Cornufer guentheri, (Boulenger, 1884) (Boulenger, 1884)

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Field observations

Cornufer guentheri is found in all kinds of vegetation, primary forest, secondary growth, village gardens, coconut and cacao plantations, sago and Pandanus swamps from sea level to 1050 m. In rugged terrain, these frogs frequently occur on forested ridges and are rare in steep sided valleys. By day the frogs rest in and under leaf litter and in debris beside stream beds. None was found in water. Frogs were active on overcast days, or under dense canopy cover and during rain but were basically nocturnal.

At night frogs were seen active on the ground or on leaf litter, often with the body held at a characteristic 45 o angle to the ground. By day or night, when a frog was disturbed, it would crouch or flatten to the ground, with its coloration and outline making it difficult to distinguish from the leaf litter. At night, a frog may close its eyes, stopping any spotlight reflection. If in leaf litter, the cryptic coloration makes the frog ‘disappear’. None of the larger frogs ever jumped away though smaller animals sometimes scurried to get under cover, but we did not observe any frog ever jumping. By contrast, the sympatric and equally common Cornufer solomonis Boulenger, 1884 escaped with long leaps, when disturbed.

Male Cornufer guentheri call from ground level; details of the call, which is similar in structure to that of other Cornufer species, are illustrated in Menzies (2006). As with other CERATOBATRACHIDAE , adults do not form breeding assemblies. Development is direct. In captivity, 5 mm diameter eggs are laid in clusters and the mean clutch size is 54 (28– 74 eggs per clutch). The eggs are buried in moist soil, and hatch 32 days later. The newly-hatched froglets are 10 mm in length. Apparently, neither parent remains with the eggs ( Barton 1991).

We did not carry out a systematic survey of the diet but dissected out stomach contents of some wild-caught specimens after preservation. We also examined the stomach contents of six museum specimens but all were empty, possibly due to the interval between collection and preservation. Among specimens collected by Parker, the stomach of one large female contained two frogs: a smaller Cornufer guentheri and an adult Batrachylodes vertebralis Boulenger, 1887 . Another large female contained a large Orthopteran insect while a third contained a small adult skink. Parker also observed two equal-sized C. guentheri holding each other by the mouth and did not let go when captured. Another C. guentheri was seen apparently trying to swallow another frog (sp. inc.) but released it when caught. Other previously published observations include those of Kinghorn (1928), Barton (1991), Schmidt (1992) and Menzies (2006).

Skull structure (fig. 1 ‒ 2)

The skull of the European Edible Frog ( Pelophylax esculentus [Linnaeus, 1758]) ( Ecker 1889), is gymnokrotaphic having extensive open areas and of light construction. To some extent, this may be a consequence of the saltatorial locomotion of P. esculentus , where minimising the weight at the anterior end of the body is advantageous. Also the efficient feeding mechanism of many frog species obviates the need, in most taxa, for powerful jaws and large teeth. The skull of Cornufer guentheri has an overall sculptured surface; the edges are extended into raised crests and various extensions form knobbly projections. The upper jaw of Cornufer guentheri has 35 small teeth on the premaxillary and maxillary bones while the lower jaw has a matching series of tooth-like serrations on the dentary.

Cranium (fig. 1)

The skull roof is composed of the nasals and frontoparietals which are synostotically united. The casque, formed by co-osification of skull roof and skin, has upturned margins (fig. 1a, 2c). There is no frontal fontanelle. The nasal region of the skull roof extends laterally to join with the preorbital process of the maxilla and the junction is marked by a transverse groove (fig. 2c). The frontoparietal also extends laterally behind the orbit to form an otic flange, partially separating the orbit from a post-orbital foramen. The sphenethmoid appears to be ossified throughout and is fused to the cultriform process of the parasphenoid in the lateral walls of the braincase. The parasphenoid is ‘T’ shaped, the cultriform process is slender and terminates between the vomers, the alae are fused with the otic processes of the pterygoid and the boundaries are indistinct (fig. 1b).

Jaws and jaw suspension

In Cornufer species other than C. guentheri , the skull, viewed dorsally, forms a rounded triangle with length usually exceeding breadth ( Table 2 View Table 2 ). The premaxilla, maxilla and quadratojugal together form a shallow curve ending in the jaw articulation. In C. guentheri , the skull is markedly triangular with the maxillary portion of the jaw almost straight and, at the maxilla-quadratojugal junction, width exceeds length by about 20 % ( Table 2 View Table 2 ). Maxilla and quadrato-jugal, which appear to be firmly sutured, do not form a smooth curve as the quadrato-jugal is deflected inwards bringing the jaw articulation closer to the mid-line than it would otherwise be (fig. 1).

The premaxilla has a broad, deeply indented palatal shelf and a dental shelf with four, five or six slightly enlarged, sometimes bicuspidate, teeth. The alary processes are rod-shaped, directed posterior at 45 o with broad connections to the anterior end of the naso-frontal through cartilages of the nasal capsule (fig. 2b ‒ c).

The pars palatina of the maxilla is broad where it contacts the premaxilla but narrows posteriorly; the pars facialis extends dorsally in two regions to form connections with the nasal and with the squamosal. The maxilla appears to be fused to the squamosal at the point where the three arms of that bone diverge, resulting in a raised, knobbly, squamosal crest (fig. 2c, 3). Connection between maxilla and quadratojugal is also marked by a sculptured extension bearing a single, sharp, outwardly-directed, curved ‘tooth’ (fig. 1b). The maxilla bears 35 small, conical teeth. The quadratojugal is as described above. The quadrate is not visible as the ventral arms of the squamosal and pterygoid and posterior end of the quadrato-jugal are synostosed at the jaw articulation (fig 1b, 2c).

The squamosal (fig. 3) is tri-radiate; the otic arm is massive, broad medially and tapering distally, synostosed with the otic capsule; the zygomatic arm forms the upper margin of the post-orbital pars facialis of the maxilla and thereafter indistinguishable from it. The ventral arm of the squamosal connects with the quadrato-jugal at the jaw articulation. The space between the ventral arm and pars facialis of the maxilla is largely filled in by a thin bony sheet (fig. 3).

The pterygoid (fig. 1b) is tri-radiate but the space between the otic and ventral arms and the otic arm of the squamosal is largely filled in by a sheet of bone, forming a deep fossa between them; the zygomatic arm has a broad connection to the central part of the maxilla. The otic arm and the alary process of the parasphenoid are synostosed and the boundaries are not distinct. The palatine and pre-vomer are not distinct. There are four slightly enlarged vomerine teeth set on a raised mound on each side (fig. 1b).

The lower mandible (fig. 4) is slender but with a distinct coronoid process that is tilted medially to a near horizontal position (fig. 4b) and forms the insertion point of the inner adductor muscles and the posterior part of the M. intermandibularis . The mento-meckelian has a few enlarged teeth; the dentary has tooth-like serrations matching the teeth on the maxilla.

The only flexible joint in the skull, apart from the jaw articulation, appears to be between the pre-maxilla and maxilla and so the skull forms a single, rigid unit.

Jaw musculature

Throat muscles (fig. 4 ‒ 5)

The M. submentalis connects the two mentomeckelian bones (fig. 5a). The M. intermandibularis ( submaxillaris of Ecker) has two accessory components; the superficial is a thick, cylindrical muscle arising from the medial side of the dentary in advance of, and slightly dorsal to, the main M. intermandibularis . It is narrow at its origin, becoming thicker then flattening at the insertion on the anterior part of the median aponeurosis, (fig. 4b, 5a). The deep component inserts on the mento-meckelian, runs parallel to the jaw and originates by a long tendon to the posterior (fig. 4c, 5a). The main part of the M. intermandibularis arises from the medial surface of the lower jaw and inserts on the median aponeurosis, partly covering the superficial accessory on the anterior edge by. The fibres of the thin anterior section run more horizontally from near the upper margin of the jaw. The posterior section is thicker, runs more diagonally, and has its origin anterior and ventral on the coronoid process (fig. 4b ‒ c, 5a ‒ b).

The M. interhyoideus is a thin sheet inserting broadly on the median aponeurosis posterior to, and slightly overlapped by, the M. intermandibularis . The origin on the prootic is cylindrical, spreading as it passes ventrally (fig. 5a).

Depressor muscles (fig. 4, 6)

There are four M. depressor mandibulae. An anterior slip, md 1, originates on the outer side of the quadratojugal and inserts separately on the end of the angulosplenial (fig. 6b). A small muscle ( md2) originates on the post-ventral rim of the tympanum, joins md3 and md4 to insert by a common tendon (fig. 6a ‒ b). A short, wide and thick muscle ( md 3) originates laterally on the ex-occipital and inserts with md2 and md4. A large muscle ( md4) has multiple origins, below the squamosal crest, on the exoccipital surface and on the fascia above the M. abdominal oblique and longissimus dorsi near the dorsal mid-line (fig. 6a ‒ c). It tapers rapidly to insert with md2 and md3 by a short strap-like tendon on the end of the angulo-splenial. Muscles md2, 3 and 4 together correspond to the M. depressor mandibulae of Pelophylax esculentus as described by Ecker (1889). Muscle md1 is not mentioned by Ecker.

Adductor muscles (fig. 4a, 6a, 7)

There are seven M. adductor mandibulae inserting via three series on the mandible. The superficial series ( ma6, ma7; fig. 4a, 6a) includes two muscles forming a rather thin sheet with fibres running vertically. The tendinous insertion extends along the lower margin of the mandible except to the anterior where the insertion diverges to the upper surface. The origin of the anterior muscle ( ma7) is along the anterior side of the tympanic annulus and that of the posterior ( ma6) is on the medial surface of the maxilla. These two muscles cover the insertions of all the other adductors. In Lithobates catesbeianus (Shaw, 1802) , Starrett (1968) named these the adductor mandibulae externus lateralis and superficialis but they were not mentioned by Ecker (1889) or Duellman & Trueb (1985).

The medial series comprises two thick muscles, ( ma4, ma5; fig. 4a, 7a), originating on the distal end of the ventral arm of the squamosal and posterior end of the quadratojugal. They run obliquely to insert broadly on the outer face of the mandible, dorsal to ma6 and ma7 (fig. 4a, 7a). The most distal of these two ( ma5) corresponds to the adductor mandibulae post articularis of Starrett (1968). There is no exact match for the adductor mandibulae post externus of which has its origin on the zygomatic arm of the squamosal, but muscle ma4 with its origin on the ventral arm of the squamosal may be the homologue. The part originating on the quadrato-jugal bone corresponds to the masseter of Ecker (1889).

The deep series (includes three substantial muscles ( ma1, ma2, ma3; fig 4a, 7b); the first ( ma1) originates high on the side of the cranium, below the supra-orbital crest, and on the anterior surface of the prootic. The fibres run ventrally then contract to a short tendon that passes medial to ma2 and inserts on the lower jaw posterior to the insertions of ma2 and ma3. This muscle corresponds to the adductor mandibulae anterior of Lithobates catesbeianus and the pterygoidus muscle of Ecker and Starrett but the tendon of insertion is considerably shorter in C. guentheri .

The second muscle ( ma2) is the adductor mandibulae posterior longus of Starrett (1968) and Duellman & Trueb (1985), or the temporalis muscle of Ecker (1889), and originates on the exoccipital from where it passes over the otic arm of the squamosal before running ventrally to insert directly on the coronoid process of the mandible. It is the largest muscle in this series. The third adductor muscle ( ma3) originates on the anterior side of the tympanic annulus and inserts with the second (fig. 4a).

Muscles of the hyoid and tongue (fig. 5b, 8)

The hyoid plate is thicker in the centre while the margins are thin; muscles are attached to the edge of the thick portion, on both dorsal and ventral sides. Only the post-medial processes are bony and there is no parahyoid bone. There are a pair of short, hook-shaped antero-medial (a-m), broad antero-lateral (a-l) and short finger-like postero-lateral (p-l) processes. The bony post-medial (p-m) processes are elongate and slightly curved (fig. 8b). The hyoid and its muscles do not appear to differ from those of Pelophylax esculentus as described by Ecker (1889) except that the attachment of the M. geniohyoideus to the jaw is more extensive. We assume that the same tongue operating system is present in Cornufer guentheri .

The M. geniohyoideus arises from the anterior half of the lower jaw and, running ventral to the hyoid plate, inserts on the cartilaginous post-lateral process of the hyoid. It has medial and lateral components (fig. 8b). The M. sternohyoideus arises from the coracoid and sternum and inserts on the edge of the thick, medial part of the hyoid (fig. 5b, 8b). The M. genioglossus , not visible in the figures, has two components originating at the mandibular symphysis and running deep to the M. hyoglossus to insert in the tongue. The M. hyoglossus is a thick, cylindrical muscle arising from the distal end of the bony post-medial process of the hyoid (fig. 5b, 8b). It runs forwards along the ventral surface of the hyoid plate to turn dorsally into the tongue (fig. 8a). The M. omohyoideus arises from the anterior border of the scapula and inserts on the ventral surface of the hyoid (fig. 8b).

Pectoral skeleton and musculature (fig. 5, 9)

The joints between coracoid, clavicle and scapula are difficult to discern; they are without joint cartilage and assumed to be immoveable while the scapula/suprascapula joint is marked by a band of cartilage and is assumed to be flexible. The scapula is elongate, fixed at 135 o vertical angle to the coracoid, and is bicapitate. The suprascapula is cartilaginous but calcified on upper and central parts.

Ventral pectoral muscles (fig. 9)

The M. supracoracoideus is a thin sheet with a broad origin on the sternum, superficial to origins of the M. episternohumeralis and coracobrachialis, inserting by a narrow tendon on the deltoid process medial to the insertion of the M. pectoralis abdominis . The M. coraco-radialis has two separate origins on sternum, deep to the M. supra-coracoideus , that join to a single tendon running in a tough sleeve deep to the tendons of the M. pectoralis abdominis and pectoralis sternalis and inserting on the radius (fig. 9b). The M. episterno-humeralis is a narrow muscle with its origin on the sternum anterior to the supracoracoideus (fig. 9a). It inserts on ventral surface of radius, about midway. The M. pectoralis sternalis originates on the sternum posterior to the supracoracoideus ; tendon runs deep to tendon of the M. epistero-humeralis and inserts on the deltoid process deep to insertion of the M. pectoralis abdominis . The M. pectoralis abdominis has a wide origin on ventro-lateral fascia of the M. abdominalis obliquus and sides of xiphisternum, inserting by a short broad tendon on post-ventral side of deltoid process (fig. 9a ‒ b).

Suspensory muscles originating on the dorsal fascia, vertebrae or suprascapula (fig. 10)

The M. dorsoscapularis has two origins; the bulk of this muscle originates on the medial surface and anterior edge of the suprascapular but there is also a small slip originating below the otic crest; narrowing and thickening to insert with the M. latissimus dorsi by a short tendon on anterior face of deltoid process. The M. latissimus dorsi is a thin fan-shaped muscle originating on the dorsal fascia near the mid-line. It unites with the tendon of the M. dorso-scapularis to insert on the deltoid process.

Suspensory muscles inserting on the medial face of suprascapula and scapula (fig. 10b)

The anterior series originating on the skull (fig. 10b) includes four muscles. The M. rhomboideus anterior arises from the dorsal side of the skull and inserts near the dorsal edge of the medial suprascapula (fig. 10b). A small fan-shaped slip arises from the fascia of the second M. adductor mandibulae and inserts with the other. The M. levator scapulae superior originates on the otic and exoccipital regions of the skull and inserts near the dorsomedial border of the suprascapula. The M. levator scapulae inferior arises with the superior but inserts near the ventral margin of the suprascapula and dorsal scapula margin. The M. cucullaris arises from the otic arm of the squamosal and runs ventrally to insert on the distal end of the coracoid.

There are four muscles originating on the vertebrae and one, the M. interscapularis , spans the suprascapula/scapula joint (fig. 10b). The M. rhomboideus posterior has two components. The first is a thin, fan shaped muscle originating on the fascia above the neural spine of vertebra 4. The second is larger and originates on the transverse process of vertebra 4. They insert together near the dorsal margin of the suprascapula. The M. serratus superior originates from the transverse process of vertebra 4 and inserts below the rhomboideus posterior. The M. serratus medius arises from the transverse process of vertebra 3 and inserts near the anterior margin of the suprascapula. The M. serratus inferior has two components with origins on the transverse processes of vertebrae 3 and 4. They run separately to inset on the anterior and posterior margins of the scapula towards the proximal end.