Palatoquadrate

The palatoquadrates are well preserved in LACM 128126, although the right one (Fig. 1A) is more complete than the left one (Fig. 1C, D), and although their mesial and distal ends are missing. Both lingual and labial sides are observable on the right one, whereas only the labial side can be seen on the left one because the lingual side is adhered to the vertebral column and plaster jacket. The ventral rim is straight except toward the mesial end which forms a gentle concavity. If complete, the maximum mesiodistal length of each palatoquadrate is estimated to be about 30 cm. The mesiodistal length of the palatine process is approximately equal to that of the quadrate process (Fig. 2). The maximum dorsoventral height of the palatine process is located toward the symphysis measuring slightly over 4.5 cm, and its minimum height is situated near the quadrate process measuring about 3.5 cm. The quadrate process is dorsally expanded near its anterior end that measures about 10 cm in estimated maximum dorsoventral height, and gradually tapers off towards the posterior end. Although the dental bulla (Compagno, 1990; Siverson [1999] referred its inner side as ‘anterior hollow’) is inferred to be present at the mesial end of each palatoquadrate (Shimada, 2002a; see below), it is not clear in the specimen. However, a trough ( = ‘lateroposterior hollow’ of Siverson, 1999) is present along the ventral half of the lingual surface (Fig. 2) that originally housed replacement tooth series. The trough is dorsoventrally wide (3 cm) near the symphysis and gradually tapers off as it continues distally for at least 20 cm. The dorsal trough margin exhibits as many as 15 dorsally directed, closely spaced indentations (Fig. 2), representing the extent of juxtaposed tooth rows. Nine undeveloped teeth are also found along the dorsal trough margin (Fig. 2). Other areas of the palatoquadrate, including the labial side (Fig. 1A), are flat and featureless.

Meckel's Cartilage

Both Meckel's cartilages are incomplete in LACM 128126, where the right one (Fig. 1B) is fragmentary and appears to be adhered against the labial side of the left one. The left Meckel's cartilage (Fig. 2) measures 23 cm in preserved mesiodistal length, and was perhaps about 24 cm if complete. It is cleaver-shaped with a concaved dorsal edge and a gently convex ventral edge. The dorsoventral height of the cartilage as preserved measures from about 3 cm to 9 cm distally. The jaw surfaces are largely flat and featureless. Because the lingual surface is not observable in both sides, the presence of the dental bulla cannot be confirmed, but it was likely present (see below). No lower replacement teeth are recognized in situ.

Vertebrae

The vertebral column in LACM 128126 consists of 35 anterior-most vertebral centra, measuring approximately 82 cm in preserved length along the column axis (Fig. 1C). The vertebrae are mostly articulated and referred to as ‘vl’ through ‘v35,’ counting sequentially from the anterior-most centrum in the specimen (Fig. 1D). They are well calcified and structurally described as ‘lamnoid vertebrae’ (sensu Applegate, 1967), exhibiting concentric lamellae around the unperforated primary double-cone amphicoelous calcification (Fig. 3; see also Applegate, 1970). The centra are asterospondylic with many tightly spaced, radiating calcified lamellae (Fig. 3D; for terminology, see Ridewood, 1921; see below for further discussion). The lamellae also include the walls of a pair of circular to oval pits for the basidorsal and basiventral cartilages (see Welton and Farish, 1993) on opposite sides along the periphery of each centrum (Fig. 3C). All centra suffer taphonomic distortion, thus offering very few meaningful measurements. The diameter of the anterior face of the first vertebra, where it attached to the occipital centrum of the skull in life, measures about 35 mm. The least distorted vertebra is v9 (Fig. 3A, B), and it measures 40 mm in diameter and 16 mm in anteroposterior length.

Teeth (General)

Teeth in LACM 128126 exhibit considerable variation in size and shape. Total tooth heights range from 5.5 to 19.9 mm, and tooth widths from 5.5 to 20.4 mm. Some teeth are almost bilaterally symmetrical, whereas others are highly asymmetrical. Nevertheless, all fully developed teeth (e.g., Fig. 4) show a moderately thick, triangular principal cusp with a pair of broad, divergent, triangular lateral cusplets, a narrow tooth neck on the lingual side between the crown and root, a well-developed bilobed root, and osteodont tooth histology (see also Welton and Farish, 1993). The principal cusp and lateral cusplets possess a razor-like cutting edge and smooth surfaces. The bilobed root has a relatively low lingual protuberance generally with one small nutritive foramen (note: some lack the foramen whereas some have a few). Further dental descriptions are given below.

General Plan and Terminology

The dentition of Cretalamna appendiculata was previously reconstructed by Welton and Farish (1993) and by Applegate and Espinosa-Arrubarrena (1996) (Fig. 5A, B). Welton and Farish's (1993) reconstruction was based on a set of isolated teeth from multiple individuals. Applegate and Espinosa-Arrubarrena (1996) based their reconstruction on some previously reported associated (but disarticulated) tooth sets (e.g., Woodward, 1911; Arambourg, 1952; for additional comments, see also Siverson, 1999:59). Because both reconstructions are based on isolated and/or disarticulated teeth from more than one shark individual, they are considered artificial (sensu Shimada, 2006a) in which their accuracy remains questionable.

I use LACM 128126 to reconstruct the upper and lower dentitions of Cretalamna appendiculata. Because the tooth set consists largely of disarticulated teeth, the reconstruction must initially be conducted by assuming that C. appendiculata belongs to Lamniformes in order to allow some basis for comparisons. The tooth type is identified for each tooth, and 11 dental measurements (Fig. 4) are taken from the best preserved tooth for each tooth type (Appendix 1).

Tooth type terminology follows Shimada's (2002a) scheme which is based on the identification of putative homologous teeth across modern macrophagous lamniforms using 1:1 topographic correspondence of occluded tooth rows and non-dental anatomical markers. Siverson and Lindgren (2005) recently questioned Shimada's (2002a) reliance on skeletonized jaws by pointing out that the tooth positions of mesially located teeth may shift based on their observation of captive sand tiger shark, Carcharias taurus, swimming in an aquarium. As described, Shimada's (2002a) notion of ‘occlusion’ in lamniforms is interlocking upper and lower tooth rows upon jaw adduction. I note here that C. taurus has multiple series of functional teeth in the mesially located tooth rows (i.e., symphysial and anterior teeth), and such functional teeth may advance so labially on a curved jaw surface (by lingering long) that their occlusional relationships may not become obvious compared to functional and well-developed replacement teeth located lingually that would occlude with the opposing tooth rows. My experience with manipulating jaws of various modern lamniforms suggests that the extent of outward and inward rotations of each jaw cartilage appears to be greater in taxa with lanceolate (grasping) teeth such as Mitsukurina and Carcharias (vs. taxa with broad ‘cutting type’ teeth: e.g., see Shimada, 2005), but the occlusional relationships of teeth, particularly those that just became functional (erect), remain consistent because the position of symphysial and mandibular articulations of jaw cartilages is fixed. Jaw kinematics in modern lamniforms is an area of research clearly in need (e.g., see Motta and Wilga, 2001; Motta, 2004; Wilga, 2005). However, the use of Shimada's (2002a) tooth type terminology here is justified because the scheme rests on the only repeatable and the least subjective method for lamniform dentitions.

Shimada's (2002a) tooth type classification recognizes up to four tooth types in each upper and lower jaw quadrant in a macrophagous lamniforms that constitutes the so-called ‘lamnoid tooth pattern’: symphysial, anterior, intermediate, and lateral tooth rows. On each jaw, the scheme assumes the presence of two anterior and at least one intermediate tooth rows, whereas the number of symphysial and lateral tooth rows may vary interspecifically and intraspecifically. The scheme does not recognize the tooth type traditionally called ‘posterior teeth’ (e.g., Applegate, 1965), but they are assimilated into lateral tooth rows. The recognition of an intermediate tooth row in the lower dentition is a matter of convention, and one may consider it as the third lower anterior tooth row (Shimada, 2002a).

Reconstruction Procedures

Because of the presence of a large number of teeth in LACM 128126, the assumption is made that the tooth set includes teeth from all four jaw quadrants. Heavily damaged teeth and those not fully mineralized (i.e., indicated by undeveloped root) are excluded (Fig. 6Aa). Also, teeth that are quite different in size and morphology compared to the majority of teeth are set aside (e.g., putative symphysial teeth: Fig. 6Ab).

The inclination of principal cusp is used to divide the majority of teeth into two groups: one group consisting of upper right and lower left teeth (Fig. 6Ac), and the other group upper left and lower right teeth (Fig. 6Ad). Simultaneously, teeth in each group are organized according to their sizes and extent of cusp inclination. Teeth found to be virtually identical in size and morphology to each other are interpreted to come from the same tooth row and are thus lumped together. It should be noted that, in lamniforms, the organization of the mesial side of the dentition is more complex than that of the distal side, and the size and cusp inclination between adjacent teeth are more subtle in distally located teeth than in mesially located teeth (see Shimada, 2002a). Thus, teeth in each group are sequentially assembled from smaller, more inclined principal cusps (i.e., presumed distally located lateral teeth) to larger, more erect principal cusps (e.g., presumed anterior teeth).

For each of the two groups of teeth, subtle morphological variation indicative of differences between upper right teeth and lower left teeth (Fig. 6Ac) as well as between upper left teeth and lower right teeth (Fig. 6Ad) are detected. One of the two subgroups in each group consists of teeth with slightly mesiodistally narrower and labiolingually thicker crown compared to similar sized teeth in another subgroup. The former subgroup (Fig. 6Bd, f) is interpreted to be the lower teeth, the latter (Fig. 6Bb, e) the upper teeth according to a similar trend found in many modern lamniform teeth (e.g., Shimada, 2002a).

Some relatively large teeth (Fig. 6Ba, g) are found to be uniquely different from other ‘large teeth’ by exhibiting a strongly curved principal cusp. These teeth are not able to be placed into any part of the dentition except as putative intermediate teeth (see Shimada, 2002a). One unique situation with LACM 128126 is that testing whether or not they are intermediate teeth is possible, because several replacement teeth are in situ on the right palatoquadrate (Fig. 2). The putative upper right dentition (Fig. 6Bb) and putative intermediate teeth (Fig. 6Ba, g) are compared against the teeth on the palatoquadrate (Fig. 6C). The result is that one set of putative intermediate teeth (Fig. 6Ba) is found to be in fact from the row of upper right intermediate teeth, whereas the other set is determined to belong to the left dentition. Furthermore, despite of some necessary minor adjustments (e.g., one of the putative lower anterior teeth reidentified as an upper anterior tooth: i.e., right-most tooth in Figure 6Bd), much of the initial arrangement of upper right teeth (Fig. 6Bb) is found to be consistent with the dental pattern seen in situ on the palatoquadrate (Fig. 6C).

One last unresolved problem is to determine as to which jaw quadrant the two putative symphysial teeth originated. Shimada (1997a, 2002a) reported the dentition of a Cretaceous cretoxy-rhinid shark, Cretoxyrhina mantelli (Fig. 5D), and the two Cretalamna teeth are found to be similar to two of the four upper right symphysial teeth in C. mantelli. Siverson (1992) also noted that symphysial teeth (his ‘parasymphysial teeth’) of C. mantelli are similar to those of Cretalamna appendiculata. Thus, they are interpreted to be upper symphysial teeth in C. appendiculata.

Reconstructed Dentition

General Remarks

The reconstructed dentition (Figs. 5C, 7), which is partially corroborated with some teeth still in situ on the right palatoquadrate (Fig. 6C), implies that the shark had a lam-noid tooth pattern (sensu Compagno, 1984; Shimada, 2002a). At least 15 upper tooth rows were present on each side, consisting of at least two symphysial, two anterior, one intermediate, and at least 10 lateral tooth rows (Figs. 5C, 7). The lower dentition is poorly represented in LACM 128126, but it apparently consisted of more than eight tooth rows on each side including two anterior, one intermediate, and at least five lateral tooth rows (Figs. 5C, 7). In dental formula, the tooth arrangement can be expressed as:

where ‘S’ and ‘s’ refer to symphysial, ‘A’ and ‘a’ anterior, ‘I’ and ‘i’ intermediate, and ‘L’ and ‘l’ lateral tooth rows (sensu Shimada, 2002a).

The reconstructed dentition of Cretalamna appendiculata in relation to the length of the jaw cartilage (Fig. 7b) indicates that adjacent tooth roots abut each other and do not overlap. This tooth row arrangement is called ‘juxtaposed dentition’ (Welton and Farish, 1993). It is a form of ‘independent dentition’ (sensu Compagno, 1988: see Shimada and Cicimurri, 2005).

It is possible that the shark could have possessed more than two rows of upper symphysial teeth given that Cretoxyrhina mantelli had up to four upper symphysial tooth rows (see Shimada, 2002a). Because the smallest upper lateral tooth (L10) is much smaller than the smallest lower lateral tooth (15), and because the reconstructed lower dental series is much shorter than the upper dental series (Fig. 7), some post-15 teeth are likely not preserved in the specimen. The presence of teeth distal to L10 in the upper dentition is uncertain although it would have been only one or two more rows, if any, given the total length of the dental series in relation to the length of the palatoquadrate (see Fig. 7B).

Comparisons of Reconstructed Dentitions

My reconstructed dentition of Cretalamna appendiculata (Fig. 5C) differs from the artificial dentition proposed by Welton and Farish (1993) (Fig. 5A) and that by Applegate and Espinosa-Arrubarrena (1996) (Fig. 5B). Welton and Farish (1993) showed one intermediate tooth that was significantly smaller in size compared to its adjacent teeth (Fig. 5A). Whereas their reconstruction did not include any rows of symphysial teeth, I believe that their illustrated intermediate tooth is actually a symphysial tooth based on its size and morphology. It is uncertain whether or not an intermediate tooth is actually included in their illustration. On the other hand, Applegate and Espinosa-Arrubarrena (1996) showed three rows of symphysial teeth in both upper and lower dentitions (Fig. 5B). This interpretation is plausible considering the fact that lower teeth are especially poorly represented in LACM 128126, and that Cretoxyrhina mantelli (Fig. 5D) had four rows of upper symphysial teeth. Another noticeable difference is that the principal cusp of the intermediate tooth illustrated by Applegate and Espinosa-Arrubarrena (1996) is erect, whereas I found the principal cusp of the tooth to be strongly inclined distally. Welton and Farish (1993) and Applegate and Espinosa-Arrubarrena (1996), illustrated three rows of lower anterior teeth (without any rows of lower intermediate teeth) and multiple rows of posterior teeth, but the differences between my and their reconstructions in regards to these teeth are a matter of different tooth type nomenclatures used.

The reconstructed dentition of Cretoxyrhina mantelli (Fig. 5D) is based on an articulated tooth set (Shimada, 1997a), representing the most objectively reconstructed fossil lamniform dentition to date (vs. Welton and Farish, 1993; Applegate and Espinosa-Arrubarrena, 1996; Kent and Powell, 1999; Siverson, 1999; Gottfried and Fordyce, 2001; Purdy et al., 2001). The tooth set is more complete than that of Cretalamna appendiculata described here. The most noticeable differences in dentition between C. appendiculata (Fig. 5C) and Cretoxyrhina mantelli (Fig. 5D) is that the latter has more rows of upper intermediate teeth than the former. There are more symphysial teeth in C. mantelli (Fig. 5D) than Cretalamna appendiculata (Fig. 5C), but this may be simply due to poorer preservation in the examined C. appendiculata specimen.

The name ‘Cretalamna’ stands for ‘Cretaceous Lamna.’ Lamna is a Cenozoic lamniform genus that includes two dentally similar extant species, L. nasus and L. ditropis (for their subtle differences, see Nakaya, 1971). Figure 5E shows the traditional tooth type classification in which the third large lower tooth from the symphysis is called the third anterior tooth and the distal-most teeth in each dentition are referred to posterior teeth. Shimada's (2002a) tooth type classification scheme refers to the former as the lower intermediate tooth and the latter as the distal-most lateral teeth. Teeth of Lamna (Fig. 5E) differ from teeth of C. appendiculata (Fig. 5C) by possessing a narrower principal cusp and lateral cusplets. However, whereas the upper intermediate tooth is much smaller in Lamna compared to that of C. appendiculata, it is noteworthy that the overall dental pattern of C. appendiculata is more similar to the dental pattern of Lamna than to that of Cretoxyrhina mantelli (Fig. 5D). Although lower symphysial teeth were not found in LACM 128126, it is also intriguing that Glickman and Dolganov (1988) documented a specimen of modern L. ditropis with a row of small symphysial teeth, similar to the identified symphysial teeth of Cretalamna appendiculata and Cretoxyrhina mantelli. As in the traditional view (e.g., Cappetta, 1987), the observed differences in tooth morphology between Lamna and Cretalamna appendiculata may indicate that C. appendiculata is not phylogenetically closely allied to Lamna within Lamniformes. If so, the observed resemblance in dental pattern between the two taxa is homoplasious due to convergent evolution. However, the tooth morphology and the number of intermediate tooth rows are significantly different between C. appendiculata and Cretoxyrhina mantelli, thus warranting the possibility that Cretalamna appendiculata does not belong to Cretoxyrhinidae as proposed by Siverson (1999).

Jaw Morphology

In Cretalamna appendiculata, the Meckel's cartilage is found to be mesiodistally shorter than the palatoquadrate. Whereas the posterior end of the upper and lower jaws met as a mandibular articulation, this condition implies that the anterior margin of the upper jaws extended forward beyond that of the lower jaws. It concomitantly implies that C. appendiculata had a subterminal mouth as the upper jaws overbit the lower jaws. However, especially because the symphysial region of the palatoquadrates is poorly preserved in LACM 128126, exactly how the jaws were suspended from the skull is uncertain for the species (e.g., see Wilga, 2005).

The shape of jaw cartilages in sharks is generally difficult to describe because the cartilages consist of curved surfaces with only few anatomical landmarks available. However, it is noteworthy that the overall outline of both upper and lower jaws in Cretalamna appendiculata resembles that of modern alopiids (Alopias) and lamnids (Lamna, Isurus, and Carcharodon; e.g., see Wilga, 2005) as well as Cretoxyrhina mantelli (see Shimada, 1997b). The jaw morphology in these lamniforms is characterized by a dorsally expanded quadrate process in palatoquadrates and a ventrally expanded posteroventral corner in Meckel's cartilage, indicating the presence of a set of massive dorsal and ventral quadratomandibularis muscles (sensu Wilga, 2005) for powerful jaw adduction. The morphology contrasts with that in the basal most lamniform taxon, Mitsukurinidae (Cappetta, 1980; Wilga, 2005), as well as in Carcharias taurus, where jaw cartilages are elongated anteroposteriorly with a restricted space for the attachment to the dorsal and ventral quadratomandibularis muscles (see Wilga, 2005). The difference is likely the reflection of the function of teeth. Teeth in alopiids, lamnids, and Cretoxyrhina, are relatively broad and have well-marked mesial and distal cutting edges suited for cutting (Shimada, 2005). On the other hand, teeth in mitsukurinids and Carcharias are narrow and delicate, which are suited for grasping (Shimada, 2005). The fact that the jaw morphology of Cretalamna appendiculata resembles that of modern alopiids and lamnids does not contradict to this observation because C. appendiculata has broad teeth with prominent mesial and distal cutting edges on both the principal cusp and paired lateral cusplets indicative of cutting function (for further discussion on feeding mechanics, see below).

Body Form

The body form of all modern lamniforms is fusiform (Compagno, 2001) but can be broadly classified into two categories: (1) a body with a conical head, lateral fluke on the caudal peduncle, and lunate caudal fin (e.g., Cetorhinus, Carcharodon, Isurus, and Lamna); and (2) a body with a head that has a flattened ventral surface, no lateral fluke on the caudal peduncle, and elongated upper lobe of the caudal fin (e.g., Carcharias and Alopias) (Thomson and Simanek, 1977). The skeletal elements preserved in LACM 128126 do not show direct evidence indicating the morphology of the head, caudal peduncle, and caudal fin of Cretalamna appendiculata. However, given the fact that all modern lamniforms have fusiform body, it is reasonable to assume that C. appendiculata also had a fusiform body. Although a broad survey of correlating the vertebral shape with the body form in elasmobranchs is needed, it is true that modern pelagic sharks with a fusiform body have circular vertebral centra, whereas at least some true benthic sharks, such as Squatina, have dorsoventrally compressed vertebral centra (see Applegate, 1967; Compagno, 1977). The centra in C. appendiculata are circular, and therefore the observation does not contradict with the interpretation that C. appendiculata possibly had a fusiform body.

Extant lamniforms that do not rely on teeth for prey capture, such as Megachasma, Cetorhinus, and Alopias, tend to possess smaller teeth relative to the jaw size (Shimada, 2005). Whereas Megachasma and Cetorhinus are filter-feeding planktonivorous sharks, Alopias spp. are macrophagous sharks but differ from other macrophagous lamniforms in their feeding habit in that they use their elongated tail as a stunning device to assist prey capture (e.g., Gubanov, 1972; Stillwell and Casey, 1976). Although the exact body form of Cretalamna appendiculata can only be speculated at the present time, the tooth size in C. appendiculata is prominent in relation to the size of its jaws indicating that teeth were important for prey capture for the fossil shark. This contention concomitantly implies that the upper lobe of its caudal fin was not needed to be elongate. Perhaps, it was moderately elongated as in modern Carcharias or was as short as in Cretoxyrhina (see Shimada, Cumbaa et al., 2006) and modern Cetorhinus, Carcharodon, Isurus, and Lamna (see Thomson and Simanek, 1977).

Body Size

LACM 128126 is a fragmentary skeleton. However, some inferences about the total length (TL) of the Cretalamna appendiculata individual in life can be made based on the diameter (40 mm) of the best preserved vertebra (v9), which is one of the largest centra in the vertebral column. One possible approach is to compare the diameter with the previously published data on the relationship between the TL and vertebral diameter (VD) in Cretoxyrhina mantelli (Shimada, 1997b). The largest centrum measures 87 mm in VD in the best skeleton of C. mantelli, which has a conservative TL estimate of 500 cm (Shimada, 1997b). Because the largest, least distorted vertebra in LACM 128126 (40 mm in VD) is 46% of the C. mantelli vertebra, the TL of the Cretalamna appendiculata individual is conservatively extrapolated to be approximately 230 cm.

Another possible approach is to examine how the vertebral size of Cretalamna appendiculata fits to known quantitative relationships between vertebral size and TL in extant lamniform taxa. Appendix 2 lists published equations correlating either the vertebral diameter (VD) or vertebral radius (VR) with the TL in modern lamniforms. Then, I mathematically estimated the TL (‘TL_est’ in Appendix 2) based on each equation using the VD of 40 mm (which reflects the diameter of the best preserved vertebra in LACM 128126) or the VR of 20 mm (which is half of the VD). The results (Appendix 2) indicate that the TL estimation for LACM 128126 depends on the inference about the length of the caudal fin. Notably, Alopias spp., which have a caudal fin nearly the same length as the pre-caudal length, give TL estimations that exceed 4 m. However, because the upper lobe of the caudal fin in C. appendiculata was probably not as elongate as in modern Alopias (see above), the fossil shark was probably much less than 4 m TL. The TL estimations for non-Alopias taxa range from 2.5 to 3.8 m (Appendix 2), and the mean of the mean TL estimations among these lamniforms is about 3 m. Given the fact that the conservative TL estimation for LACM 128126 based on Cretoxyrhina mantelli is 2.3 m, it is likely that the Cretalamna appendiculata individual measured 2.3–3 m TL.

Cretalamna as a Lamniform

On the basis of tooth morphology, which is characterized by a large, triangular principal cusp, a large bilobed root, and osteodont histology, Cretalamna appendiculata has been classified into the order Lamniformes (e.g., Cappetta, 1987). Applegate (1970) illustrated a transverse cross section of vertebra of C. appendiculata, which revealed numerous well-calcified radial cartilage lamellae radiating from its center, and LACM 128126 confirms the presence of such radial lamellae (Fig. 3D). Compagno (1990:370) and Shirai (1996:18) described this feature as “primary exochordal radii” and “endochordal radii radiating from the notochordal sheath,” respectively, and regarded as one of a few synapomorphies of Lamniformes, even though this character also occurs parallel in non-parascylliid orectolobiforms (Shirai, 1996). Whereas there is no indication of C. appendiculata belonging to Orectolobiformes, the presence of radiating lamellae in vertebrae coupled with the general dental morphology strongly indicates that C. appendiculata is a lamniform.

The monophyly of Lamniformes is supported by genetic data (Martin and Naylor, 1997; Naylor et al., 1997) as well as by a few morphological characters (e.g., Shirai, 1996). Besides the radial lamellae in vertebrae, Compagno (1990) listed three other morphological synapomorphies of Lamniformes: (1) the lamnoid tooth pattern, (2) reduction of labial cartilages, and (3) elongated ring type intestinal valve with over 15 turns. Shirai (1996) and Carvalho (1996) recognized the ring type intestinal valve as a synapomorphy of Lamniformes but not the others. However, Carvalho (1996) noted the presence of dental bullae on the jaw cartilages as one of three potential, additional synapomorphies of Lamniformes (note: other two characters are lateral rostral fenestrae and rostral apex). Subsequently, Shimada (2002a) regarded the presence of upper and lower dental bullae as a shared derived character of Lamniformes with the assumption that (1) the dental bullae in the two microphagous forms, Megachasma and Cetorhinus, are secondarily lost, and that (2) the presence of dental bullae in Hemipristis elongatus (Carcharhiniformes) is a result of convergence.

The presence of dental bullae is directly related to the presence of the lamnoid tooth pattern (Shimada, 2002a). Shimada (2002a) noted that the dental bullae are more definable than the lamnoid tooth pattern but also added that listing both the dental pattern and bullae as two separate synapomorphies for Lamniformes would be redundant. The reconstructed dentition of Cretalamna appendiculata (Fig. 5C) revealed that the fossil taxon had the lamnoid tooth pattern. The reconstruction was initially conducted under the assumption that C. appendiculata belongs to Lamniformes. However, it must be emphasized that the presence of the lamnoid tooth pattern is confirmed independently on the basis of replacement teeth preserved in situ on the right palatoquadrate (Fig. 6C). Therefore, postulating that Cretalamna appendiculata had a lamnoid tooth pattern is no longer a circular argument but an empirical observation.

Shimada (2002a) noted the probable presence of the dental bullae in the palatoquadrates of a Cretaceous mitsukurinid, Scapanorhynchus lewisii, based on illustrations presented by Davis (1887: plate 14) and Cappetta (1980: plates 8–10). However, probably because the jaw cartilage, particularly the part that makes up the dental bulla, is thin and is likely difficult to be fossilized, dental bullae have not been recorded in other fossil lamniforms, including the Cretalamna appendiculata individual described here. Nevertheless, the close proximity between the replacement tooth of the A2 (pointed by the left-most line in Figure 6C) and that of the I1 (pointed by the second line from the left in Figure 6C), compared to the distance between the other replacement teeth in situ on the jaw, appears to reflect the ‘crowded’ tooth rows at the mesial end of the palatoquadrate. This condition appears to indicate the presence of a dental bulla (e.g., see Shimada, 2002a:fig. 3). If so, C. appendiculata had a dental bulla on each palatoquadrate.

The functional diversity of feeding mechanism in elasmobranchs is large, but their feeding apparatus is surprisingly simple by consisting of only 10 cartilaginous elements (Motta, 2004). Therefore, one can argue that the skeletal evolution is conservative in elasmobranchs. Yet, an enormous morphological diversity seen in teeth of extinct and extant elasmobranchs (e.g., Compagno, 1984; Cappetta, 1987) indicates that their dental morphology readily responds to selection pressures present in their environment. In his study of correlating the osteological data with the soft tissue anatomy in various archosaurs, Witmer (1997:2) called attention to Moss's (1968) phrase ‘soft tissues evolve, bones respond’ in order to highlight the notion that “bones must be studied in concert with their associated soft tissues.” I take a similar approach by stating that ‘teeth evolve, jaw cartilages respond’ with the notion that jaw cartilages must be studied in concert with their associated dental components. Specifically, the presence of dental bullae is assumed to be associated with the lamnoid tooth pattern. This notion justifies the inference about the presence of the dental bulla in each jaw cartilage of Cretalamna appendiculata on the basis of Witmer's (1997) ‘extant phylogenetic bracket approach.’

The extant phylogenetic bracket approach uses a phylogeny of a monophyletic group of extinct and extant taxa to infer unfossilized features in extinct taxa from anatomical correlates universally found in taxa represented in the cladogram (Witmer, 1997). The systematic relationship among lamniform taxa is still in debate due to conflicting results particularly between morphological (Compagno, 1990; Shirai, 1996; Shimada, 2005) and molecular (Naylor et al. 1997) studies. Nevertheless, Figure 11 shows a consensus tree among those studies, and Cretalamna is tentatively inserted as a distinct taxon with more derived features (e.g., cutting type teeth: see Shimada, 2005) than in Mitsukurinidae but not belonging to Lamnidae (see above) or the filter-feeding Cetorhinidae. The lamnoid tooth pattern (denoted as character ‘A’ in Figure 11) is present in all extant lamniforms except the only two filter-feeding taxa, Megachasmidae and Cetorhinidae (note that one of the alopiid species, Alopias super-ciliosus, appears to possess the lamnoid tooth pattern only during its embryonic stage: Shimada, 2002a). Likewise, the dental bulla (denoted as character ‘B’ in Figure 11) is present in each upper and lower jaw cartilage in all extant lamniforms except Megachasmidae and Cetorhinidae (note that dental bullae are present in Alopias superciliosus: Shimada, 2002a). Assuming that the absence of the lamnoid tooth pattern and dental bullae in Megachasmidae and Cetorhinidae is a secondary loss due to its specialization in planktonic feeding (Shimada, 2002a, 2005), the extant phylogenetic bracketing dictates the proposition that C. appendiculata that had the lamnoid tooth pattern likely possessed dental bullae because all extant macrophagous lamniforms possess the lamnoid tooth pattern and dental bullae.

It is noteworthy that another fossil lamniform, Cretoxyrhina mantelli, was found to possess a lamnoid tooth pattern on the basis of an articulated tooth set (Shimada, 1997a), but there was no direct evidence for the presence of dental bullae (Shimada, 1997b, 2002a) although Siverson (1999) inferred that it had the bullae (his ‘anterior hollow’) based on observed ‘crowding’ of mesially located tooth rows. However, if one replaces ‘Cretalamna’ in Figure 11 with ‘Cretoxyrhina,’ the same extant phylogenetic bracketing indicates that C. mantelli must have had a dental bulla in each jaw cartilage. There are other fossil lamniforms with tentative reconstructed dentitions (e.g., Welton and Farish, 1993; Applegate and Espinosa-Arrubarrena, 1996; Kent and Powell, 1999; Siverson, 1999; Gottfried and Fordyce, 2001; Purdy et al., 2001). However, it must be noted that the extant phylogenetic bracket approach cannot be employed for these extinct sharks, because their reconstructed dental patterns are artificial based on one or more isolated or disarticulated tooth sets (sensu Shimada, 2006a) using the dental pattern of extant lamniform sharks as templates. For extinct lamniforms to be incorporated into the extant phylogenetic bracketing study, the presence of the lamnoid tooth pattern must be substantiated first by direct evidence.

Paleoecology

LACM 128126 occurred in the uppermost part of the Smoky Hill Chalk Member of the Niobrara Chalk. The stratigraphic member is 180 m thick and formed under the Western Interior Sea, an epicontinental sea that extended in a north-south direction in the middle of the North American continent over about five million years (Obradovich and Cobban, 1975, p. 50). The Smoky Hill Chalk is rich in fossil vertebrates comprising at least 16 chondrichthyan and 54 osteichthyan fish taxa (Shimada and Fielitz, 2006) as well as various tetrapod taxa including at least six marine turtles, 10 squamates (one dolichosaur and nine mosasaur taxa), four plesiosaurs, three pterosaurs, seven birds, and three non-avian dinosaurs (see table 13.1 of Everhart [2005], which is based on Russell [1988], Stewart [1990], and Carpenter [2003]; Shimada and Bell, 2006). However, it must be noted that the ‘Niobrara fauna’ is a time averaged assemblage in which not all listed species coexisted at any given point of time during the deposition of the rock member (Stewart, 1990; Shimada and Fielitz, 2006).

The 16 chondrichthyan taxa recognized in the Smoky Hill Chalk include Cretalamna appendiculata, but remains of this species are not common in the stratigraphic member particularly compared to some other shark taxa such as Cretoxyrhina and Squalicorax (Shimada and Fielitz, 2006). Williston (1900a, p. 37; 1900b, p. 247) was the first to suggest the occurrence of Cretalamna appendiculata in the Smoky Hill Chalk of Kansas, but no specific specimens of this species were recorded (see also Schultze et al, 1982). To date, only two specimens of C. appendiculata from the stratigraphic member are known in literature besides LACM 128126 (Shimada and Fielitz, 2006). They are both from Gove County and are housed in Fort Hays State University, Sternberg Museum of Natural History (FHSM), Hays, Kansas: FHSM VP-14851, an isolated tooth that occurred 3 m below Hattin's (1982) lithostratigraphic Marker Unit 4, and FHSM VP-14852, another isolated tooth occurred at Marker Unit 7 (Hamm et al., 2003). Whereas LACM 128126 is an early Campanian specimen (see above), these two FHSM specimens substantiate the occurrence of the species from the late Coniacian to mid-Santonian parts of the Smoky Hill Chalk. Everhart's (2005, table 13.1) observation also indicates that C. appendiculata occurs throughout the rock member.

The occurrence of Cretalamna appendiculata in the Smoky Hill Chalk is a small part of the fossil record of the species in Kansas. Its occurrence spans almost the entire range of Late Cretaceous marine deposits including the Dakota Sandstone (Everhart et al., 2004), Graneros Shale (Shimada, 1996), Lincoln Limestone (Liggett et al., 2005), Fairport Chalk (Hattin, 1962: 54), Blue Hill Shale (Shimada, 2006b), Codell Sandstone (Hattin, 1962:97), Fort Hays Limestone (Shimada 1996), and Sharon Springs Shale (Carpenter, 2003). These rocks, including the Smoky Hill Chalk, span chronostratigraphically from the middle Cenomanian to the middle Campanian (Kauffman et al., 1993), and represent diverse depositional settings from near shore (e.g., Dakota Sandstone and Codell Sandstone) to offshore environments (e.g., Lincoln Limestone, Fairport Chalk, and Smoky Hill Chalk) (Hattin et al., 1987).

Globally, Cretalamna appendiculata is known from marine rocks that are Albian (Early Cretaceous)—Ypresian (Paleocene) in age (e.g., Cappetta, 1987). This stratigraphic range makes C. appendiculata one of the longest persisting lamniform species in the geologic record that occurs across the Cretaceous-Tertiary (K/T) boundary (e.g., see Cappetta, 1987). Its success reflected in its long temporal distribution may be in part due to its ability to inhabit in a wide range of environmental settings. Like the fossil record in Kansas, C. appendiculata is found in various marine rocks representing diverse depositional environments (for list of references, see Shimada, Schumacher et al., 2006).

Cretalamna appendiculata was probably an ecological generalist that may have also contributed to its long temporal range. Its teeth are mostly broad and are equipped with sharp cutting edges along the principal cusp and lateral cusplets, which are suited for cutting. Although adjacent teeth are juxtaposed without any spaces, a broad root with a pair of wide lateral cusplets offers a large space between the principal cusps of adjacent teeth. Such spacing, which ‘broke up’ the dental series, probably promoted an effective penetration of the principal cusps into a food item. Unfortunately, no direct evidence (e.g., tooth marks or embedded teeth) for the diet of C. appendiculata are known in the fossil record, unlike the record of some contemporaneous sharks such as Squalicorax spp. and Cretoxyrhina mantelli (Druckenmiller et al., 1993; Schwimmer et al., 1997; Shimada, 1997c; Everhart, 2004, 2005; Shimada and Everhart, 2004; Shimada and Hooks, 2004). However, because the size and morphology of the jaws and teeth of C. appendiculata are similar to the modern Lamna spp. which commonly feed on small to medium-sized bony fishes (Compagno, 2001), it is plausible that the primary diet of C. appendiculata was also small to medium-sized bony fishes. Whereas the estimated TL of 2.3–3.0 m makes C. appendiculata a ‘medium-sized shark’ and is typical for extant lamniforms (e.g., Compagno, 2001), small to medium-sized bony fishes that were likely abundant in seas inhabited by the shark (e.g., Shimada, Schumacher et al., 2006), are considered generalized diets in sharks (e.g., Compagno, 1984). On the other hand, although Cretoxyrhina mantelli was successful during the Cenomanian through early Campanian, its body size reaching about 7 m TL (Shimada, Cumbaa et al., 2006) with a set of massive teeth (e.g., Shimada, 1997a) could have made this species prone to the changing selection pressure in its evolving environments. Likewise, Squalicorax spp. were successful throughout the Late Cretaceous, but the last species in the genus, S. pristodontus, was a massive shark (at least 3 m TL: Shimada and Cicimurri, 2005) with a highly specialized dentition (highly serrated teeth arranged like a saw blade) that could have contributed to its demise at the end of the Cretaceous. However, the exact reason for the extinction of C. appendiculata during the Cenozoic is uncertain, although it may be due to competition against many other generalized sharks, such as carcharhiniforms that became dominant in the oceans worldwide around that time (e.g., Cappetta, 1987).