What is the difference between mysticetes and odontocetes

There are many similarities between the two sub-orders such as they both give live birth, have blowholes, and use sounds to communicate with each other, as well as many others. However, there are some glaring differences between the two that allow us to really define what a dolphin exactly is. The first major difference is that Mysticetes use baleen, large overlapping plates used to filter feed , to eat small organisms like krill.

Odontocetes swallow their prey whole and use their teeth for grabbing and gripping instead of chewing. Secondly, Mysticetes are also mostly solitary animals only coming together for mating or when food is plentiful in a given spot whereas most Odontocetes, especially dolphins shown above, travel in pods. Thirdly, Odontocete jaws are much more asymmetrical different on either side of the jaw than Mysticetes so that they can receive sound waves from echolocation much better while they are feeding and locate their prey.

Fourthly, sounds made by these two are different; Mysticetes produce much lower frequency songs to navigate and communicate whereas Odontocetes produce many more high frequency clicks and whistles used to locate prey on top of communication and navigation.

Fifthly, Mysticetes have double blowholes shown to the right while Odontocetes have single ones. And lastly is their size! Odontocetes, with the exception of sperm whales, are generally much smaller than their Mysticete relatives. Kopelman, A. All rights reserved.

Facebook Twitter Email. Related Projects. How Fish Hear. February 26th, 0 Comments. Brief Intro to the Hearing Systems of Fish. February 12th, 0 Comments. Skull Morphology. Asymmetry in odontocetes is always unidirectional, with a posterior and sinistral shift in the bones, linked to the hypertrophied melon, phonic lips, and nasal sacs, all of which are associated with high-frequency sound production and echolocation [ 16 , 17 ].

Most of this asymmetry appears in the dorsal opening of the nares [ 14 , 15 , 18 ] and appears to be correlated with the degree of elevation in the cranial vertex [ 11 ]. Species with high cranial vertices such as physeterids, kogiids, and ziphiids tend to have the most asymmetrical crania, likely because a functional component of asymmetry pertains to soft facial anatomy and consequently drives evolution of the underlying bony structures [ 11 ].

Odontocete asymmetry is thought to have evolved as a result of an evolutionary hyperallometric investment into sound-producing structures to facilitate the production of high frequency vocalisations [ 11 , 19 — 22 ], but alternative explanations have been put forward.

MacLeod et al. However, this has been argued against because reduction of tooth size and loss of shearing occlusion started after asymmetry was well developed, suggesting that swallowing prey whole may not be the driver of asymmetry [ 16 ].

Alternatively, cranial asymmetry in basilosaurids and protocetids is thought to be linked to aquatic directional hearing [ 16 ]. The limited or lack of asymmetry in mysticetes, which do not echolocate and instead specialise in low and infrasonic frequencies [ 23 — 25 ], suggests directional cranial asymmetry is more likely related to echolocation than hearing [ 15 ]. Previous studies have focused on either odontocete cranial shape and function [ 13 ], archaeocete asymmetry [ 16 ], or mysticete symmetry with modern odontocetes and archaeocetes for comparison [ 15 ].

There is, however, little resolution on how cranial asymmetry evolved during the transition from archaeocetes to modern whales Neoceti [ 16 ], and little is known about archaeocete asymmetry and its relationship, if any, to that of odontocetes [ 26 ]. To assess when and how often asymmetry may have arisen, where and if it is present in the archaeocete skull, and how it relates to the evolution of echolocation, it is necessary to adopt a comparative approach by broadly sampling across living and extinct cetaceans.

Here we use geometric morphometric techniques to quantify asymmetry in the skull across modern and fossil species of Cetacea. We then use these data to reconstruct the evolution of asymmetry across cetaceans and test for shifts and jumps in the rate of evolution of cranial asymmetry across the cetacean phylogeny.

Finally, we use these results to test potential factors associated with the evolution of asymmetry in specific cetacean clades, including presence or absence of echolocation, echolocation frequency, and inhabiting acoustically complex or high-pressure environments such as shallow rivers, cluttered icy waters, and deep ocean. The mysticetes and terrestrial artiodactyls dominate the lower end of the ranking with eight of the last ten positions occupied by extant balaenids and balaenopterids and one fossil pelocetid Additional file 1 : Table S1.

For the whole cetacean data set, the most asymmetric landmarks are the nasals, the maxilla at the sutures with the nasals and premaxilla, and the posterior, dorsal premaxilla Table 2. For this reason, each cetacean suborder and the terrestrial artiodactyls were analysed separately. The most asymmetric landmarks for odontocetes are the dorsal maxilla suture with nasal and premaxilla , nasals, and the posterior-dorsal maxilla Table 2.

Cetacean subgroups differ greatly in identity of the most asymmetric landmarks and magnitude of landmark asymmetry Table 2. This difference is evident when comparing average landmark asymmetry across the groups Fig. List of the five landmarks with the greatest variation across the cranium for all cetaceans, archaeocetes, odontocetes, mysticetes, and terrestrial artiodactyls.

Each image shows the position of the five landmarks of greatest variation for each respective group. Skulls not to scale. Landmarks superimposed onto a stylised skull which represents an average specimen for that group.

Cooler yellows show less asymmetry, warmer oranges and reds show more asymmetry. The white landmarks are fixed reference landmarks and therefore show no movement. Landmarks on skulls a and d consist of pale yellows indicating low asymmetry. The landmarks on skull b are pale yellow, with darker yellows on the jugal, orbit, and rostrum indicating a higher level of asymmetry.

Oranges and red landmarks in the nasal, posterior premaxilla, and posterior maxilla on skull c the odontocete indicate a high level of asymmetry. The basilosaurid and protocetid archaeocetes show a high level of asymmetry, akin to the levels seen in fossil and extant odontocetes Additional file 1 : Table S1. The contribution of rostral landmarks to overall cranial asymmetry in these archaeocete families ranges from The average amount of asymmetry concentrated in the rostrum is higher in archaeocetes In contrast, fossil odontocetes are more symmetrical than most extant odontocetes Additional file 1 : Table S1.

Excluding rostral landmarks had the most impact on archaeocetes and mysticetes, as some of the highest levels of asymmetry in those clades are found in the rostral region Table 2 ; Additional file 1 : Tables S2—4.

However, overall, removal of the rostral landmarks had only a minor effect on results Additional file 1 : Fig. S1-S3 [ 29 ], Table S5b. Principal component analysis of landmark asymmetries showed that odontocetes exhibit a wide range of cranial asymmetry Fig. Mysticetes and terrestrial artiodactyls overlap in asymmetry morphospace, whilst archaeocetes have a higher level of asymmetry, similar to more moderately asymmetric odontocetes Fig. See Additional file 1 : Fig. S5 for identification of each specimen in the morphospace.

A morphospace labelled with a specimen key is provided in the Additional file 1 : Fig. S5—principal components plot with PC1 and PC2 plotted for each specimen. Asymmetry decreases towards the base of Neoceti, and mysticetes show the lowest level of cetacean asymmetry observed in the entire data set Fig.

As expected, odontocetes show higher values of asymmetry but span nearly the full range of asymmetry morphospace Fig. The highest asymmetry is seen in the platanistids, monodontids, and physeteroids Fig. There are some exceptions within odontocetes, however, such as lower levels of asymmetry in the other extant river dolphins Inia , Pontoporia , and Lipotes [ 21 , 31 ].

Lower asymmetry is also observed in the family Eurhinodelphinidae [ 32 ], the extant phocoenids [ 26 , 33 ], and genus Sousa [ 14 ] Fig. Phylogeny based on Lloyd and Slater [ 29 ]. There is a probability 0. There are no high probability shifts in asymmetry in the mysticete suborder, nor in the archaeocetes. There is no measurable probability of a shift occurring in the archaeocete protocetids and basilosaurids. A slower or decreasing rate of asymmetry evolution is reconstructed within Mysticeti.

Surprisingly, no shifts are reconstructed in the ziphiids, an odontocete family with bizarre asymmetrical premaxillary crests in most species e. Ziphius cavirostris. Reconstructed probability of shifts in cetacean cranial asymmetry. Reconstructed probability along each branch of the phylogeny under the assumption of relaxed Brownian motion with a Half-Cauchy distribution for the prior density of the rate scalar.

Circles indicate a shift in the trait on either the branch or in the whole clade. The colour of the circle indicates the shift direction with red indicating forward shifts and blue indicating backwards shifts.

The size of the circle indicates the probability of the shift occurring in that position in the clade with the largest circle here, 0. The colour of the branch itself indicates posterior rates for that branch with red showing higher, increasing rates and blue showing lower, decreasing rates.

The background rate is shown as grey. A trace of the chain is provided in Additional file 1 : Fig. S10—Gelman diagnostics for the two chains. Smaller jump probabilities 0. Globicephala spp. S6, S7 and Model diagnostics [ 34 — 36 ]. All model diagnostics are provided in Additional file 1 : Fig. S6—10; Table S6 and Model diagnostics section [ 34 — 36 ]. Reconstructed probability of jumps in the rate of cetacean cranial asymmetry.

The model also predicts the number of jumps which may have occurred. The size of the circle indicates the probability of the jump occurring in that position in the clade with the largest circle here, 0. SS13 Table S5c [ 29 ]. Models are detailed in Table 4 —models testing whether changes in cetacean cranial asymmetry are associated with other discrete traits.

Hereafter, results with the Benjamini-Hochberg correction are discussed. Our analyses of cranial asymmetry through the evolutionary history of whales suggests that the common ancestor of living whales mysticetes and odontocetes did not possess an asymmetric cranium, and thus, it is unlikely that echolocation was present at that stage of whale evolution or at any point in mysticete evolution.

Cranial asymmetry is highest in crown odontocetes and first becomes a major feature of odontocete crania in the Early Oligocene soon after their divergence from mysticetes. This period has previously been identified as one of unusually high diversity and evolution in neocete skull morphology [ 13 , 37 , 57 ] alongside an explosive and rapid radiation of crown cetaceans [ 38 , 57 , 58 ].

Rostral asymmetry is observed in some archaeocetes and is potentially related to directional hearing, possibly increased by deformation in some cases. We found this same rostral asymmetry in this and other archaeocetes along with asymmetry in the jugal, orbit, and squamosal.

This rostral asymmetry disappears in Neoceti and later arises in the nasofacial region in odontocetes. This distribution could be inferred as torsion in the archaeocete rostrum as part of a complex of traits which led to directional hearing [ 16 ].

This asymmetry then disappears during the transition from archaeocetes to early neocetes Fig. It is unclear whether this is due to an actual shift from a primitive form of aquatic directional hearing in specific archaeocetes the basilosaurids and protocetids, as suggested by Fahlke et al. Asymmetry unrelated to function is reported for other mammals e. Further, it could be related to specific feeding strategies such as bottom-feeding or other lateralized behaviours.

When looking at the primary landmarks displaying asymmetry in the basilosaurids and protocetids, there is no indication that these are dominated by rostral torsion more than in the other archaeocetes Additional file 1 : Table S2 , and instead, asymmetry appears to be spread in no particular pattern across the jugal, squamosal which are possibly more susceptible to deformation , rostrum, and orbit for these families.

Rostral asymmetry in the archaeocetes is at least partly caused by fossil distortion in some specimens [ 27 ], but perhaps may also be biologically present in more archaecoete families than previously thought. We found no high probability shifts Fig.

We did, however, find evidence for small temporary and rapid change jumps in asymmetry in the later archaeocetes Fig. Echolocation, telescoping, and ecological specialisation rapidly evolved shortly after the divergence of Neoceti from Basilosauridae [ 4 , 38 ], and there may have been a rapid regime change from directional hearing occurring at the same time, possibly with associated asymmetry.

Asymmetry is lowest in basal mysticetes such as Coronodon havensteini and the aetiocetids and remains low in mysticetes from the Oligocene to present. There are no high probability shifts in asymmetry in the mysticetes.

Rather, Mysticeti largely display a slower or decreasing rate of the trait. There are some increases in asymmetry observed in individual mysticetes, for example in Balaenopteridae indet NMNZ MM and Aglaocetus moreni FMNH P , but this likely represents taphonomic distortion in the rostrum rather than biological asymmetry. Quantifying cranial asymmetry in living and extinct mysticetes allows reconsideration of the evolution of echolocation in this clade.

The consensus is that cranial asymmetry in whales evolved due to the production of high-frequency vocalisations [ 19 — 21 ].

The consistent level of symmetry in the mysticetes corroborates the hypothesis that mysticetes never evolved sophisticated echolocation [ 25 , 62 ] and also contradicts the hypothesis that this suborder secondarily lost their echolocation capabilities [ 63 ]. Our analysis further suggests that echolocation was likely not present in the common ancestor of mysticetes and odontocetes [ 25 , 62 ] but evolved early in the common ancestor of odontocetes shortly after they diverged from mysticetes [ 4 ].

As reported in Fahlke and Hampe [ 15 ], mysticete crania are similar in magnitude of asymmetry to terrestrial artiodactyls Table 2 ; Fig. In mysticetes, the highest level of cranial asymmetry was found across the rostrum anterior and posterior maxilla and premaxilla , likely due to deformation. In some extant specimens, we observed that the tip of the rostrum has dried out and partly split apart. Even with drying-out and potential taphonomic deformation, the levels of asymmetry in mysticetes were lower than asymmetry seen in archaeocetes and much lower than that of odontocetes.

Cranial asymmetry first appears as a significant morphological trait in the Early Oligocene odontocetes Xenorophidae Fig. Odontocete asymmetry is overwhelmingly concentrated in the nasals including the posterior suture with the frontal, maxilla, and premaxilla. Most early odontocetes are less asymmetric Fig. Other extant odontocetes with low cranial asymmetry include Sousa , Sotalia , and Steno Fig.

Phocoenids also exhibit a low level of cranial asymmetry Fig. This low asymmetry is likely tied to their relatively low peak-power biosonar [ 22 , 64 ].

Further, many descriptions of eurhinodelphinids have suggested that their crania are only slightly asymmetric [ 32 , 65 ], as is supported here Fig. Thus, it should be considered that although some later fossil odontocetes had symmetrical skulls, they may have had asymmetrical nasal sacs as is observed in these extant species.

This result adds further evidence to the idea that xenorophids and other odontocetes iteratively evolved specialisations for the production of high-frequency sounds necessary for echolocation [ 4 — 6 , 39 ]. The distinct cranial morphology and by inference, distinct soft tissue morphology found in xenorophids e. The position of xenorophids as the earliest diverging clade within Odontoceti demonstrates that echolocation, telescoping, and ecological specialisation rapidly evolved shortly after the extinction of the Basilosauridae [ 5 , 6 , 38 ].

Since then, cranial asymmetry has increased and remained generally high throughout the odontocete lineage Fig. The latter two families share marked asymmetry in the premaxillae with the right maxilla narrower than the left in dorsal view [ 67 ]. Further, asymmetry is recorded in the frontal and maxillary crests of fossil platanistids such as Zarhachis flagellator [ 67 ] although the supraorbital crests are not as developed as the extreme maxillary crests in the extant Platanista gangetica which is one of the most asymmetric of all odontocete skulls [ 68 ].

There is also marked skull asymmetry in the distantly related squalodelphinid, Notocetus vanbenedeni , which also sits within the superfamily, Platanistoidea [ 66 , 67 ]. We hypothesise that this regime may be linked to the pressures which arise from inhabiting acoustically complex environmental niches. The physeteroids were the first of the major odontocete crown lineages to rapidly diverge and are easily recognisable due to a highly asymmetric facial region and supracranial basin [ 26 ].

Their large body size and hypertrophied nasal structures produce a low-frequency multi-pulsed sound [ 45 ], which facilitates long range detection of prey [ 22 ]. This is highly advantageous when searching for patchy prey, especially as the physical properties of the water itself alter sound velocity and potentially constrain sensory morphology [ 69 ]. Platanista gangetica , the sole modern survivor of Platanistidae sits alone among river dolphins for having a highly asymmetric cranium and echolocating at broadband low frequency BBLF.

The unique, autapomorphic bony maxillary crests of Platanista may help achieve a higher directionality than expected for a cetacean that clicks nearly an octave lower than similar sized odontocetes [ 43 ], a feature that would be useful in the turbid, cluttered rivers they inhabit.

Other species in this highly asymmetric model include both monodontids: belugas Delphinapterus leucas and the narwhal Monodon monoceros. Their unique sound repertoire narrowband structured, NBS is ideal for projecting and receiving signals in icy, shallow waters, where the animals can detect targets in high levels of ambient noise and backscatter [ 44 ] Additional file 1 : Table S8 [ 24 , 40 — 42 , 45 — 54 , 64 , 70 — 72 ].

Jumps detected in the delphinids all belong to the subfamily Globicephalinae Fig. In particular, the highly asymmetrical Globicephala Table 1 ; Additional file 1 : Table S1 has evolved a deep-dive pattern to target a deep-water niche occupied by large, calorific, and fast squid, and its acoustic behaviour is more akin to deep divers than to oceanic delphinids [ 73 ].

The cochlea of Globicephala is also morphologically different to other delphinids [ 69 ], which could also represent adaptation to the extreme acoustic environment of the deep ocean. Further, studies suggest that Pseudorca which also has a highly asymmetric cranium Table 1 echolocates with different vertical and horizontal plane patterns to other delphinids [ 74 ].

Surprisingly, no jumps or shifts are seen in the deep-diving ziphiids beaked whales , an odontocete family with bizarre asymmetrical premaxillary crests and an asymmetric prenarial basin Additional file 1 : Fig.

Previous studies have suggested that the beaked whale genus Berardius the most basal crown genus shows the least bilateral asymmetry in the skull [ 78 , 79 ], and we saw a similar result here. We attribute the underrepresentation of asymmetry in the ziphiid skull to the use of landmarks alone.

Whilst detecting asymmetry in the shifting of the nasal, premaxilla, and maxilla to the left side of the skull, this method underrepresents the degree of asymmetry in the morphology of the bones themselves. The premaxilla is landmarked with points at the posterior dorsal premaxilla and the dorsal medial maxilla suture with nasal and premaxilla which accurately captures asymmetry in the positioning of the bone and its attachment but fails to capture the tapering of the highly asymmetric premaxillary crest itself Additional file 1 : Fig.

Future studies in this area should be done with curve sliding semi-landmarks and surface patches to more accurately capture the complex morphology [ 80 ] of the premaxillary crests and premaxillary sac fossae in ziphiids which are not represented using fixed landmarks alone. This model suggests a different evolutionary regime for each of the most asymmetric groups. As above, it could be hypothesised that the highly asymmetric species live in unique, acoustically complex environments all of which have rather extreme specific environmental selection pressures.

The reduction in the p value after phylogenetic correction for the regime and regime-split models suggests that the factors influencing asymmetry may be shared by closely related taxa. Echolocation frequency has been widely suggested as a key driver of asymmetry in the cranium [ 16 , 17 ] and soft tissues [ 81 ]. Although not the best model fit, we suggest that this relationship be investigated in more detail, for example with a more detailed analysis of species-specific echolocation frequencies and associated categories across Cetacea [ 17 ].

It is important to note that these methods assume a Brownian motion model, which oversimplifies the actual evolutionary model underlying the evolution of asymmetry shown here to be better described by an OU model. We found no support for several other potential drivers for observed patterns of cranial asymmetry, independent of phylogeny.

There is no significant effect of geologic age of the specimen e. This result is likely because, despite odontocete crania becoming more asymmetrical in most extant families, mysticetes do not. This is not surprising as there is generally a clear phylogenetic relatedness in whether a cetacean is symmetric mysticete or asymmetric odontocete. Again, this is not surprising as there is a clear phylogenetic relatedness in whether a cetacean can echolocate, i. There is a small chance that skulls used in this study may be more asymmetrical, i.

Where possible, we chose skulls based on their overall quality and representation of the species. This was not possible for fossils which are often represented by one specimen, but deformed skulls were removed from the study so as not to falsely imply there is biological asymmetry in the skull when there is none. Further, the sex of the specimen may slightly alter the degree of asymmetry in the skull. Female false killer whales, for example, have a slightly more asymmetrical skull than males [ 82 ], and this may partially explain why the individual in this study appears to have a higher level of asymmetry than the other delphinids.

However, the sex of this specimen USNM is listed as unknown. It is important to note that adult male narwhal exhibit an extreme form of asymmetry in the tusk and vestigial teeth [ 83 ]. The specimen in this study USNM is female and therefore lacks a highly asymmetric tusk, however, the paired tusks embedded in the maxillae may still exhibit asymmetry [ 83 ] and may affect the overlying bone structure. This has not skewed the results seen here as the top 6 landmarks of asymmetry in the Monodon skull are in the nasals and posterior premaxilla and maxilla i.

Feresa attenuata and Peponocephala electra. Lastly, an argument against the hypothesis that echolocation drives asymmetry in the odontocete skull is that bats also echolocate and do not have cranial asymmetry as the natural condition [ 18 ]. However, the extreme differences in the environments in which bats and cetaceans echolocate, as well as other ecological and morphological differences between the two clades, complicate any meaningful comparison [ 85 ].

It should be noted that both odontocetes and bats share a remarkable convergence on narrow biosonar beams across species independent of body size [ 22 , 86 ], with the ability to do this in odontocetes likely a result of cranial asymmetry. With the most widely supported explanation of asymmetry being sound production, our results support the hypothesis that craniofacial asymmetry along with concavity in the facial area, hypertrophied naso-facial muscles, air sacs, melon, and premaxillary sac fossa [ 26 ] arose in odontocetes to support high-frequency echolocation.

Further, echolocating in complex environments continues to be a primary factor driving the evolution of asymmetry in the odontocete skull, as supported by the independent evolutionary regimes for the most asymmetric odontocetes. Our study represents the first comprehensive analysis of cranial asymmetry spanning the evolutionary history of cetaceans. We demonstrate that the common ancestor of living cetaceans had little cranial asymmetry and thus is unlikely to have possessed the ability to echolocate.

Odontocetes display increasing cranial asymmetry from the Oligocene to present, reaching their highest levels in extant taxa. Separate evolutionary regimes are supported for three odontocete clades monodontids, physeteroids, and platanistids that inhabit acoustically complex environments, suggesting that echolocation and cranial asymmetry are continuing to evolve under strong selection in these niches.

Surprisingly, no increases in asymmetry were recovered within the highly asymmetric ziphiids. We attribute this to the extreme, asymmetric shape of the premaxillary crests and sac fossae in these taxa not being captured by landmarks alone.

Mysticetes have maintained a low level of cranial asymmetry since their origin, and if asymmetry reflects ultrasonic sound production ability, it is unlikely that mysticetes were ever able to echolocate. Archaeocetes have a high level of asymmetry in the rostrum which could be linked to directional hearing, as reported by Fahlke et al.

Smaller shifts were found in the Squalodelphinidae and Platanistidae. Additional episodes of rapid change were found in the Mid-Late Oligocene, a period of rapid evolution in cranial asymmetry in odontocetes. These results support studies suggesting that biosonar, the signature adaptation of odontocetes, and associated asymmetry were acquired at or soon after the origin of this clade [ 4 — 6 , 39 ].

Additionally, 10 terrestrial artiodactyls representing 7 of the 10 Arctiodactyla families were included to provide a baseline for symmetry as cetaceans are nested within Artiodactyla. Specimen details Additional file 1 : Table S9 and museum abbreviations are provided in Additional file 1.

Specimens were selected to cover the widest possible phylogenetic spread, representing 38 families and genera from the Eocene to the present.

The Early-Middle Eocene is represented by the land-dwelling family Pakicetidae through to semi-aquatic Ambulocetidae and Remingtonocetidae. The Pelagiceti are represented by the fully aquatic Basilosauridae of the Late Eocene through to the modern Neoceti. This includes representation of some early stem toothed mysticetes such as the Mammalodontidae and the Aetiocetidae. Three of the four extant mysticete families are represented. The more crownward odontocetes of the Miocene are represented by the Eurhinodelphinidae, Kentriodontidae, Albireonidae, Squalodelphinidae, Squalodontidae, and Allodelphinidae among other extinct families.

All ten extant odontocete families are represented. See Additional file 1 : Table S9 for details. Because many extant and all fossil specimens lack information on sex, sexual dimorphism could not be considered. All specimens are adult except for one subadult, Mesoplodon traversii. Specimens were selected based on completeness but some bones were broken e. For this reason and because fossils often have a higher proportion of missing data, we also ran analyses without any fossils and without rostral landmarks.

Specimens with obvious taphonomic or other deformation were excluded from further analysis Additional file 1 : Table S Excluded specimens include the basilosaurid Cynthiacetus peruvianus which shows sinistral torsion in the rostrum. Although a potential natural feature in protocetids and basilosaurids [ 15 , 16 ], it is suggested that rostral distortion in this particular specimen MNHN. PRU10 is at least partly the original morphology of the skull and potentially a result of some taphonomic distortion [ 27 ].

Inevitably, some fossil specimens have sections of reconstructed bone. Their inclusion in the study was based upon the extent and accuracy of the reconstruction and the unavailability of alternative specimens. Skulls were scanned using a Creaform Go! SCAN 50 depending on the size of the skull. Scans were cleaned, prepared, and merged in VX Elements v. Models were decimated down to 1,, triangles, reducing computational demands without compromising on detail for further morphometric analyses.

In many studies of morphology when the skull is incomplete, it is possible to digitally reconstruct bilateral elements by mirroring across the midline plane if preserved on one side [ 87 — 89 ].

However, due to the substantial asymmetry observed in many taxa in this study, mirroring a complete half of the skull was not possible Fig. Misalignment of mirrored landmarks when using the mirrorfill function on a specimen without bilateral symmetry. Landmarks mirrored in the geomorph package [ 90 ] on an asymmetric specimen.

Note the incorrect mirroring of landmarks on the nasal and to a lesser extent on the lateral point of the maxilla near the orbit circled in this specific specimen. Inset shows the same skull with the landmarks correctly placed. We placed 57 landmarks on both the left-hand side LHS and right-hand side RHS of the skull, and 9 landmarks on the midline, totalling landmarks covering both the dorsal and ventral sides of the skull Fig.

Type I and II landmarks [ 91 ] were selected to comprehensively represent the full cranium Fig. In some ziphiids, e. Mesoplodon carlhubbsi , the teeth or tusks erupt midway along the mandible [ 92 ] whilst other species present multiple pairs of tusks [ 93 ]. In others e. Hyperoodon ampullatus , teeth typically erupt as a single pair on the anterior mandible which often protrudes beyond the upper jaw [ 92 ].

Without the mandible, it is challenging to pinpoint the positioning of the back of the toothrow, and even then, the presence and number of teeth is negligible in some species. Further, these tusks only erupt in adult males. As previously noted, some specimens have missing data. Geometric morphometric analyses and plotting functions implemented in geomorph v. We then used the estimate.