chilotes were detected. The 'Early Juvenile to Late Juvenile' transition of the shape change in H. chilotes occurred slightly earlier than in O. niloticus. We also compared the shape changes during growth of the two cichlids. Principal component analysis showed both commonalities and differences in the morphological changes between the cichlids. Our data suggest that most shape change may have a similar pattern during the growth of O. niloticus and H. chilotes, and that the differences in adult shapes may be due to differences arising early in development, not to the difference of shape change during growth. These data provide a basis for understanding the developmental mechanisms underlying this adaptive trait of East African cichlids.     

Relationship between gape size and feeding selectivity of fish larvae from a Neotropical reservoir

Larvae feeding selectivity of Iheringichthys labrosus , Hypophthalmus edentatus and Plagioscion squamosissimus was assessed, examining the role of mouth gape in prey selection. Fish larvae were sampled in the Itaipu Reservoir (Brazil–Paraguay). Iheringichthys labrosus and H. edentatus larvae, with small and similar gape sizes, exhibited slightly different diets; I. labrosus preferred cladocerans ( Bosmina hagmanni , Bosmina huauriensis and Bosminopsis deitersi ) and the rotifer Brachionus calyciflorus . Hypophthalmus edentatus , however, primarily ingested the cladocerans B. hagmanni , Ceriodaphnia cornuta , Daphnia gessneri and Diaphanosoma spinulosum . Plagioscion squamosissimus , with a greater gape size, preferred Calanoida. The mechanistic processes that determine food selectivity of fish larvae in temperate aquatic systems were similar in the Neotropical system. The trophic spectrum of these species is characterized by small- to intermediate-sized prey. Plagioscion squamosissimus larvae, which have larger mouths, exploit primarily larger prey differing from the most abundant species or size classes; consequently, their diet is quite different from I. labrosus larvae and modestly similar to H. edentatus larvae, opportunistic feeders that they eat more abundant prey.     

The functional significance of inherited differences in feeding morphology in a sympatric polymorphic population of Arctic charr

Artificially fertilised eggs from wild-caught Arctic charr parents of two sympatric morphs (benthivorous and planktivorous) from Loch Rannoch, Scotland were reared in the laboratory under identical conditions. During the subsequent 2 years, aspects of their trophic anatomy and feeding behaviour were compared. As previously described for wild-caught fish, charr derived from the benthivorous morph had an increasingly wider mouth gape for a given body length than those derived from the planktivorous morph. The functional significance of these differences in gape was tested by comparing the maximum size of prey that could be handled by each of the two morphs. In both forms, a larger gape enabled larger food particles to be eaten, but the elevation of the regression of maximum prey size on gape was higher in the benthivorous form, indicating the existence of additional morphological and/or behavioural differences influencing the size of prey consumed. When offered a choice between a typical benthic prey item and a typical pelagic food item, charr of benthivorous origin were more likely to feed on the former, whereas those of planktivorous origin were more likely to feed on the latter. Thus inherited differences in gape place constraints on foraging ability and are associated with inherited differences in dietary preference. We conclude that the functional significance of the foraging specialisations indicate a strong selection pressure for the evolution of the divergence and propose that heterochronic growth is the mechanism resulting in the divergence of tropic anatomy.