A new fossil dolphin with tusk-like teeth from New Zealand and an analysis of procumbent teeth in fossil cetaceans
ABSTRACT
Studies involving anatomical description and taxonomy of fossil odontocetes offer insights into their evolutionary history and diversity. This study analyses tusk-like teeth in odontocetes including the description of a new species, Nihoroa reimaea, from the Waitaki valley, North Otago, New Zealand. Dental features of Nihoroa reimaea, a gracile, longirostrine odontocete with procumbent tusk-like anterior teeth and slightly denticulate cheek teeth, are described in detail. A comparative analysis of tusk-like teeth from New Zealand specimens and from elsewhere in the world was performed allowing a classification of tusk-like teeth in odontocetes and highlighting the differences between true tusks and rooted procumbent teeth. Correlation analyses revealed significant associations between rostrum proportions and tooth crown morphology. This study contributes to the understanding of tusk-like teeth and illuminates their significance in odontocete evolution. Nihoroa reimaea expands our knowledge of fossil cetaceans and highlights the importance of New Zealand's exceptional fossil record of odontocetes with tusk-like teeth.
Introduction
The study of fossil cetaceans provides valuable insights into the evolutionary history and diversity of these wide-ranging marine mammals. Fossil remains offer a unique opportunity to investigate the anatomy, phylogenetic relationships, and ecological adaptations of ancient cetaceans. New Zealand, in particular, has an abundance of such material (Robinson under review) and the collection of the University of Otago Geology Museum is rich in late Oligocene basal odontocetes (Fordyce 1994; Tanaka and Fordyce 2014, 2015a, 2015b, 2016, 2017; Tanaka et al. 2022; Coste et al. 2023). The Oligocene is an important period in cetacean evolution as it represents a transitional period in odontocete evolution with an increased diversity of species and morphologies (Fordyce and de Muizon 2001; Coombs et al. 2022; Bennion et al. 2023; Roston et al. 2023). In this paper OU 22162, a new species of Oligocene basal odontocete, is described and insights into its taxonomy, anatomy, and evolutionary significance are discussed. This new species further expands the diversity of tusk-like teeth in Cetacea which prompted the opportunity for an initial categorisation of tusk-like teeth to shed light on their morphological variation and potential evolutionary patterns.
Procumbent tusk-like teeth are observed in several fossil odontocetes worldwide (Kellogg 1923, 1927; Fordyce 1994; Tanaka and Fordyce 2014, 2015b; Fitzgerald 2016; Albright et al. 2018; Boessenecker et al. 2020; Coste et al. 2023). Historically there has been no precise terminology for these teeth which are commonly referred to as tusks. These tusk-like teeth exhibit unique characteristics, such as procumbence and extra-oral positioning, whilst having long closed roots, which sets them apart from other teeth such as the true ever-growing tusks of narwhals (Monodon monoceros) (Nweeia et al. 2009, 2012; Nasoori 2020).
Elucidating the definition, classification, and phylogenetic significance of tusk-like teeth in odontocetes is crucial for understanding their evolutionary significance and ecological implications. In this study we focused on the incisors or first erupted teeth in the upper jaws. This paper explores the association between different shape and rostral proportions and procumbent tusk-like teeth crowns focused on the diverse New Zealand ‘tusked’ odontocetes and describes a new species of Oligocene odontocete from NZ with tusk-like teeth.
Methods
Preparation, anatomical description and illustration
Mechanical preparation was carried out by A. Grebneff using pneumatic chisels and hand tools under a Zeiss SR binocular microscope. Anatomical terminology follows Mead and Fordyce (2009) and Cozzi et al. (2017). Specimens were photographed with NikonD90, D700 and D800 cameras with 60 and 105 mm macro lenses and coated with sublimed ammonium chloride before photography when practicable.
Phylogenetic analysis
The phylogenetic analysis used the Coste et al. (2023) matrix. The matrix contained 98 operational taxonomic units (OTUs) and 284 characters. Matrix data, list of characters and unpruned tree can be found in the Supplementary materials. Mesquite 3.61 (Maddison and Maddison 2008) was used to handle character and tree data. Analyses were carried out in TNT version 1.5 (Goloboff and Catalano 2016), where data was constrained with a set tree of extant taxa, based on the topology of the McGowen et al. (2009) molecular phylogeny. All characters were treated as having equal weights and unordered, and used the New Technology Search with settings to recover the minimum length trees 1000 times. The consensus tree was obtained using the majority rule with a 50% cut off.
Procumbent teeth characterisation: data collection and measurements
The characterisation of procumbent teeth was performed to elucidate features that define tusks and tusk-like teeth, and how skull and rostral characteristics associate with dental attributes. Specimens were collated from publications using the words ‘procumbent’ and ‘tusk’, as well as from references to other specimens within those. Specimens with procumbent teeth from the University of Otago Geology Museum (OU) collections formed the core of the analyses. In total, 67 specimens were included; 17 specimens with tusk-like teeth from New Zealand, 16 specimens with tusk-like teeth from elsewhere, 2 specimens with true tusks, and 32 tuskless odontocetes ().
Table 1. List of specimens measured and details on the source of the images used of extant or fossil specimens, and tooth type measured.
Measurements () were obtained using ImageJ 1.52t from photographs and published figures and are given to the closest 0.5 mm. The angles of insertion of the teeth were calculated using the length of the crown and parallel to the rostrum when in-situ, or the length of the root ex-situ.
Figure 1. Photographs of Nihoroa reimaea skull with marked out measurements. Loose incisors from OU 22262.
A series of Pearson’s correlations were carried out in IBM SPSS Statistics version 29 (IBM Corp 2022) on rostral/dental data so their relationships can be interpreted (Lund and Lund 2018). Correlation strength was determined using Cohen (1988). To remove size effects, all length-based variables were presented as ratios or percentages – the only untransformed variable was the angle of the teeth. Monodon and Odobenocetops were excluded from analyses involving the proportion of crown and rostral length as their extreme values skewed the analyses.
An additional comparison made was the relationship between the angle of the crown to the rostrum and the root. However, very few of the extant specimens had any photographed ex situ incisors or other teeth available. Scatter plots were generated in IBM SPSS Statistics version 29 (Figure 4 and supplementary materials).
Results
Systematics and description
Cetacea Brisson, 1762.
Odontoceti Flower, 1867.
Nihoroa reimaea, gen. et. sp. nov.
Holotype
OU 22162 is a near-complete skull missing its left nasal, both lacrimojugals and pterygoids. Both tympanoperiotics and ossicles are present other than the right stapes; the right earbones are in place in the skull, the left was removed to allow further description. Six tusk-like teeth are in place in the rostrum and a further 16 teeth are loose. The right nasal is loose and detached from the skull. No mandibles or postcranial material were recovered with the specimen.
Etymology
Genus Nihoroa: From the Māori language, ‘Niho’ for teeth and ‘Roa’ for being long, in reference to the long crowns of its teeth.
Species reimaea: From the Māori language, ‘Rei’ for ivory or tusk and ‘Maea’ for emerging, in reference to incompletely emerged tusk-like first incisors.
Pronunciation guide: nee-ho-row-ah rey-ma-eh-ah.
Locality and horizon
Nihoroa reimaea (OU 22126) was collected by R. E. Fordyce, A. Grebneff, C. Samson and G. Ferguson in late January 1992 from the top of a cliff overlooking the north-western bank of the Awamoko stream in Tokarahi, North Otago, New Zealand (44.95°S 170.65°E). The specimen was retrieved from glauconitic Otekaike Limestone, with the sediment being a calcarenite, fine light yellow-white sand, bioclastic limestone. Nihoroa reimaea was retrieved from a stratigraphically higher location than Awamokoa tokarahi (Tanaka and Fordyce 2017), which was retrieved from the transitional lithology between the Kokoamu Greensand and Otekaike Limestone. The suggested age for Awamokoa tokarahiwas 25.0–25.4 Ma. Foraminifera from the matrix of OU 22126 include specimens of the planktonic Globoquadrina dehiscens with a first appearance datum at the start of the Waitakian stage, indicating that OU 22126 is no older than 25.2 Ma (Raine et al. 2015), and likely closer to 24–23 Ma. Details of the fossil collection are available on the New Zealand Fossil Record File Database (www.fred.org.nz) under fossil record number I41/f0198 (and I41/f0129).
Diagnosis
Nihoroa reimaea is a gracile, longirostrine, polydont, heterodont odontocete (). Its long rostrum carries long, procumbent anterior tusk-like teeth and small, slightly denticulate cheek teeth. The premaxilla forms the anterior 30% of the dorsoventrally compressed rostrum. It has asymmetrical antorbital notches and a short, rounded cranium. Its premaxilla extends along 84% of the condylobasal length (CBL), with long thin splint-like extensions reaching past the posterior margin of the nasals. The crowns of the first incisors (I1) are over 1.75 times the length of the second incisors (I2) on either side. It has a narrow mesorostral groove and wide maxillary flanges. N. reimaea differs from Waipatia maerewhenua (Fordyce 1994) in having a much smaller cranium (172 mm wide and 153 mm long, versus Waipatia’s 246 mm long and 251 mm wide). N. reimaeaalso has a narrower more dorsoventrally flattened rostrum and less differentiated cheek teeth than W. maerewhenua. N. reimaea differs from Otekaikea (Tanaka and Fordyce 2014, 2015b) in having a lower vertex, less expanded premaxilla on the face and a broader posterior process of the periotic. N. reimaea differs from Ediscetus (Albright et al. 2018) in having a less pronounced intertemporal constriction, a lower temporal crest, less inflated premaxilla and quadrangular nasals rather than triangular. N. reimaea differs from Nihohae matakoi (Coste et al. 2023) in having fewer tusk-like teeth, shorter I2 and I3s, a less pronounced intertemporal constriction, an anteroposteriorly shorter vertex and more rounded zygomatic processes which reach further forward towards the postorbital processes. N. reimaea has an alveolar groove with indistinct interalveolar septa and likely had over 20 teeth per quadrant in comparison to Nihohae matakoi’s 17 teeth per quadrant. Compared to Nihohae matakoi, N. reimaea has a more rectangular pars cochlearis of the periotic and has an anterior incipient spine on the tympanic bulla.
Figure 2. Line art overlayed on photographs of Nihoroa reimaea coated with sublimed ammonium chloride. Skull in A, dorsal view; B, ventral view; C, left lateral view; left periotic in D, ventral view; E, dorsal view; F, lateral view and G,medial view; left tympanic bulla in H, dorsal view; I, ventral view; J, medial view and K, lateral view; L, all the loose teeth in labial view; M, malleus; N, dorsal view of right nasal.
Abbreviated description
The skull of N. reimaea is near complete, missing the left nasal, most of lacrimojugals and pterygoids. There is minimal burial-related transformation, though some crushing of the posterior of the cranium is evident. There is mild asymmetry of the supraorbital portion of the frontals, the antorbital notches and the zygomatic processes of the squamosals (see ).
The rostrum is straight in both lateral and dorsal views and comprises 69% of the condylobasal length (CBL), is relatively wide at the antorbital notches, and tapered. The mesorostral groove is open, with the vomer not visible dorsally due to sediment present. Thin, sharp-edged maxillary flanges are present posteriorly on the rostrum. The ventral surface of the rostrum carries deep sulci laterally to the sutures of premaxilla and maxilla with the vomer. The alveoli are poorly defined and form irregular alveolar grooves on each side of the rostrum (A–C).
The cranium of N. reimaea is slightly wider (173 mm) than it is long (152 mm). It has a narrow and flat vertex and its facial fossae are shallow and angled outwards, medially to the nares. The supraoccipital is convex. In dorsal view, the narial passages are level with orbits and in lateral view the orbits lie above the ventral surface of the rostrum. Ventrally, the preorbital and postorbital processes are prominent. The orbit is narrow and shallow, becoming deeper posteriorly, and the orbitotemporal crest is straight and partly roofs the temporal fossa. The temporal fossa of N. reimaea opens posteriorly and is sub-vertical. The ventral surfaces of the zygomatic processes are level with the orbit, with the tips nearly reaching the postorbital processes. The pterygoid sinus fossae are well preserved and clearly defined.
The premaxilla forms the apical 27% of the rostrum, extending past the anterior end of the maxilla, and is narrowest 85 mm from the tip. The premaxillae each carry three incisors – one large partly erupted tusk-like tooth and two smaller tusk-like teeth. The posteromedial splints are slightly asymmetrical, thin and extend past the nasals and the frontal at the vertex.
The cranial portion of the maxilla does not fully cover the frontal, leaving the edge of the orbit and orbital processes exposed. There is no maxillary crest or thickening over the orbits. Ventrally, the maxilla forms most of the rostral surface and extends to form the anterior and lateral edges of the infraorbital foramen, whilst the medial and posterior margins are formed by the palatine. The maxilla carries a long alveolar groove which carried up to 20 teeth per quadrant, plus the three incisors in each premaxilla.
The palatine is exposed in the posterior of the palate, extending from the posterolateral edges of the nares to near the posterior end of the tooth row. The palatine forms the anterior and lateral walls of the nares. Only the right nasal of N. reimaea is preserved, separately from the skull (N). In dorsal view, the posterior margin of the nasal is convex and runs smoothly into the medial margin, the anterior margin is mildly sinusoidal, and the lateral splint protrudes anteroventrally. Viewed dorsally, the nasals would have formed a point anteromedially.
The ethmoid complex is visible (A). The ectethmoid forms the posterior wall of the nares dorsally. The mesethmoid is 13 mm wide between the nares and rises to underlie the nasals. Dorsally, the vomer is visible only posteriorly in the mesorostral groove. It forms the lateral walls of the mesorostral groove and the medial walls of the nares, contacting the mesethmoid. Ventrally, it is visible in the vomerian window. Posteriorly, it emerges from under the palatine and extends posteriorly to reach the basioccipital.
The lacrimojugal is exposed ventrally and laterally in a small area of the skull. The lateral exposure forms a small portion of the medial wall of the antorbital notch. The ventral exposure extends medially towards the palatine and is covered by the maxilla. Only the base of the thin styliform process of the jugal is preserved.
The frontals are mildly asymmetrical. An interfrontal suture is visible at the vertex where the frontals are near horizontal. The surface of the frontals at the vertex is rugose. Ventrally, the frontals form the orbits and the anterior of the temporal fossae. The sphenopalatine foramen is posterior to the infraorbital foramen with its anterior and medial walls formed by the palatine, the posterior wall by the orbitosphenoid and the anterior wall by the frontal. The optic canal is outlined by two walls, the anterior of orbitosphenoid and the posterior of frontal.
Dorsally, the visible parietal is a small triangle lateral to the nuchal crest forming the anterolateral wall of the braincase. There is no identifiable interparietal. Ventrally, the parietal forms the posterior portion of the temporal fossa. A small sliver of parietal is present in the basicranium, lining the lateral margin of the cranial hiatus. The squamosal areas visible dorsally are the zygomatic processes which are subparallel to the skull, their anterior tips naturally separate from the postorbital processes. The squamosal forms some of the posteroventral portion of the lateral wall of the temporal fossa.
The squamosal is intricate in the basicranium. It forms and carries the glenoid fossa, a robust postglenoid process, a well-developed and thin falciform process and a large periotic fossa divided by the supratubercular ridge. The squamosal also holds the external auditory meatus, which is deep and narrow expanding laterally, a tall anterior meatal crest and shorter posterior meatal crest. When in place, the posterior process of the periotic sits internally to the lateral edge of the skull and is covered by the bulla, making it amastoid.
The supraoccipital and exoccipitals are crushed though minimally deformed. The nuchal crest is low laterally and level with the frontals at the vertex. The surface of the supraoccipital is convex, with four shallow anterolateral depressions immediately behind the nuchal crest. The exoccipitals are excavated by the dorsal and ventral condyloid fossae at the base of the condyle pedicles. The articular surfaces of each condyle are small, wider than tall and gently rounded. Laterally, the exoccipitals form the paroccipital processes. Medially to the paroccipital processes form the jugular notch with the hypoglossal foramen.
The basioccipital extends out from under the vomer. The basioccipital crests are about 35-37 mm long, thick and robust. The alisphenoid forms the anterior, medial and lateral walls of the foramen ovale. It runs to the base of the basioccipital crest and carries the carotid foramen. It has distinct fossae for the pterygoid sinus and middle sinus. Posteriorly, it carries a groove for the mandibular nerve, extending from the foramen ovale to one in the falciform process. The basisphenoid overlies the vomer, pterygoids and the anterior of the basioccipital. The orbitosphenoid forms the posterior and lateral walls of the ethmoid foramen and the anterior and ventral walls of the optic canal.
The description of the periotic and bulla were primarily based on the removed left elements. The periotic has a robust anterior process, a tapered and triangular posterior process and a weakly inflated pars cochlearis (D–G). The anterior process is bulbous and rounded dorsally, and slightly concave ventrally. The lateral and medial margins are sub-parallel, tapering abruptly at the apex. There is a well-defined C-shaped parabullary sulcus and the anterior keel is low and rounded. The pars cochlearis is sub-rectangular in ventral view. The internal acoustic meatus opens mediodorsally and is comma-shaped. The fenestra rotunda is near circular. The aperture for the cochlear aqueduct is small and sub-circular, connecting with the fenestra rotunda. The aperture for the vestibular aqueduct is tear-shaped. On the body of the periotic, the fenestra ovalis is subcircular and holds the stapes is in-situ. The posterior process is triangular in dorsal view. The posterior bullar facet is roughly planar with a low ridge bisecting it. Ventrally, the apex carries two shorter ridges matching grooves in the posterior process of the tympanic bulla.
The tympanic bulla is heart shaped in ventral view with a short incipient anterior spine (h–K). It has a bilobed posterior face with a broad and deep interprominential notch, ventrally passing into the median furrow. The involucrum is marginally excavated by the tympanic cavity. In lateral view, the sigmoid process is low and angled anteriorly, and rounded in anterior view. The accessory ossicle is in place leaning on the involucrum. The posterior process of the bulla has subparallel lateral margins. The anteriormost facet for the posterior process of the periotic is smooth and has shallow grooves. Anterolaterally, there is a small facet laying against the posterior meatal crest. The posteriormost facet is rugose to suture with the squamosal.
Five teeth are in-situ; the three right incisors (I1, I2, I3) and the first and third left ones (I1, I3). There are 16 loose teeth of varying sizes (L). N. reimaea has an alveolar groove with indistinct interalveolar septa with likely over 20 teeth per quadrant. No mandible was found. Of the preserved loose teeth, 7 are single-rooted, 3 of which are tusk-like. Two are double-rooted and the 7 remaining are either double-rooted or have partly double-rooted teeth.
The first incisors have crown heights of 55 and 54 mm. The incisors are heavily embedded in the premaxilla and only minimally erupted, with 16–23 mm of the crown exposed. This represents 42% of the right I1 and 31% of the left I1exposed. The I2 and I3 are procumbent anterolaterally with shorter crowns. These anterolaterally oriented teeth are shorter in N. reimaea than in Nihohae matakoi, and the postcanines are near vertical. The posteriormost teeth are double-rooted, have shorter crowns and an entocingulum and encocingulum. The posterior teeth are flattened, triangular with minute denticles at the bottom of their keels. The teeth show minimal evidence of apical wear on the enamel.
Phylogenetic analysis
The phylogenetic analysis, identical to Coste et al. 2023, resulted in 50 retained most parsimonious trees with a best score of 2077. A consensus tree was obtained using the 50% majority rule in TNT () placing Nihoroa reimaea amongst the waipatiids grade of basal odontocetes and most closely related to Nihohae matakoi, OU 22262 and Ediscetus osbornei.
Figure 3. Tree modified from Coste et al. (2023). Phylogenetic analysis 50% majority consensus tree of unweighted analysis with branch lengths labelled above the centre of each and Bremer support values below nodes. The clades Physeteroidea, Ziphioidea, Inioidea, Phocoenidae and Delphinidae are collapsed.
An analysis of rostral characteristics and procumbent teeth in cetaceans
There were nine significant Pearson's product-moment correlations between seven variables as seen in , and which are explained in more detail below.
Table 2. Pearson correlations for the study variables.
Percentage premaxillary length to percentage rostral length
The percentage of premaxillary length (relative length of rostrum formed only by the premaxilla in proportion to complete rostrum length) vs. the percentage of rostral length (relative length of the rostrum as proportion of CBL) had a moderate negative correlation r(48) = −0.326 (p ≤ 0.05). This indicates that 11% of the variation in relative premaxillary length is explained by relative rostral length (A).
Figure 4. Scatter plots for correlations A, percentage premaxillary length to percentage rostral length, B, Relative crown height to percentage premaxillary length, C, Relative crown height to crown angle and D, Crown shape ratio to crown angle. On D, the clusters of odontocetes with tusk-like teeth and without are outlined in long and short dashes respectively.
For fossil odontocetes with tusk-like teeth from elsewhere in the world, the proportion of the rostrum formed by premaxilla increases with rostral length, whilst the opposite was observed for New Zealand fossils with tusk-like teeth. Other cetaceans without tusks had weak correlations for these variables.
Shape ratio of the rostrum at the premaxilla to percentage rostral length
There is a strong correlation between shape ratio of the rostrum at the premaxilla (ratio of the height to width of the rostrum measured at the anteriormost end of the maxilla) with the percentage rostral length, r(48) = 0.509 (p ≤ 0.01). This correlation explains 26% of the variation between these rostral proportions and is similar for all studied odontocetes (see supplementary materials).
Shape ratio of the rostrum at the mid-length to percentage premaxillary length
The correlation between the shape ratio of the rostrum at mid-length (ratio of the height to width of the rostrum measured at mid-length) and the percentage of premaxillary length is strong, r(48) = 0.513 (p ≤ 0.01), and also explains 26% of the variation between these variables. There is no apparent distinction amongst studied specimens (see supplementary materials).
Relative crown height to percentage premaxillary length
There is a moderate correlation between the relative crown height (proportion of the greatest crown height to rostral length) and the percentage of premaxillary length, r(40) = 0.410 (p ≤ 0.01) and this explained 17% of the variation between these two variables (B). For New Zealand specimens there was a strong positive correlation between the variables, with the percentage premaxillary length explaining 80% of the variation in the relative crown height. Tusked cetaceans from elsewhere had a negative correlation explaining 33% of the variation.
Crown shape ratio to percentage premaxillary length
There was a negative moderate correlation between the crown shape ratio (greatest crown height to greatest crown width ratio) and the percentage of premaxillary length, r(48) = −0.368 (p ≤ 0.05), explaining 14% of the variation seen between these variables. The New Zealand specimens showed a strong correlation, with the percentage of premaxillary length explaining 77% of the variation in the crown shape ratio. However, for the other specimens with tusk-like teeth and those without tusks, they show moderate correlation between these variables with about 10% of the variation explained.
Shape ratio of the rostrum at the premaxilla to shape ratio at mid-length
There was a strong correlation between the shape ratio of the rostrum at mid-length and the shape ratio of the rostrum at the premaxilla, r(48) = 0.576 (p ≤ 0.01), which explains 33% of the variation between the variables. This correlation was stronger for odontocetes with tusk-like teeth from New Zealand and elsewhere, where in both cases it explained over 70% of the variation seen.
Relative crown height to crown shape ratio
The correlation between the relative crown height and crown shape ratio was negative and strong, r(40) = −0.581 (p ≤ 0.01), explaining 35% of the variation between the variables. New Zealand specimens had a strong negative correlation, with 77% of the variation in crown shape ratio being explained by relative crown height, whilst other odontocetes with tusk-like teeth from elsewhere and other cetaceans presented no significant correlation in these two variables.
Relative crown height to crown angle
The relative crown height to the crown angle (averaged from angle to the rostrum and/or root) was the strongest correlation observed, r(40) = 0.635 (p ≤ 0.01), explaining 40% of the variation between these two variables (C). This correlation was moderate for NZ odontocetes with tusk-like teeth, explaining 20% of variation observed. Other odontocetes with tusk-like teeth from elsewhere and other cetaceans showed no significant correlation. However, the increase in crown height was linked to the procumbence of the teeth.
Crown shape ratio to crown angle
The correlation between crown shape ratio and crown angle was strong, r(54) = −0.599 (p ≤ 0.01), explaining 36% of the variation between them (D). New Zealand specimens had a strong correlation explaining 71% of the variation, while other tusk-like tooth-bearing cetaceans had a strong correlation explaining 29% of the variation and other cetaceans show no correlation.
Discussion
This paper expanded the diversity of described odontocetes with tusk-like teeth through the addition of a new species, Nihoroa reimaea, increasing our understanding of their phylogenetic relationships and shape diversity. A second aim was to further our understanding of tusk-like teeth in odontocetes, in particular to elucidate how NZ specimens relate to those from other parts of the world. The definition and classification of tusk-like teeth in odontocetes need further study, to differentiate those from tusks such as the ones seen in Monodon or Odobenocetops (de Muizon et al. 2002; de Muizon and Domning 2002). Here, the focus was on incisors, or the first erupted teeth in the upper jaws. The analyses focus on the association between shape and rostral proportions and procumbent tusk-like tooth crowns of NZ odontocetes with those from other parts of the world and odontocetes without procumbent teeth. If proven useful, these characters could be incorporated into future phylogenetic analyses of fossil odontocetes.
The phylogenetic placement of Nihoroa (following Coste et al. 2023) has it within the ‘waipatiid’ grade, including Waipatia (Fordyce 1994; Tanaka and Fordyce 2015a), Otekaikea (Tanaka and Fordyce 2014, 2015b), Awamokoa (Tanaka and Fordyce 2017), Urkudelphis (Tanaka et al. 2017), Ediscetus (Albright et al. 2018) Nihohae and now Nihoroa (). All the species in this group show evidence of having long and thin procumbent teeth.
The laterally-oriented incisors of Nihoroa were possibly used for slashing and stunning prey as in Nihohae matakoi (Coste et al. 2023). However, the under-eruption of the first incisors may suggest that these teeth were less-used in feeding. In other cetaceans, a similar pattern of retained/poorly erupted teeth is known in female narwhals (Monodon monoceros) (Nweeia et al. 2012) and in female beaked whales (family Ziphiidae) (MacLeod 2018). In narwhals, the tusks remain embedded in the maxilla, erupting in less than 15% of the females, whilst in males tusks are commonly present (Nweeia et al. 2012; Heide-Jørgensen 2018). It has been hypothesised that the primary function of the narwhal’s tusk is for sexual display and additional sensory ability (Nweeia et al. 2009, 2014; Graham et al. 2020; Vicari et al. 2022). The tusk, however, is not necessary for survival as some males and most females lack them (Heide-Jørgensen 2018). In many species of beaked whales, males have a single pair of tusk-like teeth, which are present in females though not fully erupted (MacLeod and Herman 2004; MacLeod 2018; Mesnick and Ralls 2018). In species such as Mesoplodon bidens, there are also modifications in the bone structure of the jaw in males to further support the antagonistic use of the teeth (MacLeod and Herman 2004). It has been suggested that similar bony modifications such as the distally expanded rostrum in certain squalodontids may be a characteristic of sexual dimorphism or ontogeny (Dooley 2005). However, considering the limited available evidence of sexual dimorphism in odontocetes with tusk-like teeth, it is not possible to determine if the teeth of Nihoroa are a sexually dimorphic display feature. Whilst it is possible that the holotype of N. reimaea OU 22126 is a female representative of its species, this cannot be reliably confirmed.
Due to the variability and incomplete nature of palaeontological data, it was not possible to run a principal component analysis to simultaneously assess the different variables. However, the series of Pearson's product-moment correlations enabled the comparison of size-adjusted variables to investigate odontocetes with tusk-like teeth in NZ and elsewhere in the world. The results show that there were statistically significant correlations among several of the variables tested.
The percentage premaxillary length to percentage rostral length shows that a greater proportion of the rostrum formed of premaxilla extending past the maxilla is linked to a greater rostral proportion and that this was particularly true in species and specimens with tusk-like teeth. This is likely explained by the fact that when procumbent incisors are present, the premaxilla needs to be of greater length to allow space for the roots of these teeth which are often long (Tanaka and Fordyce 2015a, 2015b).
Other correlations relating to one of the shape ratios of the rostrum at mid-length and at the premaxilla express the association between the shape and its length and proportions. Overall, when the rostrum and premaxilla are longer, the rostrum becomes more laterally compressed. The correlation between relative crown height and the percentage of premaxillary length is strong and positive for New Zealand specimens but negative for other tusk-like bearing cetaceans. When considering that most New Zealand specimens included are likely waipatiids (14/17), this proportion of crown length and premaxillary length should be further investigated as a potential phylogenetic tool. On the other hand, other cetaceans with tusk-like teeth have a moderate negative correlation which is likely due to the greater variety of groups included. This includes specimens such as Leviathan (Lambert et al. 2010), Prosqualodon (Flynn 1920, 1923; Tanaka et al. 2022) or the extant beluga/narwhal hybrid (Vicari et al. 2022), which all have proportionally short premaxilla and long crowns. This is particularly evident in the beluga/narwhal hybrid which has the rostral proportions of its parents but much longer tusk-like teeth embedded in the maxilla of the specimen.
The association between tooth shape and the proportion of premaxilla in the rostrum shows that long and thin teeth are correlated to an increased premaxilla. If the tooth shape/pointiness is considered a feature of tusk-like teeth, then the association between these variables may differ. The correlation between relative crown height and crown shape ratio shows that as the crown becomes taller, it also gets proportionally thinner. This is more evident in New Zealand specimens and moderately in other tusk-like tooth-bearing cetaceans, which represent all data points where the crown is more than 10% of the rostral length. As the crown gets longer, the base must be relatively thinner to fit into the rostrum.
The correlations concerning the angle of the crown to the root or rostrum compare it to crown height and crown shape ratio. Though these two measures are similar, they relate to the angles slightly differently. In the correlation of angle with relative crown height, the relationship was stronger when the categories were considered independently. As crown height increases in proportion to rostral length, the tusk is increasingly angled forward, likely because past a certain length it becomes difficult to close the mouth with vertically-oriented teeth. The correlation of crown angle with crown shape ratio was less strong than the correlation to crown height. However, this association was strong for New Zealand specimens and it explains 71% of the variation between these two variables. The linear relationship between crown shape and angle for waipatiids is likely a useful diagnostic factor. The use of correlations when analysing similar variables seems to be a valuable tool for understanding the differences between tooth form in cetaceans and may be useful, in conjunction with traditional methods, for preliminary diagnoses of waipatiids.
Our results also suggest a selection method for determining whether a tooth should be considered tusk-like or not. The first variable to consider is the crown angle; specimens described as having ‘tusks’ or procumbent teeth have a measurable angle of over 130°, except for Squalodon calvertensis which has a measured crown to root angle of 126° but has been described as having tusk-like teeth. Conversely, there are only three species analysed here not considered to have tusk-like teeth as they are not extra-oral and have crowns sitting at over 130°; Delphinapterus leucas (137.5°), Delphinus delphis (130.7°) and Globicephala melas (134.4°). Secondary to this, the shape of the tooth should be considered though it seems to be less diagnostic than the angle. Most specimens considered to have tusk-like teeth have crown height to width ratios of less than 0.6 and those without have ratios generally above 0.4. Though these data overlap (D), when considered together, these variables allow cetaceans analysed here to be sorted into two clear clusters – with and without tusk-like teeth.
This paper expands our knowledge of odontocetes with tusk-like teeth by introducing Nihoroa reimaea and investigating the relationships between tusked dolphins and their diversity. The study examined tusk-like teeth in odontocetes from New Zealand and compared them to those from elsewhere, suggesting most NZ taxa possess waipatiid-like teeth. Phylogenetic analysis adds Nihoroato the ‘waipatiid’ grade, expanding the number of known cetaceans with long and thin procumbent teeth. The analysis of Nihoroa's incisors suggested a potential use for slashing and stunning prey as in Nihohae, but the limited eruption and reduced number teeth suggests they may have had a lesser role in feeding. The correlations between variables provided valuable insights into the proportions, shapes, and positioning angles of tusk-like teeth. These correlations can aid in understanding tooth forms in cetaceans and may be useful for diagnosing waipatiids. Crown angle and crown shape ratios are key factors in determining whether a tooth should be considered tusk-like. This research contributes to our understanding of odontocetes with tusk-like teeth and provides a framework for future studies investigating a broader range of cetacean dentitions. With an increased understanding of tusk-like teeth and additional information on feeding variables such as tooth wear patterns, bite force estimates and jaw and teeth size, it may be possible to use finer statistical methods to infer the potential uses of tusk-like teeth.
Disclosure statement
No potential conflict of interest was reported by the author(s).