The biology and systematics of a new species of Xenopus (Anura: Pipidae) from the highlands of Central Africa

A new species of clawed toad, Xenopus wittei sp. nov. is described from highland areas overlapping the borders of Rwanda, Uganda and Zaire. A comprehensive morphological analysis of the species has been carried out. X. wittei sp. nov. is very closely related to X. vestitus and both have a tetraploid chromosome complement (2 n = 72), the only examples within the genus. Nevertheless, the two species may be distinguished by easily observed features, including head morphology and dorsal body colours and patterns, and are also identified by their karyotypes and mating calls.
Data on distribution, ecology and parasite infection are derived from both museum collections and fieldwork in Central Africa. X. wittei sp. nov. is sympatric in parts of its range with X. vestitus, X. laevis victorianus and X. I. bunyoniensis , with up to three of the taxa sharing the same habitats. The implications of potential species interactions are discussed. Experimental hybridization has confirmed the reproductive isolation of the two tetraploid species. Parasitological studies show that they may be distinguished from one another by host specific species of the nematode Chitwoodchabaudia and from other Xenopus species by the absence of the tapeworm genus Cephalochlamys . Large scale population movements of the Xenopus species have apparently occurred in the Central African highlands during the period of available records. Sympatry observed amongst the present day representatives of Xenopus may have been an important factor in the evolution of these toads since X. wittei sp. nov. and X. vestitus are interpreted as allopolyploids, the products of interspecies hybridization.     

The karyotype of the tetraploid species Xenopus vestitus Laurent (Anura: pipidae)

Xenopus vestitus possesses 72 chromosomes, wherease in the majority of known Xenopus species there are 2n = 36. During meiosis, 36 bivalents are usually observed at metaphase I and 36 chromosomes at metaphase II. Arranged according to size and centromere position, the chromosomes form the same basic morphologic groups typical of the genus Xenopus. However, the groups are composed of quartets of four similar chromosomes instead of diploid pairs of homologs. The exception to this arrangement involves chromosomes bearing secondary constrictions, which in X. vestitus are represented by two different pairs of homologs, one of which shows, in 39% of the observed mitoses, somatic association and is, therefore, considered to carry the nucleolar organizer. X. vestitus represents either a case of ancient autotetraploidy or, more likely, one of allotetraploidy of more modern origin.     

Skeletal development in Xenopus laevis (Anura: Pipidae).

Postembryonic skeletal development of the pipid frog Xenopus laevis is described from cleared-and-stained whole-mount specimens and sectioned material representing Nieuwkoop and Faber developmental Stages 46-65, plus postmetamorphic individuals up to 6 months old. An assessment of variation of skeletogenesis within a single population of larvae and comparison with earlier studies revealed that the timing, but not the sequence, of skeletal development in X. laevis is more variable than previously reported and poorly correlated with the development of external morphology. Examination of chondrocranial development indicates that the rostral cartilages of X. laevis are homologous with the suprarostral cartilages of non-pipoid anurans, and suggests that the peculiar chondrocranium of this taxon is derived from a more generalized pattern typical of non-pipoid frogs. Derived features of skeletal development not previously reported for X. laevis include 1) bipartite formation of the palatoquadrate; 2) precocious formation of the adult mandible; 3) origin of the angulosplenial from two centers of ossification; 4) complete erosion of the orbital cartilage during the later stages of metamorphosis; 5) development of the sphenethmoid as a membrane, rather than an endochondral bone; and 6) a pattern of timing of ossification that more closely coincides with that of the pelobatid frog Spea than that recorded for neobatrachian species.     

Evolution of globin expression in the genus Xenopus (Anura: Pipidae)

Comparison of electrophoretic globin phenotypes of 18 different Xenopustaxa reveals four different basic types of banding patterns. Each typeincludes species that also are similar in their morphological,cytogenetical, and biochemical characteristics. Three of these patternsreflect distinct evolutionary lines, while the fourth may be interpreted asthe intersection of two of these lines. The composition of the basicpattern of the highly polyploid species is consistent with an allopolyploidorigin of most of these species. The number of distinct globinpolypeptides--four in the only extant diploid species, X. tropicalis, andfive or more in most of the tetraploid species, including X.laevis--suggests that primordial globin genes had undergone duplicationeither before or after the tetraploidization event. Finally, the individualglobin phenotypes are excellent molecular markers that are of great help inidentifying the various species but not the subspecies.     

The taxonomic status,host range and geographical distribution of Dollfuschella Vercammen-Grandjean, 1960 (Digenea: Halipeginae) from Xenopus spp. (Anura: Pipidae)

The taxonomy, host range and geographical distribution of halipegine digeneans from Xenopus spp. are reviewed. Dollfuschella Vercammen-Grandjean, 1960 is reinstated from synonymy with Halipegus Looss, 1899 because of the presence of a sinus-sac and weakly developed permanent sinus-organ. Only one representative of the genus, D. rodhaini Vercammen-Grandjean, 1960, is recognised, of which H. rhodesiensis Beverley-Burton, 1963 is considered a synonym. Based on previous literature records this parasite occurs in Xenopus laevis laevis from South Africa and Zimbabwe, X. l. victorianus from Zaire and Uganda, X. l. bunyoniensis from Rwanda, X. wittei from Zaire and Uganda, X. vestitus from Uganda and X. muelleri from Zaire (in some cases host identification has been revised). New host and/or locality records are from X. l. poweri in Zambia, X. l. victorianus in Rwanda, X. l. bunyoniensis in Uganda and X. clivii in Ethiopia. All known hosts belong to a clade characterised by multiples of 2n = 36 chromosomes. There are no records of halipegines from the other major lineage within Xenopus, X. tropicalis-like species with multiples of 2n = 20 chromosomes. This latter group occurs in lowland tropical rain forest from west Africa in contrast to the hosts of D. rodhaini which are found typically in grassland and wooded savanna and in montane forest biotypes. The distribution of D. rodhaini might, therefore, be limited by phylogenetic specificity to the definitive host group or by other ecological factors (e.g., availability of suitable molluscan hosts). Its wide geographical and host range, in common with some other parasite species from the 2n = 36 Xenopus lineage, may result from the lack of ecological or geographical barriers between different definitive host species and subspecies. However, significant geographical variation in egg-size occurs between northern D. rodhaini populations (north of about 15° S) and those from X. l. laevis in southern Africa. This is not considered sufficient for taxonomic recognition but it could reflect the operation of some isolating factor: parasite divergence concurs with evidence that X. l. laevis is evolutionarily relatively distant from the other (more northerly) members of the laevis Rassenkreis.     

Morphology of skin incubation in Pipa carvalhoi (Anura: Pipidae)

South American Pipidae show a unique reproductive mode, in which the fertilized eggs develop in temporarily formed brood chambers of the dorsal skin after eggs have been deposited on the back of the female. We studied the skin incubation of Pipa carvalhoi using light microscopy and scanning electron microscopy. The skin consists of a stratified epithelium with a one‐layered stratum corneum, and the dermis. The dermis of the dorsal skin of nonreproductive and reproductive females lacks a distinct stratum compactum, which is typical for most anuran skins. The entire dermis shows irregularly arranged collagen bundles like a stratum spongiosum. Before egg laying, the skin swells, primarily by thickening and further by loosening of the middle zone of the dermis. In the epidermis, large furrows develop that are the prospective sites of egg nidation. The epidermis, which forms a brood chamber around the developing egg becomes bi‐layered and very thin and lacks a stratum corneum. Further, the dermis loosens and becomes heavily vascularized. Egg carrying females do not have mature oocytes in their ovaries indicating a slow down or interruption of egg maturation during this period. Similarities with the brood pouch of marsupial frogs are discussed. J. Morphol., 2009. © 2009 Wiley‐Liss, Inc.     

Patterns of spatial and temporal cranial muscle development in the African clawed frog, Xenopus laevis (Anura: Pipidae)

The African Clawed Toad, Xenopus laevis, has been a major vertebrate model organism for developmental studies for half a century. Because most studies have focused on the early stages of development, this has had the effect that many aspects of organogenesis and later development remain relatively poorly known in this species. In particular, little is known about cranial muscle development even at the level of morphology and histological differentiation of muscle anlagen and muscle fibers. In this study, we document the morphogenesis and histological differentiation of cranial muscles in X. laevis. We provide a detailed account of the timing of development for each of the cranial muscles, and also describe a new muscle, the m. transversus anterior. The cranial musculature of X. laevis larvae generally develops in a rostrocaudal sequence. The first muscles to differentiate are the extrinsic eye muscles. Muscles of the mandibular and hyoid arches develop almost simultaneously, and are followed by the muscles of the branchial arches and the larynx, and by the mm. geniohyoideus and rectus cervicis. Despite the fact that differentiation starts at different stages in the different muscles, most are fully developed at Stage 14. These baseline data on the timing of muscle differentiation in the X. laevis can serve as a foundation for comparative studies of heterochronic changes in cranial muscle development in frogs and other lissamphibians.     

Paramphistome digeneans from shape Xenopus species (Pipidae) in Africa: taxonomy,host-specificity and biogeography

The taxonomy, host range and geographical distribution of paramphistome digeneans from Xenopus spp. in sub-Saharan Africa are reviewed. Two representatives of Progonimodiscus Vercammen-Grandjean, 1960 are recognised, both of which are narrowly or primarily specific to Xenopus. An analysis of morphometric and meristic characters indicated geographical variation in Progonimodiscus doyeri (Ortlepp, 1926), with two allopatric forms showing significant, but continuous, variation in testis size and vitelline follicle number. P. colubrifer n. sp. is distinguished from P. doyeri by the form of muscular elevations on the acetabulum accessory peduncle. It infects Xenopus ( Silurana) tropicalis-like toads from lowland tropical rain forest zones in Nigeria, Togo and the Ivory Coast, while P. doyeri occurs in hosts of the subgenus Xenopus from a wide variety of biotypes. Previous literature records indicate the presence of the southern P. doyeri morphological variant in X. laevis laevis in South Africa and Zimbabwe and the northern variant in X. l. victorianus, X. fraseri aff. and X. muelleri in the Democratic Republic of Congo, X. wittei in Uganda and the Democratic Republic of Congo, and the ranid Conraua crassipes in Cameroon (the only record of Progonimodiscus from a non-pipid host). New host and/or geographical records for this species are of the northern form in X. l. victorianus, X. l. bunyoniensis and X. vestitus in Uganda, X. l. sudanensis in Cameroon, X. borealis in Kenya, X. pygmaeus in the Democratic Republic of Congo, X. fraseri aff. in Cameroon, X. wittei aff. in the Democratic Republic of Congo and X. muelleri in Cameroon and Nigeria. While the geographical limits of the two P. doyeri variants are not known with precision, existing data are consistent with a turnover in the region of 15°S, where a notable discontinuity occurs in the distributions of other Xenopus parasites. Species of Diplodiscus Diesing, 1836 in Xenopus hosts are rare. Diplodiscus peregrinator n. sp. was recovered from X. tropicalis at a single locality in the Ivory Coast and distinguished by a combination of body size, egg size, genital pore position and acetabulum morphology. D. fischthalicus Meskal, 1970 was not found during the present study.     

Representatives of Batrachocamallanus n. g. (Nematoda: Procamallaninae) from Xenopus spp. (Anura: Pipidae): geographical distribution,host range and evolutionary relationships

The great majority of the Procamallaninae occur in teleosts from tropical regions; however, representatives of this group are also frequent parasites of aquatic clawed toads (Xenopus spp.) in Africa. The taxonomic status of procamallanines from different Xenopus spp. and their geographical distribution is reviewed. Batrachocamallanus n. g. is created to include forms from amphibians with large numbers of mucrons on the female tail and relatively small body size. B. occidentalis n. sp. and B. siluranae n. sp. are described, while Procamallanus brevis Kung, 1948, originally recorded from an unidentified African amphibian, is considered a synonym of B. slomei (Southwell & Kirshner, 1937) n. comb. Due to the presence of spiral thickenings on its buccal capsule, B. xenopodis (Baylis, 1929) n. comb. has previously been placed in the genus Spirocamallanus Olsen, 1952. However, this species shares the apomorphic presence of numerous mucrons on the female tail, and almost identical cephalic morphology, male caudal structures and female reproductive system with other procamallanines from clawed toads. This suggests that they represent a monophyletic grouping. There is also only limited morphometric differentiation between B. xenopodis and the other proposed representatives of Batrachocamallanus (supported by a multivariate analysis of male and female specimens), which further indicates a close relationship between them. Great variability in the presence and type of buccal capsule thickenings occurs within Batrachocamallanus. Members of this genus most closely resemble the African species Procamallanus laeviconchus (Wedl, 1862), which exhibits a smooth buccal capsule similar to that of B. siluranae. Buccal capsule thickenings of the remaining Batrachocamallanus spp. probably arose independently from those described in other procamallanines. Such characters may be evolutionarily unstable and an unsuitable basis for generic classification in this subfamily. Although B. siluranae is the only Batrachocamallanus species to occur in X. tropicalis-like hosts (which represent a separate lineage from other clawed toads), its distribution, and that of its congeners, may be determined more by host-independent ecological or biogeographical factors than by an association with host phylogeny. Thus, B siluranae occurs in Xenopus spp. from tropical rain forest (including those from other host lineages) while the other forms are typically found in savanna or montane forest, and in the cases of B. slomei and B. xenopodis at least, do not show narrow host specificity to particular clawed toad taxa. Although more than one Batrachocamallanus spp. were found in X. laevis, X. muelleri and X. fraseri-like clawed toads, co-existence at the same locality never occurred, perhaps indicating a high degree of interspecific ecological segregation.     

Anomalous rates of evolution of pancreatic polypeptide and peptide tyrosine-tyrosine (PYY) in a tetraploid frog, Xenopus laevis (Anura:Pipidae)

The South African clawed frog Xenopus laevis is believed to have arisen as a result of a tetraploidization event occurring approximately 30 million years ago. Two molecular forms of pancreatic polypeptide (PP) have been isolated from an extract of the pancreas of this species and two molecular forms of peptide tyrosine-tyrosine (PYY) from the intestine. Despite the fact that the amino acid sequence of PP has, in general, been very poorly conserved during the evolution of tetrapods (only Pro(5), Pro(8), Gly(9), Ala(12), Tyr(27), Arg(33) and Arg(35) are invariant among species studied so far), the two Xenopus PPs differ by only a single amino acid substition (Asp(22)-->Glu). In contrast the two molecular forms of PYY differ by six amino acid substitutions (Glu(15)-->Gln, Thr(18)-->Ala, Leu(21)-->Met, Ile(22)-->Thr, Ile(28)-->Val, Val(31)-->Ile). The data imply that strong evolutionary pressure is acting to conserve the functional domain in both genes encoding PP and so suggest that PP may have an important physiological role in amphibians (although the nature of this role has yet to be determined). The more rapid mutation of the functional domain in the genes encoding PYY, a peptide whose amino acid sequence has been quite well conserved in tetrapods and whose physiological significance is well established, suggests that one of the PYY genes may be evolving towards a new function or towards becoming a pseudogene.     

Glomerular development and growth of the renal blood vascular system in Xenopus laevis (Amphibia: Anura: Pipidae) during metamorphic climax.

Microcorrosion casts of the renal vascular system of tadpoles of the Clawed Frog, Xenopus laevis, were observed by scanning electron microscopy. Glomerular differentiation was studied qualitatively and quantitatively during developmental stages 56-66 (metamorphic climax). The general structure of the renal vascular system corresponds to the pattern commonly found in anurans; however, the arterial supply has conspicuous connecting vessels that supply groups of glomeruli. In the dorsal part of the kidney, qualitative differentiation of glomerular structures precedes quantitative growth. The ventral part of the kidney has larger, well-developed renal corpuscles of nearly adult appearance. Four developmental stages of glomerulogenesis are distinguished morphologically and their glomerular and vascular growth is analyzed.     

Evolutionary relationships,host range and geographical distribution of Camallanus Railliet & Henry, 1915 species (Nematoda: Camallaninae) from clawed toads of the genus Xenopus (Anura: Pipidae)

Representatives of the genus Camallanus Railliet & Henry, 1915 occur mainly in teleost fishes, although a significant number of species have also been recorded from anuran amphibians. The taxonomy, host range, geographical distribution and phylogenetic relationships of Camallanus spp. from African clawed toads (Xenopus spp.) are reviewed. Besides C. kaapstaadi Southwell & Kirshner, 1937, which shows a widespread distribution in sub-Saharan Africa and occurs in X. laevis subspecies, X. wittei, X. fraseri-like toads, X. borealis and X. muelleri, three new species were found: C. siluranae n. sp. from X. tropicalis in west Africa, C. macrocephalus n. sp. from X. borealis in Kenya, and C. xenopodis n. sp. from X. laevis laevis in South Africa and X. borealis in Kenya. C. johni Yeh, 1960 described from Xenopus sp. in Tanzania is considered a species inquirenda. C. kaapstaadi and C. macrocephalus are very closely related and both occur in the oesophagus of their hosts, unlike other Camallanus spp. which are found in the intestine or more rarely the stomach. Some of the unusual morphological features of these species may be an adaptation to attachment in the oesophagus. The host of C. siluranae, X. tropicalis, belongs to a separate species group (as has been established by recent molecular and cytological studies) to those of C. kaapstaadi, C. macrocephalus and C. xenopodis. Morphological affinities suggest that Camallanus spp. from clawed toads are not monophyletic with those from other amphibians and that C. siluranae is distantly related to, and probably not monophyletic with the remaining species from clawed toads. The Camallanus fauna of Xenopus spp. may thus be derived from at least two independent colonisations, of different host clades, by parasite lineages occurring in teleost fishes.     

Histological microstructure of the claws of the African clawed frog, Xenopus laevis (Anura: Pipidae): implications for the evolution of claws in tetrapods

Claws are consistent components of amniote anatomy and may thus be implicated in the success of the amniote invasion of land. However, the evolutionary origin of these structures in tetrapods is unclear. Claws are present in certain extant non-amniotes, such as Xenopus laevis, the African clawed frog. The histology of the soft tissue component of the claws of X. laevis is described and compared with the amniote condition in order to gain new information on the question of homology of claws in these two groups based on patterns of keratinization.The X. laevis claw sheath is composed of a localized thickening of the corneous region of the epidermis that envelops the terminal phalanx. Noted differences between the non-cornified layers of the epidermis of the claw and non-claw region are the overall grainier appearance of the cells and an increased abundance of desmosomes in the intermediate spinosus cells. The biochemical identity of the sheath keratin(s) is inferred to be different from that of non-claw region epidermis, based on histological differences and differences in stain affinity between the two regions. The microstructure of the frog claw differs from that of amniotes in several respects, including the lack of a specified zone of growth near the base of the claw. Amphibians and amniotes, therefore, have very different patterns of claw sheath growth. Observations do not support homology of claws on a structural level in these two groups; however, further experimental work may confirm a conserved pattern of cornification in these structures in tetrapods.     

Tooth replacement in adult Xenopus laevis (Amphibia: Anura)

To determine the time scale of tooth replacement in adult Xenopus laevis (Daudin), three large females of similar size were kept in aquaria at 25 °C for ten weeks. They were anaesthetized twice weekly with MS 222 and impressions of their upper jaws were taken using thin sheets of dental gold-casting wax. Because the erupted tips of the teeth were small (100 μm), the impressions were enlarged by projection so that the presence or absence of a tooth at each locus in the jaw could be recorded. Each half of each animal's jaw was analysed separately and a statistical analysis of the records yielded results for the duration of the Replacement Cycle and Functional Life of the teeth. The range of the median Replacement Cycle time between specimens was 910–1,010h, that of the Functional Life 580–700 h and that of the Gap Period (the time over which loci were unoccupied by functional teeth) 230–420 h. A tentative time scale for the complete tooth development cycle (from tooth germ initiation to complete resorption) was calculated by extrapolation from the results and ranged from 59.07 to 71.29 days.     

Evolution of pipoid frogs:intergeneric relationships of the aquatic frog family Pipidae (Anura)

The 27 species of the aquatic frog family Pipidae are currently arranged in four genera: Xenopus (15 species), Hymenochirus (four species), and the poorly known genus Pseudhymenochirus (one species) occur in Africa; Pipa (seven species) is found in South America and lower Central America. Despite extensive work on the biology of Xenopus from various disciplines, the evolutionary relationships of Xenopus to other pipids have not been resolved. Phylogenetic analyis of morphological features of pipid frogs indicates that, contrary to earlier opinions, Hymenochirus and Pipa are closest relatives (sister-groups); these genera are placed in the subfamily Pipinae. Also, the currently recognized species of Xenopus do not form a natural group; the species tropicalis and epitropicalis are more closely related to Hymenochirus + Pipa than to the remaining species of Xenopus . The two discordant species are transferred to the genus Silurana , which is relegated to the new subfamily Siluraninae; it is the sister-group of the Pipinae. The remaining species of Xenopus constitute a monophyletic group that is placed in the subfamily Xenopodinae as the sister-group of the other genera of pipids.     

Mechanism of multinucleate myotomal muscle fibre formation in Hymenochirus boettgeri (Anura, Pipidae)

During somitogenesis in Hymenochirus boettgeri, somites separate from non-segmented mesoderm. Somite formation involves changes in position of myotomal cells from perpendicular to parallel relative to axial organs; the changes are asynchronous and show a dorsoventral gradient. After the rotation has been completed, the myotomal cells (primary myoblasts) occupy the whole length of the myotomes. MyoD is present in nuclei of non-segmented mesoderm cells, of myotomal cells during their rotation and of myoblasts occupying the whole length of the myotomes. The effect of MyoD which activates muscle-specific genes is confirmed by the appearance of skeletal α-actin in mononucleate myoblasts in which myofibrils and the sarcotubular system develop. Differentiation of primary myoblasts results in development of mononucleate, morphologically mature myotubes. Differentiating myotubes are initially not accompanied by any other cells. In further developmental stages, mesenchymal cells appear in intermyotomal fissures and then in myotomes. Their role depends on their position: mesenchymal cells remaining in the intermyotomal fissures differentiate into fibroblasts while those that have migrated into the myotomes, between the myotubes, transform into secondary myoblasts. Their myogenic function is evidenced by the presence of MyoD in their nuclei. These cells fuse with the already existing mononucleate myotubes, resulting in an increase in their size and number of nuclei. Accepted: 30 January 2001     

Various DNA content in myotube nuclei during myotomal myogenesis in Hymenochirus boettgeri (Anura: Pipidae)

During myotomal myogenesis in Hymenochirus boettgeri primary myoblasts differentiate into morphologically and functionally mature, mononucleate myotubes. Further muscle development in the studied species is due to fusion of mesenchymal cells with the latter, resulting in the presence of two classes of nuclei in the myotube: large of myotomal origin and small of mesenchymal origin. Densitometric measurements of DNA content revealed that the myotube nuclei at stages 35 reached values close to 4C DNA (3, 3C DNA), while at a later stage (42) the values were equal to 4C. Conversely, the secondary myoblast nuclei following the fusion with the myotube at stage 42 had 2C DNA--a content comparable to that found in erythrocyte nuclei. PCNA (Proliferating Cell Nuclear Antigen)--marker of S-phase of cell cycle, detected in the myotube nuclei (at stages 35, 42) appears during DNA replication.     

The free swimming Pipa larvae,with a review of pipid larvae and pipid phylogeny (Anura: Pipidae)

This paper describes the morphology of the free swimming Pipa larvae, compares them with Xenopus, Hymenochirus, and to some extent, Rhinophrynus larvae, and presents a morphological diagnosis of pipid larvae. Pipa and Xenopus have very similar chondrocrania. Hymenochirus is superficially different but has the same diagnostic features. The differences appear related to its small size and predatory habitus. Other aspects of anatomy, especially the filter apparatus are very different in each genus. The filter apparatus of Pipa is somewhat reduced and seems modified for the retention of relatively large (20+microns) particles. Similar adaptations may have been annectant to predations in Hymenochirus, which lacks a filter apparatus. However, varying states of seven character complexes, which cut across the varying ecology, show that there are two basic pipid lineages, each currently confined to Africa or South America, respectively. Recent finds of fossil South American Xenopus indicate that these two lineages separated before the continents did. This does not warrant the recognition of two subfamilies because Xenopus and Hymenochirus are too different. Pseudhymenochirus is not an intermediate between them; it is a primitive Hymenochirus. Eight character states separate pipid and rhynophrynid larvae.     

The functional significance of the pterygomaxillary ligament in Xenopus Iaevis (Amphibia: Anura)

A pterygomaxillary ligament, which runs postero-inferiorly from the posterior end of the maxilla to the pterygoid, is described macroscopically and microscopically in the aquatic anuran amphibian Xenopus laevis (Daudin). The quadratojugal is absent in this species, and consequently the short maxilla has no posterior articulation and is potentially mobile. However, the pterygomaxillary ligament limits upward and lateral displacement of the maxilla when pressure is applied upwards during closure of the lower jaw. The ligament absorbs forces applied to the posterior end of the maxilla and permits their dissipation via the pterygoid to the neurocranium.