JOURNAL OF MORPHOLOGY 177255-268 (1983)

Miniaturization and its Effects on Cranial Morphology in Plethodontid , Genus (Amphibia, ): II. The Fate of the Brain and Sense Organs and Their Role in Skull Morphogenesis and Evolution

JAMES HANKEN Museum of Vertebrate Zoology, University of California, Berkeley, California 94720

ABSTRACT Relative size and arrangement of the brain and paired sense organs are examined in three species of Thorius, a genus of minute, terrestrial salamanders that are among the smallest extant tailed tetrapods. Analogous measurements of representative species of three related genera of larger trop- ical (, Chiropterotriton) and temperate (Plethodon)salamanders are used to identify changes in gross morphology of the brain and sense organs that have accompanied the evolution of decreased head size in Thorius and their relation to associated changes in skull morphology. In adult Thorius, relative size (area measured in frontal plane, and length) of the eyes, otic capsules, and brain each is greater than in adults of all of the larger genera; relative size of the nasal capsules is unchanged or slightly smaller. Interspecific scaling phenomena-negative allometry of otic capsule, eye and brain size, isometry or slight positive allometry of nasal capsule size, all with respect to skull length-also are characteristic of intraspecific (onto- genetic) comparisons in both I: narisovalis and Pseudoeurycea goebeli. Predominance of the brain and eyes in Thorius results in greater contact and overlap among these structures and the nasal capsules in the anterior portion of the head. This is associated with anterior displacement of both the eyes and nasal capsules, which now protrude anterior to the skull proper; a change in eye shape; and medial deformation of anterior braincase walls. Posteriorly, predominance of the otic capsules has effected a reorientation of the jaw suspensorium to a fully vertical position that is correlated with the novel presence of a posteriorly directed squamosal process and shift in origin of the quadropectoralis muscle. Many of these changes in cranial morphology may be explained simply as results of mechanical (physical) interactions among the skeletal, nervous, and sensory components during head development at reduced size. This provides further evidence of the role of nervous, sensory, and other “soft” tissues in cranial skeletal morphogenesis, and reinforces the need to consider these tissues in analyses of skull evolution.

“In vertebrates, a quite simple change A well-established principle of vertebrate in epigenetic mechanism may have a pro- development is the great degree to which found and extensively different result. nervous and sensory components may pro- Moreover, the result is an integrated or- mote, or even direct, skeletal differentiation ganism’’ (Davis, ’64, p. 5). “Many conspicuous features in the skel- eton depend only on the capacity of bone James Hanken’s present address is Department of Environ- to respond to extrinsic factors” (Davis, ’64, mental, Population, and Organismic Biology, University of Col- p. 12). orado, Campus Box B-334, Boulder, CO 80309.

0 1983 ALAN R. LISS, INC. 256 J. HANKEN and morphogenesis. This is perhaps nowhere and the evolution of urodeles from early am- better seen than in the skull. Here, the influ- phibians have been attributed to changes in ence of the brain and sense organs ranges the relative size and position of the brain, from purely mechanical effects, which in otic capsules, eyes, and nasal capsules (Car- large part may determine both the shape and roll, '70; Carroll and Holmes, '80). differentiation of individual skull elements In this study I examine the changes in gross (Bassett, '72; Hall, '81; Moss, '61; Vilmann morphology, relative size, and geometrical and Moss, '79), to more subtle (presumably "packaging" of the brain and sense organs chemical) inductive effects that influence the that have accompanied extreme size reduc- timing and orientation of bone formation tion, or miniaturization, in an lin- (Schowing, '68a; Simons and Van Limborgh, eage, and the relationship of these changes '79). Earlier views of skull development (see to the associated modifications in skull archi- reviews by Goss, '72, '80; Hoyte, '66; Van tecture. My analysis focusses on salaman- Limborgh, '72) held that cranial skeletal ders of the plethodontid genus Thorius, a morphology was determined intrinsically, group comprising at least 15 terrestrial or i.e., within the developing skeletal tissues, arboreal salamanders that are among the and was relatively unaffected by surround- smallest extant tailed tetrapods. Data com- ing tissues. However, the prevailing view of prise a series of measurements of gross di- skull development (often subsumed under the mensions (length, area) and arrangement of headings "functional craniology" or "func- the brain and the three paired primary sense tional cranial analysis"; see Dullemeijer, '68, organs-otic capsules, eyes, and nasal cap- '72, '74; Van der Klaauw, '46; Moss, '68a,b, sules-relative to the surrounding skull in '72a,b; Moss and Young, '60) stresses the Thorius and selected genera of larger sala- critical role of neighboring tissues. These manders. Earlier (Hanken, '83), I presented include sensory, nervous, circulatory, connec- an analysis of the patterns of osteological tive, and muscular tissues that interact to variation in the cranium of Thorius, stress- determine the ultimate size and shape of ing particularly those features that charac- many skull elements, the intrinsic growth of terized the evolution of decreased head size. which is labile and relatively indeterminate. Three general characteristics were apparent: The predominant role of nervous and sen- reduced or limited development of many in- sory components in skull development has dividual elements; increased variability; and been confirmed in numerous experimental morphological novelty, particularly involv- studies which include a wide variety of ver- ing the jaw suspension and braincase. The tebrates, including fishes (Pinganaud-Perrin, first two characteristics were considered as '73), chicks (Coulombre and Crelin, '58; either direct or indirect consequences of trun- Schowing, '68b,c; Silver, '62; Simons, '79; Si- cated development or paedomorphosis in mons and Van Limborgh, '791, Thorius relative to larger generalized sala- (Burr, '16; Corson, '66; Leibel, '76; Richard- manders; little more will be said about them son, '32; Twitty, '32; Washburn and Detwiler, here. However, I will stress the relationship '43), and mammals (Moss, '61; Sarnat, '82; among altered proportions and distribution Young, '59). These studies share a primary of the brain and sense organs and the novel goal: identification of basic processes and aspects of cranial morphology. Three pri- mechanisms of vertebrate head morphogen- mary questions will be addressed: 1)What is esis. In vertebrate paleontology and compar- the relative size of the brain and sense or- ative morphology, appreciation of the gans in Thorius compared with those of "interactive" nature of head development larger salamanders? 2) How is packaging of has provided a very effective framework for these structures accommodated in a skull of analysis of skull evolution. For example, drastically reduced size? 3) Do modifications often drastic skull rearrangements that char- in the shape, size, andlor position of the brain acterize phyletic evolution in many mam- and sense organs impose or effect any struc- malian lineages have been interpreted as tural rearrangements of the surrounding direct consequences of alteration in the skull? shape, size, or orientation of the brain or Two implicit assumptions, both supported sense organs (DuBrul, '50; Van der Klaauw, by earlier studies (see above), underlie this '52; Radinsky, '68). Similarly, significant analysis. First, many prominent features of modifications in skull architecture that typ- adult skull morphology are a direct result of ify both the amphibian-reptilian transition physical (mechanical) interactions with the HEAD MORPHOLOGY AND EVOLUTION IN SALAMANDERS 257 brain and sense organs. Second, many phy- imens examined are listed in Hanken ('80) logenetic changes in skull morphology rep- and are deposited in the Museum of Verte- resent secondary responses of skeletal brate Zoology, University of California, elements to modifications which primarily Berkeley. involve changes in the relative size and/or Specimens were cleared and differentially position of nervous and sensory components. stained for bone and cartilage using the Al- A later paper will present detailed aspects of cian Blue-Alizarin Red S procedure (Dinger- brain and sense organ structure and function kus and Uhler, '77; Wassersug, '76) as (Grunwald, Hanken, and Roth, unpublished modified by Hanken and Wassersug ('81). observation). Skulls were photographed in dorsal view us- ing a dissecting microscope fitted with a MATERIALS AND METHODS photo tube (Wild M8S Zoom Stereomicro- Relative size and geometrical arrangement scope). Specimens were immersed in glycerin of the brain and each pair of sense organs and covered with a cover slip to stabilize were quantified in each of five adult females them. All skull photographs were printed to of three species of Thorius (mean snout-vent approximately the same size on 20 x 25 cm length, SVL, in mm, measured to the poste- photographic paper from which measure- rior end of vent, is in parentheses): T penna- ments of head proportions were made. tulus (19.21, T minutissimus (24.61, and T Relative sizes of the nasal capsules, eyes, narisovalis (28.7). These species collectively otic capsules, and brain were measured by span the range of mean adult body size found first drawing the outline of each structure (in in the genus, and thus would be expected to frontal plane) as it appeared on the photo- demonstrate any size-related interspecific graph (Fig. 1, left). The nasal capsule outline trends in head packaging. For uniformity, included all associated nasal cartilage visible only females were included. However, there dorsally. Otic capsule outline was defined by are no apparent sex-related differences in the clearly visible semicircular canals. head packaging other than those directly re- Braincase dimensions were substituted for lated to body size differences; in a given spe- those of the brain itself. Operationally, the cies of Thorius, mean SVL of adult females braincase outline was defined as a six-sided typically is slightly larger than that of adult polygon symmetric about the longitudinal males. Five adult females each of three larger skull axis with the following three, paired, plethodontid species, Chiropterotriton pris- corner reference points: 1)dorsal articulation cus (45.7), Plethodon vehiculum (49.01, and between the orbitosphenoid bone and poste- Pseudoeurycea goebeli (54.41, were used as rior nasal cartilage, 2) anterior articulation reference material for evaluating head pack- of the occipito-otic and parietal bones, and 3) aging in Thorius. These species possess a posterior articulation of the occipito-otic and relatively generalized plethodontid morphol- supraoccipital bones at the level of the syn- ogy that may be similar to that of the ances- otic tectum. Overall head outline was defined tral form from which Thorius was derived by the perimeter contributed in turn by the (Wake, '66; Wake and Elias, '83). Ontoge- premaxilla, nasal capsule, eye, mandible, netic comparisons in these species were based quadrate and squamosal (when visible), oc- on examination of five juvenile T narisovalis cipito-otic and occipital condyle. The area (SVL = 12.1-13.4,~= 12.6) and five juvenile (cm2) of each structure then was measured P goebeli (SVL = 15.1-48.6, z = 32.8). Spec- from the photographs with a Dietzgen Plan-

Abbreviations Anterior edge of nasal capsule NC, Nasal capsule Posterior edge of nasal capsule Oc, Occipital condyle Anterior edge of eye OC, Otic causule Posterior edge of eye P1, Plethoaon vehiculum Braincase Pm, Premaxilla Anterior edge of braincase Ps, Pseudoeurycea goebeli Posterior edge of braincase Q, Quadrate Chiropterotriton priscus SL, Skull length Anterior edge of otic capsule Sq, Squamosal Posterior edge of otic capsule T.m., Thorius macdougalli Eye T.n., Mandible T.P., 258 J. HANKEN ume. This condition is met for the nasal capsule, eye, and otic capsule, but brain shape differs markedly between Thorius and larger plethodontid genera such as Pseu- doeurycea (G. Roth, personal communication) and Hydromantes (Grunwald, ’81).However, the shape differences, particularly the in- crease in relative brain height in Thorius, are such that differences in relative brain area underestimate proportional differences in brain volume (Grunwald, Hanken, and Roth, unpublished observation). Linear distribution and overlap of the three sense organs and brain were quantified by drawing the median longitudinal skull axis on each photograph and extrapolating a line perpendicular to this axis from the anterior and posterior edges of the nasal capsules, eyes, otic capsules, and braincase (Fig. 1, right). Proportion (%) of skull length occupied by each structure was defined operationally as the length of the skull axis segment delim- ited by its anterior and posterior edges, di- vided by skull length (sl), multiplied by 100. Occipital condyle shape and position relative Fig. 1. Measurement techniques. Left: Outline of to the otic capsules are similar in all species braincase and sense organs used to measure the propor- tion of head area occupied by each. Dashed line denotes considered here, thus providing a standard- head outline used to obtain overall head area. Values ized landmark for the rear skull margin in were calculated for one side (usually left) and doubled to all comparisons. give total estimate in each skull. Right Photograph of stained skull indicating the respective anterior and pos- RESULTS terior edges of the braincase and sense organs extrapo- lated to the median longitudinal skull axis. Length and Relative size and distribution of the brain position of each structure were standardized to a skull and sense organs generally are the same in length of 100 units from the posterior edge of the occipi- comparisons among the three, larger refer- tal condyle (0) to the anterior edge of the premaxilla (100). Numbers identify the paired reference points used ence species; interspecific variation is slight to delineate braincase outline (see text). relative to the differences between each of these species and any of the three species of Thorius (Tables 1-3). For this reason, I will imeter no. 132. Proportion (%) of head area discuss in detail a comparison between a sin- occupied by the paired sense organs and gle reference species, Psuedoeurycea goebeli, brain was calculated by dividing the photo- and Thorius. All generalizations presented graph area occupied by each of these struc- are equally true for the remaining compari- tures by the corresponding estimate of head sons of both Plethodon and Chiropterotriton area, multiplied by 100. Absolute area (mm2) to Thorius. of the brain and sense organs was derived Head area using the formula, AREA = (SL/sl)’ x areap, in which SL = skull length (anterior edge of The eyes, otic capsules, and brain are pre- premaxilla to posterior edge of occipital con- dominant in Thorius (Table 1; Figs. 2A,B, 3). dyle) measured directly on the specimen, sl In dorsal view, the eyesoccupy 30-50% and = skull length measured on the photograph, the otic capsules 20-50% more head area and area = area measured on the photo- than in Pseudoeurycea. The brain, which is graph. To the extent that the shapes of the approximately 60% larger in Thorius, shows brain and sense organs, in transverse plane, the largest relative size increase. In contrast, are similar in Thorius and the larger genera, the nasal capsules are either the same rela- proportional differences in relative head area tive size, or smaller, in Thorius. Figure 4 occupied by these structures will reflect cor- depicts interspecific scaling of the brain and responding differences in relative head vol- sense organ area in Thorius and larger sala- HEAD MORPHOLOGY AND EVOLUTION IN SALAMANDERS 259

TABLE 1. Proportion (7%) of head area occupied hy the nasal capsules, eyes, otic capsules, and brain in Thorius and three other plethodontid genera' Species Nasal c. Eyes Otic c. Brain 111 pennatulus 14.0 (.3) 26.5 (.6) 17.0 (23) 36.9 (.5) T niinutissim.us 15.8 (.4) 23.4 (.6) 13.5 (.6) 36.5 (.2) T narisoualis (ad.) 16.7 (.7) 22.8 (.6) 13.6 (.4) 37.4 (.4) 2: narisovalis (iuv.) 14.5 (.5) 26.7 (.4) 15.7 (.7) 42.0 (.5) Pseudoeurycea goebeli 17.1 (.5) 17.5 (.7) 11.1 (.3) 23.1 (.7) Chiropterotriton priscus 15.9 (.6) 17.7 (.7) 11.1 (.4) 26.3 (.5) Plethodon uehiculum 15.2 (.7) 15.9 (.4) 11.9 (.5) 27.4 (.4)

'N = 5 for all samples; standard error is in parentheses.

TABLE 2. Proportion (%) of skull length occupied by the nasal capsules eyes, otic capsules, and brain in Thorius and other plethodontid genera'

Species SL Nasal c. Eyes Otic c. Brain 71 pennatulus 3.31 (.04) 27.5 (.4) 33.5 (.6) 36.5 (.8) 64.4 (.9) 111 minutissirnus 3.93 LO51 28.9 (1.0) 30.7 (.3) 33.8 (.4) 63.6 (.7) 111 narisoualis (ad.) 4.25 (.07) 28.9 (.7) 31.9 (.7) 34.7 (.9) 63.2 (.4) T narisoualis (iuv.) 2.53 (.06) 28.2 (3) 35.8 (.5) 35.5 (1.0) 67.6 (.7) Pseudoeurycea goebeli 10.58 (.16) 30.6 (.8) 26.7 (.6) 26.7 (.6) 59.8 (.4) Chiropterotriton priscus 7.84 (.27) 27.8 (.7) 27.3 (.4) 27.5 C.4) 61.6 (.6) Plethodon uehiculurn 7.53 (.14) 28.5 (.1) 25.7 (.3) 28.0 (.4) 59.4 (.4) 'N = 5 for all samples; standard error is in parentheses. SL, skull length in mm.

TABLE 3. Relative linear position' of the anterior and posterior edges of the nasal capsules (a, a'), eyes ib, h 7, otic capsules (c, c'), and brain (d, d'J in Thorius and other plethodontid genera Nasal capsules Eyes Otic capsules Brain Species a a' b b' C C' d d' 2: pennatulus 103.8 (.3) 76.3 (.3) 84.6(.5) 51.1 (.2) 38.7 (.4) 2.3 (.4) 79.0 (.4) 14.2 (.4) 2: minutissirnus 104.3 (.5) 75.4 (1.3) 83.7 (.7) 53.0(.6) 37.3 (.2) 3.5 (.4) 77.2 (.9) 13.6 (1.0) 7: narisoualis (ad.) 104.1 (1.1) 75.2 (1.2) 81.9 (.9) 50.0(.8) 37.8 (.7) 3.1 (.4) 79.2 (.5) 16.0 (.5) 111 narisoualis (iuv.) 103.5 (.9) 75.3 (.9) 83.4 (.9) 47.6 (.4) 40.0 (1.2) 4.5 (.4) 82.0 (.7) 14.4 (1.0) Pseudoeurycea goebeli 101.3 (.2) 70.6 (.6) 72.4(.6) 45.7(.7) 28.6 (.6) 1.9(.1) 72.8 (.6) 13.0 (.7) Chiropterotriton priscus 102.3 (.3) 74.5 (.9) 78.4(.9) 51.1 (.6) 30.5 (.7) 2.9(.3) 75.8(1.1) 14.2 (3) Plethodon uehiculurn 101.3 (.4) 72.9 (.4) 77.3 (.7) 51.6 (.4) 30.7 (.5) 2.7 (.1) 73.6 (.7) 14.2 (.9)

'In each specimen, linear position was standardized to a skull length of 100 units from the posterior edge of the occipital condyle (0) to the anterior edge of the premaxilla (100).N = 5 for all samples; standard error is in parentheses. Proportion (%I of skull length occupied by each structure (Table 2) equals a-a', b-b', etc.

manders. Slopes of the least squares regres- tion parallel to the posterior braincase walls. sion lines calculated from mean values for Brain shape change includes substantial in- adults of each species equal 1.05 (nasal cap- crease in width beginning at the level of the sule), 0.81 (eye), 0.85 (otic capsule), and 0.79 eyes and continuing posteriorly to a maxi- (brain); r for each comparison exceeds 0.99. mum at the anterior articulation with the Thus, eye, otic capsule, and brain size each otic capsule. scale with negative allometry with respect to Interspecific comparisons among adult skull length, whereas nasal capsule size Thorius demonstrate the same trends that scales isometrically or possibly with slight characterize the comparison of Thorius and positive allometry. larger genera: negatively allometric scaling Accompanying increased relative size of the of eye, otic capsule, and brain size; isometric brain, eye, and otic capsule in Thorius is a scaling of nasal capsule size. change of shape of each organ in dorsal view (Fig. 2A,C). The eye, which is more or less Linear distribution circular in Pseudoeurycea, is ovoid, with the Measurements of relative length mirror long axis perpendicular to the optical axis. those of relative area; eyes are 15-25%, otic Otic capsules are more elongate in the direc- capsules 27-37%, and the brain 6-8% longer 260 J. HANKEN

Fig. 2. Photographs of stained skulls. A, B. Thorius within. Scale bar = 1 mm. The quadrate and squamosal, pennatulus (M 5114; SL = 19.6 mm). C. Pseudoeurycea which together constitute the jaw suspensorium, are vis- goebeli (MVZ 130543;SL = 54.9 mm). A and C are dorsal ible in dorsal view in Pseudoeurycea (C) but not in Thor- views; B is a lateral view. The eyes in A and B are ius (A); they are visible in lateral view in Thorius (B). partially depigmented, revealing the prominent lens HEAD MORPHOLOGY AND EVOLUTION IN SALAMANDERS 261 i BRAIN 1 EYES

nw a I3 V V 0 20 U W (L U n Uw -I2345 I2 345 I c I OTlC C. -1 I234 5 -12345

Fig. 3. Proportion (%) of head area occupied by the horizontal bar; range, vertical bar; 95%confidence inter- nasal capsules, eyes, otic capsules, and brain in Pseu- val, rectangle. N = 5 for each sample. doeurycea goebeli and three species of Thorius. Mean, in Thorius than in Pseudoeurycea, whereas forward displacement of the anterior end of the nasal capsules are the same relative size the braincase; the rear braincase margin is or as much as 10% shorter (Table 2). Relative at approximately the same relative position eye and otic capsule length are larger, and in all skulls. The eye, however, both is signif- relative nasal capsule length is smaller in 71 icantly (19%)longer and lies further forward. pennatulus compared with both I: minutissi- The nasal capsules, although showing little mus and I: narisoualis; differences among change in relative length, also have been the latter two species are not significant (P displaced anteriorly with respect to the rear > .95). Relative brain length is approxi- skull margin (occipital condyles); in each mately the same in all three species of Thor- comparison of Thorius with Pseudoeurycea, ius. the posterior nasal capsule margin is shifted Examination of the relative position of each forward by as much as 6% of head length. structure reveals more subtle differences (Ta- The anterior margin, which typically over- ble 3, Fig. 5). Beginning at the rear of the hangs the skull only slightly in larger sala- skull, the substantial (30%)increase in rela- manders, protrudes well beyond the tive length of the otic capsule in Thorius is premaxilla (cf. Fig. 2A,C). achieved mostly by anterior expansion (the anterior border lies further forward). Simi- Ontogenetic comparisons larly, the moderate (6%)increase in relative Examination of juvenile I: narisovalis and length of the brain is achieved primarily by Pseudoeurycea goebeli provides information 262 J. HANKEN

h E Ps E A .’ -3 A/ N. / -a PI J c LLJ [L a 0: T.n. A a 2‘ z .. I..’ /. I I IIIII I I 1 I i Ill 5 10 5 10

h E c E 5 -. a w K a W> w I / ,I I I 1 I1111 5 10 SKULL LENGTH (mm)

Fig. 4. Allometric scaling of (A) nasal capsule, (B) otic squares regression line in each graph. Individual juve- capsule, (C) eye, and (D) brain area relative to skull nile specimens of Pseudoeurycea goebeli (A)and T narz- length in Thorius and larger salamanders. Scale is log- soualis (a)are also indicated. Regression slopes are log. Closed circles represent the mean value of five ad- discussed in text. ults of each species; these were used to compute the least

about the ontogeny of head packaging. In which have not diminished in proportion to both species, negative allometry of otic cap- the absolute dimensions of the head, occupy sule, eye, and brain size (area) contrasts with a greater proportion of head area and head the isometry or slight positive allometry of length in Thorius than in larger salaman- nasal capsule size (Tables 1, 2 Figs. 4, 5B,C). ders. In effect, the skull has shrunken around As a result, the predominance of the brain these relatively expanded nervous and sen- and eyes in tiny juveniles may be quite ex- sory components (Fig. 2A,B). Furthermore, treme: in some juvenile Thorius, the brain the geometrical packaging and arrangement occupies nearly 43% of head area. of the brain and sense organs suggest 1)space available to contain these structures is se- DISCUSSION verely limited; 2) there is “competition” for Head packaging head space among these structures, espe- Cranial miniaturization in Thorius is char- cially anteriorly; and 3) the predominant acterized by extreme reduction or loss of brain, otic capsules, and eyes have imposed much of the ossified skeleton, especially the structural rearrangements on much of the anterior elements, which typically is present skull that remains. in larger, adult salamanders (Hanken, ’83). In the occiput, otic capsule enlargement is In contrast, the otic capsules, eyes, and brain, achieved primarily by expansion anteriorly HEAD MORPHOLOGY AND EVOLUTION IN SALAMANDERS 263

together with the narrow squamosal, the A. P. goebeli 30.6 I I quadrate thus constitutes the jaw suspenso- SL= 10.6 mm NASAL C. 26.7 I rium (Fig. 2C). In Thorius, the quadrate maintains its conservative articulations with EYE 26.7 I both the otic capsule and lower jaw but, be- cause of lateral expansion of the hindbrain OTIC '' 59.8: I and otic capsule relative to the remaining BRAIN I skull and lower jaw, it is rotated to a nearly I 1 I I I vertical orientation beneath the otic capsule (Fig. 2A,B). This is seen clearly by comparing the two skulls illustrated in Figure 2 (A,C); 1 B. T nurisovdis (aq.1 28.9 I the jaw suspensorium is visible from dorsal SL=4.3mm I view in Pseudoeurycea, but is nearly hidden ,I 31.9 from dorsal view in Thorius. The significance 34.7 I of the reorientation of the jaw suspensorium I elements for mechanisms of jaw opening and I

63.2~~ closure is unknown. Rotation of the quadrate and squamosal I I I I represents only one of a suite of three fea- tures involving the jaw suspensorium which are unique to Thorius. The two remaining c. TI nurisovulis ( juv.) 28.2 features are 1) a posteriorly directed, bony, SL =2.5mm I squamosal process or spur, and 2) a shift in ' 35.8 the origin of the quadropectoralis muscle

I from the quadrate-as is typical of most 35.5 I plethodontid salamanders-to the squamo- I 67.6 sal, where it attaches to this process (Tanner, '52). I suggest that, as with the reorientation of the jaw suspensorium, these two features represent secondary consequences of in- 0 I 3 crease in relative size of the brain and otic SKULL LENGTH (Oh) capsule. The influence of muscle attachment Posterior Anterior in promotion of bone growth is well demon- Fig. 5. Schematic representation of the relative length strated (e.g., Washburn, '47); in Thorius, de- and position of the nasal capsule, eye, otic capsule, and velopment of a bony spur may represent only brain in (A) adult Pseudoeurycea goebeli and (B) adult a passive response of the developing squa- and (C)juvenile Thorius narisoualis. Relative length is expressed as a percentage of total skull length from the mosal to altered muscle patterning-quad- posterior edge of the occipital condyle (0) to the anterior rate to squamosal-effected by reorientation edge of the premaxilla (100). N = 5 for each sample. of the jaw suspensorium. Accordingly, the developing squamosal of other, closely re- lated plethodontid genera should have the (Fig. 5). This brings the anterior margin of potential for evoking this response, and a the otic capsule closer to the eye, but these posteriorly directed process has been de- structures remain well separated. Similarly, scribed in a single Oedipina complex (Wake, transverse broadening of the otic capsule is '66)although the origin of the quadropectora- effected by lateral expansion of the capsule lis muscle in this specimen is not known. from its origin on the posterolateral brain- Packaging of the eyes, nasal capsules, and case wall, but this apparently neither re- brain in Thorius must be discussed as one stricts nor is restricted by lateral expansion phenomenon because of mutual interactions of the hindbrain. Transverse broadening of among these structures in the anterior por- the hindbrain and otic capsule, however, has tion of the head. Relative size of the eyes and substantial effects on the configuration of brain both have increased, but most trans- lateral skull elements. In larger salaman- verse expansion of the brain is achieved pos- ders, such as Pseudoeurycea, the quadrate teriorly, behind the eyes (Fig. 2A,C). articulates on the lateral surface of the otic Anteriorly, lateral brain expansion appears capsule from which it descends ventrolater- to be limited by the eyes which, as seen in ally to an articulation with the lower jaw; dorsal view, overlap and intrude medially 264 J. HANKEN against the braincase. In response, anterior in the mechanical environment during devel- braincase walls deflect inwardly as they con- opment (Corsin, '66; Pinganaud-Perrin, '73; form to the margin of the eyes. (The accom- Silver, '62; and above). While such manipu- panying change in eye shape suggest that lations have not been performed on pletho- the eyes actually are pressing against the dontid salamanders, the diversity of braincase.) This results in a change in gross vertebrates which have been studied, com- shape of the braincase from the parallel-sided bined with the general similarity of results, structure that is typical of the skull of larger suggest that mechanical interactions are a salamanders in which the eyes are well sep- conservative characteristic of vertebrate de- arated from the braincase (Figs. 2C, 6). Cor- velopment that may be applied to analyses respondingly, location of the relatively longer of cranial evolution of plethodontid salaman- eyes more anteriorly may be ascribed to me- ders. This supports the interpretation of mod- chanical interactions with the anterior ifications in jaw suspensorium and braincase braincase walls which have displaced the eye morphology in Thorius presented above. forward. This, in turn, brings the eyes into Perhaps the most pertinent experiments contact and overlap with the nasal capsules are two that utilized a heteroplastic grafting which are likewise displaced further forward procedure in which sensory placodes were and now protrude conspicuously beyond the grafted between larvae of two species of am- anterior margin of the skull (Figs. 2A,C, 5). bystomatid salamanders, Ambystoma macu- Numerous experimental studies have dem- latum and the larger A. tigrinum. In such onstrated the degree to which the jaw sus- manipulations, the grafted organs typically pensorium, braincase, and other skull demonstrate great fidelity to their character- elements can be influenced by perturbations istic, i.e., species specific, growth rate and attain their normal adult size regardless of their new host environment. In this manner, the response of the developing host skull to I( ) the presence of an uncharacteristically small T. norisovoiis P. goebeli or large sense organ may be examined. Washburn and Detwiler ('43) made recipro- cal grafts of optic placodes and observed pro- nounced effects on the shape of the chondrocranium and the orientation of the nasal and otic capsules of the host skull. In hosts of both species, normal skull develop- ment was altered to accommodate the smaller or larger eye. Leibel ('76) trans- planted otic vesicles. When grafted into the presumptive otic region of A. maculatum, the presumptive otic vesicle of A. tigrinum at- tained its normal, larger size and, by moving the dorsal articulation of the quadrate lat- erally, effected a reorientation of the jaw sus- pensorium from a ventrolateral to a fully vertical displacement. Thus, by simply re- placing the normal otic vesicle with one of a larger growth rate and finite size, Leibel ex- perimentally produced a series of skeletal modifications that mimic those seen in the phylogeny of plethodontid salamanders. Functional considerations 0 In a consideration of the geometry of the vertebrate skull, Gans ('74) observed that Fig. 6. Schematic representation of braincase shape maintenance of skull functions (e.g., protec- (dorsal view) in adult Thorius narisovalis (M 3558; SL = tion of the brain, food acquisition, support of 4.25 mm) and Pseudoeurycea goebeli (MVZ 130531; SL the sense organs) becomes more difficult as = 10.8 mm). Braincase dimensions were standardized to the same skull length to illustrate relative braincase external dimensions are constrained. He sug- size, shape, and position in the two genera. gested that different structures will respond HEAD MORPHOLOGY AND EVOLUTION IN SALAMANDERS 265

differently to changes in scale in order to First, the periotic canal and periotic cistern maintain functional efficiency. In several are reduced greatly. Second the periotic sac, amphisbaenid reptiles, for example, reduc- which normally extends from the inner ear tion of skull diameter is associated with ex- into the braincase, is enlarged to such an treme modifications of skull architecture extent that it actually invades the occipital presumably to maintain rigidity demanded condyles from within (this condition has ap- by the dominant role of the head in burrow- peared independently in Batrachoseps, an- ing. The skull of another amphisbaenid, other genus of diminutive plethodontid Bipes, is widest at the level of the otic cap- salamanders). Last, the internal diameter of sules, which bulge inward against the lateral the semicircular canals is relatively great, walls of the braincase. Gans believed the otic confirming the predicted negative allometry capsules to be the limiting factor in skull between canal bore and overall size based on reduction in these because of the functional analysis of inner ear design (Jones need to maintain a certain minimum inter- and Spells, '63). nal diameter of the semicircular canals for It is important to remember, however, that their proper function as sensory acuity de- some increase in the relative size of the brain creases as the distance between these paired and sense organs is likely as a result of the structures is reduced (Gans, '60, '74; Jones extrapolation of conservative growth rela- and Spells, '63; see also Carroll, '70). Rieppel tionships to small head size. Negative allom- ('81) proposed analogous interpretations of etry of brain size relative to body size is several pronounced modifications in head typical of vertebrates generally (Bauchot, morphology that have accompanied extreme '78); Radinsky ('81)recently documented neg- size reduction in scincoid lizards; these mod- ative allometry for otic bulla volume and or- ifications include reduction and loss of post bit area in interspecific comparisons of temporal fossae and the upper temporal ar- carnivorous mammals. Similar trends are cade, and increase in the relative size of ad- seen in ontogenetic comparisons of Thorius ductor musculature, braincase, and otic and Pseudoeurycea (Fig. 4). Therefore, capsule. greater relative brain, eye, and otic capsule Predominance of the eyes, brain, and otic size may be expected in Thorius, with respect capsules of Thorius suggest that, here too, to larger plethodontid salamanders, even in these nervous and sensory structures are ap- the absence of functional considerations (see proaching their lower size limit for eEcient Cheverud, '82a, for a discussion of the com- skull function. This has been confirmed by plex relationship between ontogenetic and recent detailed anatomical studies of the eye interspecific allometry). Predominance of the and brain in which morphological differences brain, eyes, and otic capsules in Thorius between Thorius and larger salamanders likely represents both allometric effects and were interpreted as functional compensa- compensations that maintain functional effi- tions for the reduced size of these structures. ciency at reduced head size. In the eye, altered curvature of the inner surface of the retina apparently serves to Integrating mechanisms of skull maintain a minimum "working distance" be- development and skull evolution tween the retina and the relatively enlarged The developing skull is considered most lens which otherwise nearly completely fills appropriately as a complex of different tis- the vitreous cavity (G. Roth, personal com- sues that respond in a predictable way to the munication). Novel features of brain ultra- influence of neighboring nonskeletal struc- structure in Thorius include a more intricate tures; its growth should be interpreted in the neuronal branching pattern (increased arbor- context of overall head development (Moss, ization) in the optic tectum, and greater '68a, '7.2~).Of particular importance is the branching angle between adjacent dendrites dynamic aspect; cranial morphogenesis com- on a given neuron (Grunwald, '81; Grun- prises a sequence of interactions between tis- wald, Hanken and Roth, unpublished obser- sues and structures that differ in their vation). The most complete study yet response capabilities, which culminates in available, though, is that of Lombard ('77), the production of the complex adult structure who investigated the inner ear morphology (Blechschmidt and Gasser, '78; Horder, '81). of plethodontid salamanders. Lombard iden- This view of cranial morphogenesis can ex- tified several modifications of the inner ear plain both the high degree of regularity and of Thorius, some of which have appeared in- correlated growth (sensu Twitty, '32) that is dependently in other small salamanders. typical of skull development under normal 266 J. HANKEN conditions (Cheverud, '82b) and provides a fornia, Berkeley. There it was supported by mechanism for the relatively great struc- the Department of Zoology, Museum of Ver- tural alterations achieved with only slight tebrate Zoology, Center for Latin American experimental alteration (e.g., DuBrul and Studies, Sigma Xi, and NSF grant DEB- Laskin, '61; Leibel, '76). 7803008 to D.B. Wake. Completion of the But most important to the evolutionary study was partially supported by the Depart- morphologist, this view provides a mecha- ment of Biology and I.W. Killam Foundation nism to achieve the often radical, yet coordi- of Dalhousie University, Halifax, Nova Sco- nated, changes in head morphology that tia, Canada. Dr. Leonard Radinsky stimu- characterize much of vertebrate phylogeny lated my early interest in functional cranial (Davis, '64; Frazetta, '75). Thus, a relatively analysis and provided many valuable sug- minor, but genetically based, alteration in gestions for improvement of early drafts of the development of a given nonskeletal head this manuscript. Dr. David Wake shared his component (e.g., the intrinsic growth rate of insights into head packaging in Thorius and the eye) may initiate a series of compensa- made the initial suggestion that I pursue this tory responses in the surrounding skull, study. For additional support and advice I thereby effecting a change in overall head thank D. Eakins, W. Grunwald, B. Hall, T. morphology. A genetic change that directly Hetherington, E. Lombard, P. 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