Initially the frontoparietals are present as thin rods of ossification dorsomedial to the orbits; the frontoparietals extend from the anterior to the posterior end of the orbit by developmental stage 44. The anterior ends of the bones remain thin and pointed; ossification progresses medially from the midpoint of the length of the orbit and posteriorly to the level of the exoccipital; a median center of ossification joins the frontoparietals posteriorly, thereby forming the posterior border of the frontoparietal fontanelle. The supraorbital f.l.a.n.g.es of the frontoparietals do not appear until all other cranial bones are ossified, or nearly so. The most rapid ossification begins laterally at the posterior edge of the orbit and decreases anteriorly over the posterior half of the orbit. This differential rate of proliferation of bone results in the pattern of development of the supraorbital f.l.a.n.g.es shown in figure 7. The nasals appear as thin slivers of bone half way between the anterior ends of the frontoparietals and the end of the snout. As ossification proceeds the nasals a.s.sume a triangular shape in dorsal view. The anterior ends are pointed; the lateral margins are parallel to the maxillaries. The posteromedial points do not reach the lateral margins of the ethmoid, and the maxillary processes extend about three-fourths the distance from the bodies of the nasals to the maxillaries. Following the union of the frontoparietals posteriorly, the nasals widen anteriorly and are narrower at the midpoints of their long axes than anteriorly or posteriorly. With further ossification the maxillary processes extend to the maxillaries and form complete bony anterior margins to the orbits; the mid-parts of the nasals widen (Pl. 1B).

[Ill.u.s.tration: FIG. 7. Developmental sequence of the frontoparietal fontanelle and a.s.sociated bony elements in _Smilisca baudird_: (A) KU 60026, 5; (B) KU 85438, 4; (C) KU 26328, 3; (D) KU 68184, 2.3.]

The parasphenoid is the first of the palatal bones to appear. At metamorphosis the bone is well developed; the anterior tip is situated just in front of the anterior edge of the orbit, and posteriorly the lateral processes extend laterally beyond the ossified parts of the auditory region. The pterygoids do not appear until metamorphosis, when ossification is evident in only the mid-parts of the posterolateral arms. Ossification follows in the mid-parts of the anterolateral arms and occurs last in the pterygoid pedicles. The palatines do not appear until all three arms of the pterygoids are at least partly ossified.

Ossification proceeds rapidly from the maxillaries medially to the unossified ethmoid, which is the last of the cranial bones to appear.

Initially it is extremely shallow; dorsally it is widely separated from the nasals, and ventrally the posterior margin meets the anterior point of the parasphenoid. In dorsal view, ossification proceeds anteriorly between the nasals and posteriorly, ventral to the frontoparietals; ventrally, ossification proceeds posteriorly dorsal to the parasphenoid.

The ventral arms of the squamosal and the supraoccipital region of the exoccipital are the first occipital bones to appear. Ossification follows in the regions of the semicircular ca.n.a.ls and occipital condyles. The dorsal end of the ventral arm of the squamosal and the posterior arm of the squamosal ossify as a unit at the same time the quadratojugal appears. Shortly thereafter the anterior arm of the squamosal ossifies, the distal part of the columella appears, and the anterior and lateral parts of the auditory region ossify.

The angular and dentary of the lower jaw appear concurrently with the dentigerous bones. Initially, the angular is short and broad; the articular surface is absent, and the anterior end is slightly overlapped by the dentary. The mentomecklians do not ossify until approximately the same time that the quadratojugal appears in the upper jaw.

_Comparative Osteology_

The genus _Smilisca_ is characterized by the following combination of cranial osteological characters: (1) A large amount of bone is involved in the skull and a minimal amount of cartilage and/or secondarily ossified cartilage; co-ossification is absent. (2) The skulls are uniformly broad with angular lateral margins, and truncate anteriorly.

(3) An internasal septum and quadratojugals are present. (4) A well-developed squamosal minimally extends one-fourth the distance from the dorsal end of the quadrate to the maxillary, and maximally is separated from the maxillary by a suture. (5) The ethmoid is large; the distance between the anterior end of the ethmoid and the anterior edge of the premaxillary varies between 15 and 20 per cent of the total length of the skull.

On the basis of cranial osteology two species-groups can be recognized within the genus _Smilisca_. The _sordida_ group, comprising _S.

sordida_ and _puma_, is characterized by a broad skull in which the lateral margins of the maxillaries are relatively straight anterior to the orbit. The moderate-sized nasals are rounded anteriorly, and bear relatively short, sometimes blunt, maxillary processes. The long axes of the nasals are not parallel to the maxillaries. The ethmoid is proportionately small in the _sordida_ group. The bony part of the ethmoid terminates near the anterior edge of the orbits and does not extend anteriorly between the nasals; the entire anterior margin of the ethmoid is separated from the nasals by cartilage. The squamosals are generally small. They are narrow in dorsal view, and minimally extend one-fourth the distance from the dorsal end of the quadrate to the maxillary, and maximally, two-thirds the distance. The tegmen tympani are relatively small (Fig. 8).

[Ill.u.s.tration: FIG. 8. Dorsal views of the skulls of the species of _Smilisca_: (A) _S. baudini_ (KU 68184); (B) _S. puma_ (KU 68636); (C) _S. phaeota_ (KU 41090); (D) _S. sila_ (KU 80625); (E) _S.

cyanosticta_ (KU 55938), and (F) _S. sordida_ (KU 36765). 1.5.]

In contrast to the tendency for reduction of cranial parts in the _sordida_ group, the _baudini_ group, const.i.tuted by _S. cyanosticta_, _phaeota_, and _baudini_, is characterized by more ossification of the cranial elements. The skull is broad; the lateral margins are less angular and are gently curved, rather than straight as in the _sordida_ group. The nasals tend to be larger with the long axes parallel to the maxillary. Anteriorly the nasals are pointed, and posteriorly they bear long, delicate palatine processes extending to the maxillary. The ethmoid is fully ossified, extends anteriorly between the nasals, and laterally is separated by a suture from the nasals if the latter are fully ossified. The squamosals are large, and wide in dorsal view. They minimally extend one-fourth the distance from the dorsal end of the quadrate to the maxillary, and maximally are sutured to the maxillary.

The tegmen tympani are ma.s.sive.

_Smilisca sila_ is intermediate between the two species-groups described. The skull is broad; the lateral margins are gently curved, and have a p.r.o.nounced angularity just anterior to the palatines which results in a broad, truncate snout. The nasals are moderate in size; because of the anterior angularity of the lateral margins, the long axes of the nasals lie parallel to the maxillary. The nasals are only slightly pointed anteriorly, and posteriorly they bear short, blunt palatine processes and medial processes in contact with the lateral corners of the ethmoid. The ethmoid is fully ossified, but does not extend anteriorly between the nasals. The squamosals are moderate in size and extend one-fourth the distance from the dorsal end of the quadrate to the maxillary. The tegmen tympani are relatively large, but proportionately short.

The cranial characters utilized in the a.n.a.lysis of species groups (general shape, nature of the nasals, ethmoid, squamosals, and tegmen tympani), together with other characters, such as the relative height and shape of the prenasal processes, the extent of the internasal septum, and the nature of the vomers, frontoparietals, maxillaries and pterygoids are useful in distinguishing the various species (Table 4, Fig. 8), as well as in establishing relationships within the species-groups.

Within the _sordida_ group, _S. sordida_ and _S. puma_ can be distinguished by the following characters: The bony part of the ethmoid terminates posterior to the anterior edge of the orbit and is thus widely separated from the nasals by cartilage in _S. puma_. In _S.

sordida_ the bony part of the ethmoid always terminates at a level equal to, or slightly in front of the anterior edge of the orbit; therefore, less cartilage exists between the ethmoid and nasals in _S. sordida_ than in _S. puma_. The width of the premaxillary comprises about 30 per cent of the width of the skull in _S. sordida_ and 20 per cent in _S.

puma_. The proportion of the length of the skull anterior to the bony part of the ethmoid in _S. sordida_ is approximately 21 per cent, as compared with about 29 per cent in _S. puma_. The prenasal processes are convex in _S. sordida_ and straight in _S. puma_.

The marked ontogenetic variation in _S. sordida_ is considered in more detail in the account of that species, but it is pertinent to the present discussion to note that with respect to some features of the skull some young breeding specimens of _S. sordida_ are intermediate in appearance between large females of _S. sordida_ and adults of _S.

puma_. In some breeding males (usually the smaller individuals) of _S.

sordida_ the bony part of the ethmoid terminates at the anterior edge of the orbit and is widely separated from the nasals by cartilage. In small individuals _S. sordida_, especially in males, and in adults of _S.

puma_ the tegmen tympani are relatively short, whereas in adult females of _S. sordida_ these elements are long and slender. In the smaller specimens of _S. sordida_ and in _S. puma_ the squamosal is small; it extends only about one-fourth of the distance to the maxillary in the smaller _S. sordida_ and about one-half the distance in _S. puma_. The more ma.s.sive squamosal in large adult females of _S. sordida_ extends at least two-thirds of the distance to the maxillary.

Table 4.--Comparative Cranial Osteology of Smilisca.

===============+==============================+======================= Character | _S. baudini_ | _S. cyanosticta_ ---------------+------------------------------+----------------------- | | Alary Processes| Four times as high as | Three times as high | lateral wing of premaxillary;| as lateral wing of | anteriorly | premaxillary; | convex. | anteriorly | | convex.

| | Nasals | Long, wide anteriorly, | Long, widest | narrowing posteriorly; | posteriorly; | attached to ethmoid. | attached to | | ethmoid.

| | | | | | Ethmoid | Long; entirely ossified; | Long, entirely | smooth margins. | ossified; | | smooth margins.

| | Frontoparietal | Small, ovid fontanelle | Large fontanelle, two | present or absent; | and one-half times as | long, pointed pos...o...b..tal | long as wide; narrow | processes curving | supraorbital f.l.a.n.g.es | along posterior | with irregular margins.

| border of orbit. | | | Squamosal | Large: anterior arm | Large; anterior arm | in contact with maxillary. | in contact with | | maxillary.

TABLE 4 (Continued) ===============+=============================+========================= Character | _S. phaeota_ | _S. puma_ ---------------+-----------------------------+------------------------- | | Alary Processes| Two and one-half | Two times as high as | times as high as lateral | lateral wing of | wing of premaxillary; | premaxillary; | anteriorly convex. | straight.

| | | | Nasals | Long, widest anteriorly | Short, narrow, not | and posteriorly, | attached to ethmoid.

| bearing posteromedial | | process; not attached | | to ethmoid. | | | | | Ethmoid | Long, entirely ossified; | Short, about two-thirds | smooth margins. | ossified; irregular | | margins.

| | Frontoparietal | Fontanelle absent; | Keyhole-shaped fontanelle; | large supraorbital | smooth margins; | f.l.a.n.g.es having | f.l.a.n.g.es absent.

| straight edges and extending| | posterolaterally. | | | | | Squamosal | Large; anterior arm | Small; anterior arm | extending 1/2-2/3 way | extending 1/2 way to | to maxillary. | maxillary.

TABLE 4 (Continued) ===============+===========================+============================ Character | _S. sila_ | _S. sordida_ ---------------+---------------------------+---------------------------- | | Alary Processes| One and one-half | Two and one-half | times as high as lateral | times as high as lateral | wing of premaxillary; | wing of premaxillary; | straight. | slightly convex | | anteriorly.

| | Nasals | Short, wide, bearing | Moderately long narrowest | small posteromedial | anteriorly and | processes; not attached | posteriorly; not attached | to ethmoid. | to ethmoid.

| | | | | | Ethmoid | Moderately long; entirely | Short; one-half to entirely | ossified; smooth | ossified; irregular | margins. | margins.

| | Frontoparietal | Large, ovoid fontanelle; | Large, elongate fontanelle; | smooth margins; | smooth margins; | f.l.a.n.g.es absent. | f.l.a.n.g.es absent.

| | | | | | | | Squamosal | Moderately large; anterior| Moderately small; anterior | arm extending | arm extending | 1/4 way to maxillary. | 1/4-2/3 way to maxillary.

Within the _baudini_ group, the skull of _S. cyanosticta_ is the most generalized of the three species; the cranial characters are intermediate between _S. phaeota_ and _S. baudini_. The lateral margins of the skull in _S. cyanosticta_ are gently curved, and have an angularity anterior to the palatine-maxillary suture; the anterior margins are less angular in _S. phaeota_, which has a broader snout.

Posteriorly in _S. baudini_ the margins are slightly curved medially, and the greatest width of the skull is between the quadratojugal-maxillary sutures on either side of the skull. The frontoparietals of _S. cyanosticta_ bear slightly irregular lateral margins and a large fontanelle. There is a tendency for obliteration of the fontanelle with increasing age in both _S. baudini_ and _S.

cyanosticta_; the lateral margins of the frontoparietals bear large supraorbital f.l.a.n.g.es in both of these species. In _S. phaeota_ the f.l.a.n.g.es are most prominent; they extend posterolaterally with straight margins along two-thirds of the length of the orbit and terminate in rather blunt points. The broad interorbital f.l.a.n.g.es result in a relatively broad external interorbital distance. In _S. baudini_ the f.l.a.n.g.es are curved posterolaterally around the orbit and terminate in sharp, thin points. The tegmen tympani of all three species are ma.s.sive.

In _S. cyanosticta_ the prootics slope posteriorly, whereas they slope anteriorly in _S. baudini_ and _S. phaeota_.

The skulls of _S. cyanosticta_ and _S. baudini_ are alike in certain respects. The squamosals of both species are large and connected to the maxillary by a bony connection; the squamosals of _S. phaeota_ are large, but extend only two-thirds of the distance from the dorsal end of the quadrate to the maxillary. In _S. baudini_ and _S. cyanosticta_ the nasals are separated throughout their lengths from the ethmoid, whereas the nasals of _S. phaeota_ are separated from the ethmoid by cartilage.

The latter separation is due to an incomplete ossification of the nasals in _S. phaeota_. The bony part of each nasal is constricted in the middle of the long axis of the bone, and the nasals are widest anteriorly; posteriorly each nasal bears a medial process, which is narrowly separated from the lateral edge of the ethmoid.

Table 5.--Variation in the Number of Teeth in the Species of Smilisca. (All Are Males; N = Number of Jaws, or Twice the Number of Individuals; Means Are Given in Parentheses After the Observed Ranges.)

=================+====+==============+==============+=========== Species | N | Maxillary | Premaxillary | Vomerine -----------------+----+--------------+--------------+----------- _S. baudini_ | 20 | 49-65 (56.0) | 9-16 (13.6) | 5-9 (7.2) _S. cyanosticta_ | 8 | 50-64 (57.9) | 10-12 (10.8) | 4-11 (7.1) _S. phaeota_ | 20 | 50-68 (58.1) | 10-15 (12.1) | 5-9 (7.3) _S. puma_ | 6 | 60-67 (63.6) | 11-13 (12.0) | 4-7 (5.3) _S. sila_ | 8 | 48-60 (52.9) | 10-14 (11.3) | 5-7 (5.7) _S. sordida_ | 12 | 39-55 (44.2) | 7-11 (9.3) | 4-6 (5.2) -----------------+----+--------------+--------------+-----------

The teeth of all species of _Smilisca_ are spatulate and bifid. The numbers of maxillary, premaxillary, and vomerine teeth are summarized in Table 5. Smaller and presumably younger specimens of all species of _Smilisca_ have fewer teeth than do larger specimens of the same species. This correlation between size and number of teeth does not exist as an interspecific trend within the genus; for example, the smallest species in the genus, _S. puma_, has the highest number of maxillary teeth. In small specimens of a given species wide gaps are present between the maxillary teeth posteriorly; in large specimens the gaps are filled by teeth, beginning anteriorly and progressing posteriorly, until the maxillary dent.i.tion is continuous.

Musculature

No extensive study of the muscular system was undertaken, but certain muscles know to be of taxonomic importance were studied.

_Jaw Musculature._--Starrett (1960) pointed out the unique jaw musculature in _Smilisca_. In this genus M. depressor mandibulae consists of two parts, one arising from the dorsal fascia and one from the posterior arm of the squamosal. Two muscles arise from the anterior arm of the squamosal and insert on the lateral face of the mandible. Of these muscles, M. adductor mandibulae posterior subexternus lies medial to the mandibular branch of the trigeminal nerve; the other, M. adductor mandibulae externus superficialis, lies lateral to the same nerve (Fig.

9). In most other hylids the latter muscle is absent. No significant variation in the position of the muscles was noted in the various species of _Smilisca_, though M. adductor mandibulae originate somewhat more anteriorly in _S. baudini_ and _S. cyanosticta_ than in the other members of the genus, all of which have a shorter anterior arm of the squamosal that does not reach the maxillary. The two separate parts of M. depressor mandibulae are not so widely separated in members of the _sordida_ group as in the _baudini_ group.

[Ill.u.s.tration: FIG. 9. Lateral view of the left jaw of _Smilisca baudini_; _A. M. E. S._, adductor mandibulae externus superficialis; _A. M. P. S._, adductor mandibulae posterior subexternus; _Col._, columella; _D. M._ depressor mandibulae; _M. B. T. N._, mandibular branch trigeminal nerve; _Sq._, squamosal. KU 64214, 5.]

[Ill.u.s.tration: FIG. 10. Ventral view of throat musculature in an adult male _Smilisca baudini_ (Superficial musculature on left, deep musculature on right); _A. C._ anterior cornua of hyoid; _Gen. L._, geniohyoideus lateralis; _Gen. M._, geniohyoideus medialis; _Hyo._, hyoglossus; _Omo._, omosternum; _Pet._, petrohyoideus; _S._, submentalis; _Sm._, submaxillaris; _St._, sternohyoideus; _V. S._, vocal sac. KU 64220, 2.5.]

_Throat Musculature._--The frogs that comprise the genus _Smilisca_ are characterized by paired subgular vocal sacs, essentially the same as those in _Triprion_ (Duellman and Klaas, 1964). The following description is based on _Smilisca baudini_ (Fig. 10).

M. submentalis lies in the anterior angle of the lower jaw, is thick, and consists of transverse fibers extending between the dentaries. M.

submaxillaris is thin and arises from the whole of the inner surface of the lower jaw, except for the anterior angle occupied by M. submentalis.

Anteriorly M. submaxillaris is broadly attached by fascia to M.

hyoglossus and M. geniohyoideus, which lie dorsal to M. submaxillaris.

Medially this attachment continues posteriorly for about one-half the length of the hyoglossus. Posteriorly M. submaxillaris is folded and attached to M. sternoradialis of the pectoral girdle. The vocal sacs are formed by a pair of posterolateral ev.a.g.i.n.ations of M. submaxillaris; a broad connection between the pouches allows free pa.s.sage of air between the pouches.