Lawomys rokusi, Veatch & Fabre & Tocheri & Sutikna & Saptomo & Musser & Helgen, 2023

Lawomys rokusi sp. nov.

urn:lsid:zoobank.org:act:E19E2F3A-BE7C-495F-B056-64DECF70D468

Figs 4 View Figure 4 , 5 View Figure 5 , 7A View Figure 7 , 12 View Figure 12 , 13 View Figure 13 , 16 View Figure 16

Diagnosis. There is only one species in the genus; thus, the specific and generic diagnoses are the same.

Holotype. LB-MUR-6482 (held in Organisasi Riset Arkeologi, Bahasa, dan Sastra, referred to hereafter as ORARBASTRA, in Jakarta, Indonesia) is a partial right mandible with the dentary of an adult animal, preserving the first ( LB-MUR-6491 ) and second ( LB-MUR-6492 ) molars as well as the incisor ( LB-MUR-6490 ) (broken at the alveolar opening). The specimen is broken along the ramus posterior to the second molar exposing the incisor canal ( Fig. 4 View Figure 4 ). The fracture surface of the break indicates it occurred recently, perhaps during excavation and/or wet sieving, with a separate refitting fragment retaining the condyle and ramus ( LB-MUR-6483 ). It was recovered from Sector XXI ( Fig. 2 View Figure 2 ) between 75 and 85 cm depth from the cave surface floor. Approximately 90% of the bone surface is covered in matrix with slight manganese staining along the bone and incisor enamel surface. In the holotype, the second molar is slightly rotated back artificially in the jaw, such that the tooth now slopes downward posteriorly rather than sitting in the natural plane of the original molar row ( Fig. 4 View Figure 4 ). There is damage to the anterior portion of the second molar where the outer enamel surface has been broken off GoogleMaps .

Paratypes. A total of 11 relatively intact dentaries ( Table 2; Fig. 4 View Figure 4 ) and two additional dentary fragments: LB 33, left ramus, young adult; LB 36, right ramus, adult; LB 37, right ramus, adult; LB 86, right ramus, adult (all deposited in Naturalis Biodiversity Center in Leiden, Netherlands); LB-MUR-6488 , right ramus with incisor ( LB-MUR-6489 ), adult; LB-MUR-6484 , left edentulous ramus, adult; LB-MUR-6485 , right edentulous ramus, adult; LB-MUR-6487 , fragment of right ramus, adult; LB-MUR-6486 , fragment of left ramus, adult; LB-MUR-2759 , left ramus with incisor ( LB-MUR-2760 ), adult; LB-MUR-4846 , left ramus with incisor ( LB-MUR-4847 ), adult; LB-MUR-5372 , left edentulous ramus, adult; LB-MUR-5415 , right edentulous ramus, adult (all deposited at ORARBASTRA). The specimens at Naturalis (labelled simply as “LB”) were collected during excavations of Liang Bua in 1965 ( Musser et al., 1986) whereas those at ORARBASTRA were collected during more recent excavations between 2010 and 2019 ( Sutikna et al., 2016, 2018) GoogleMaps .

Type locality. The holotype, paratypes, and referred material were all recovered at Liang Bua (8.534167°S 120.460278°E), Flores, Indonesia.

Age. The holotype, paratypes, and referred material all derive from Holocene sediments, although three specimens may be slightly older (i.e., terminal Pleistocene) given specific stratigraphic uncertainties. However, we suspect that targeted re-examination of the entire murine assemblage from Liang Bua will likely result in the identification of this new species in the older layers of the site as well.

Referred specimen. LB-MUR-5835, an isolated right lower incisor.

Etymology. The specific epithet honours zooarchaeologist Rokus Due Awe ( Fig. 6 View Figure 6 ), who was born 20 March 1942 in a small hamlet called Gisi (or Kampung Gisi) located in the Mataloko Subdistrict of the Ngada Regency (East Nusa Tenggara, Indonesia). To his family and childhood friends, he was affectionately known as “Due”, but to his many colleagues from Indonesia and around the world whom he met through his love of archaeology, he was “Pak Rokus”. He first became interested in archaeology when he participated in Theodor Verhoeven’s surveys and excavations in the early 1960s. Verhoeven was a Catholic missionary and archaeologist who taught ancient Latin and Greek at the Mataloko Seminary while Rokus was a student in elementary school. After graduating from high school in 1962, Rokus began to assist Verhoeven in his archaeological exploration of Flores. In 1963 and 1964, they conducted surveys and excavations in central Flores at the So’a Basin, recovering Stegodon remains at Boaleza and Lembah Menge, for example, as well as other areas further to the north, including Wangka, Teong, Liang Rundung, Liang Mbikong, and Liang Toge.

In July of 1965, Rokus walked more than 50 km over several days from his home in Mataloko to Liang Bua to meet Verhoeven. Together, they excavated Liang Bua for the first time, recovering large numbers of stone artifacts, faunal remains, and pottery, as well as several modern human burials. After finishing their excavations, Rokus and Verhoeven travelled to Reo, on the north coast, and took a small boat west to Labuan Bajo, where they excavated at Liang Momer in August. In 1966, Rokus helped Verhoeven survey Timor at Belu and Watu Besi. As the 1970s approached, Verhoeven retired and returned to the Netherlands while Rokus studied history at the Institut Keguruan dan Ilmu Pendidikan (Institute of Teacher Training and Education, a campus of the University of Cendana, Kupang) in Ende, Flores, and graduated with his baccalaureate degree in August 1973.

After completing his studies, Rokus went to Jakarta to meet Raden Pandji Soejono, who was the head of the Department of Prehistory at Djawatan Arkeologi (now known as ORARBASTRA). Verhoeven had mentioned Rokus in his previous correspondence with Soejono about the archaeology of Flores. In 1975, Rokus received a permanent job as civil servant at Djawatan Arkeologi and returned to Flores the following year with Budianto Aziz to conduct an archaeological assessment of Liang Bua for Soejono. Rokus then participated in multiple excavations at the site between 1978 and 1989, as well as later between 2001 and 2014. He was the first person to correctly identify the hominin bones and teeth that ultimately became part of the hypodigm of Homo floresiensis following the discovery of the famous partial skeleton (LB1) in 2003 ( Morwood et al., 2004; Brown et al., 2004). Rokus’ deep passion for and dedication to archaeology as well as the study of faunal remains resulted in his involvement in archaeological research across the entire Indonesian archipelago right up until his untimely death on May 18th, 2015. We pay tribute to his life’s work and significant contributions to Indonesian archaeology by naming a unique endemic Flores murine in his honour.

Description and comparisons

Mandibular morphology

Lawomys shares several mandibular traits with other shrew rats from the Indo-Pacific. For example, Lawomys has a tiny coronoid process similar to some of the Sulawesi shrew rats from the Echiothrix Division (cf. Echiothrix , Paucidentomys , Hyorhinomys , Tateomys , and Melasmothrix ). In contrast, shrew rats from the Philippines (tribe Chrotomyini : Chrotomys Division) and New Guinea (tribe Hydromyini : Hydromys Division) have a more developed coronoid process ( Fig. 7 View Figure 7 ). The coronoid process constitutes the origin of the lateral temporalis muscle, which is likely extremely reduced in these Wallacean shrew rat lineages. Another trait found in several Indo-Pacific shrew rats is a relatively large mandibular condyle joint, with an articular surface that extends across its entire dorsal edge ( Fig. 7 View Figure 7 ). In most murids, the articular facet is localized on the anterodorsal edge of the condyle, as in Pseudohydromys ellermani ( Fig. 7B View Figure 7 ). However, in some worm-eating murids this articular surface extends to the outer edge of the condylar process. The medial ridge of this facet is similarly developed in Chrotomys and Rhynchomys in the Philippines as well as Echiothrix and Hyorhinomys in Sulawesi ( Fig. 7C–E View Figure 7 ). In association with this large articulation, a large incisor canal bulges on the lateral side of the mandibular condyle in Lawomys , a trait also observed in Chrotomys , Hyorhinomys , and to a lesser extent in Echiothrix . This feature is also seen in some New Guinea species such as Pseudohydromys ellermani and P. fuscus . The large retromolar fossa of Lawomys is also found in Echiothrix as well as in Rhynchomys and Chrotomys , but it is not as developed as in Hyorhinomys stuempkei ( Fig. 7C,E View Figure 7 ). This trait is unusual in murids and reflects the large surface insertion of the medial temporalis muscle. On the medial side, the mandibular foramen ( Fig. 7 View Figure 7 ) has a similar position and morphology, dorsal to the incisor bulge and posteroventrally to the coronoid process. This foramen, which is well developed in Lawomys , allows the passage of the mandibular branch of the trigeminal nerve. Its shape, size, and position in conjunction with large proodont lower incisors are like that seen in worm-eating rats (e.g., Chrotomys and Hyorhinomys ). The morphology of this foramen is rather divergent in Pseudohydromys in which it is slit-like and closer to the condyle and its posteroventral border ( Fig. 7B View Figure 7 ). On the medial side of the angular process in Lawomys , the internal pterygoid fossa is large and the angular shape overall is once again very similar to that of Chrotomys and Hyorhinomys . Another diagnostic trait found in Lawomys concerns the masseteric ridge ( Fig. 7 View Figure 7 ). The anterior part of this ridge extends rather anteriorly and inserts more ventrally as compared to typical murids. Such a morphological state is only found in the most derived forms of worm-eating shrew rats. Perhaps due to its large body size, Lawomys is characterized by a well-developed anteriorly positioned ridge reflecting large anterior and posterior deep masseters.

Mandibular 2D geometric morphometrics and lever-arm distances

The mandibular morphology of Lawomys rokusi was quantitatively compared to that of other murids from the oceanic islands of Flores, Sulawesi, Sunda, Luzon, as well as Australo-Papua using 2DGM and visualized through a PCA ( Fig. 8 View Figure 8 ). Species with dorsoventrally narrower jaws load on the positive side of PC1, which explains 30.0% of the variance. These taxa exhibit an angular process extending posteriorly to the condyloid process, an elongated and narrow anterior portion of the jaw, and proportionally shorter coronoid and angular processes. Lawomys loads negatively on PC1 along with murine jaws that are dorsoventrally higher and characterized both by massive condyloid and angular processes, a condyloid process that expands posteriorly to the angular process, and a proportionately wider ramus including the coronoid and angular processes. Along PC1, the positive end mainly includes carnivorous murids such as vermivorous Rhynchomys and Tateomys while the negative end has more mixed species with lineages belonging to herbivorous (e.g., Papagomys armandvillei and Komodomys rintjanus ), omnivorous (e.g., Rattus hainaldi and Lorentzimys nouhuysi ), and carnivorous lineages ( Hyorhinomys stuempkei and Pseudohydromys spp. ) ( Fig. 8 View Figure 8 ). PC2, which explains 24.6% of the variance, distinguished carnivorous murids that cluster more negatively by having jaws with a more proodont lower incisor and a thinner angular process that is well circumscribed from their larger and longer condyloid process. Large herbivorous murids cluster more positively on PC2 due to jaws that have a more opisthodont lower incisor with a wider angular process as well as a shorter and wider condyloid process. Lawomys clearly stands apart on this axis and plots close to carnivorous Pseudohydromys (cf. P. ellermani , P. occidentalis , P. pumehanae ), Echiothrix ( E. centrosa , E. leucura ), Hyorhinomys stuempkei , and Chrotomys ( C. whiteheadi , C. mindorensis , C. silaceus , C. sibuyanensis ). Along PC3, which explains 16.6% of the variance, mandibles with reduced coronoid and angular processes and an elongated condylar region plot toward the negative end whereas mandibles that have a shorter condyle along with longer coronoid and angular processes plot toward the positive end. Lawomys rokusi stands apart from the carnivorous cluster on the positive end of PC3 by having both a short coronoid process and a large condyloid process like the Sulawesi shrew rats (e.g., Echiothrix spp. ). Overall, the PCA of jaw shape data reveals significant ecomorphological differences among dietary categories which is confirmed by ANCOVA results on centroid size (F = 15.55; P <0.0001, SI Table 4) and MANOVA analyses (F = 5.4; P <0.0001, SI Table 5). Interestingly, a cluster analysis based on Procrustes distances indicated a similar mandibular ecomorphology between Lawomys and multiple Pseudohydromys species, with the Lorentzimys omnivorous lineage recovered adjacent to Lawomys and Pseudohydromys ( Fig. 9 View Figure 9 ).

A comparison between jaw centroid size (logged), length of the lower first molar (logged), and the incisor angle shows some interesting trends in murid dietary and morphological adaptations ( Fig. 10A,B View Figure 10 ). Centroid size tracks overall jaw size, with larger mandibles plotting towards the positive end and smaller mandibles plotting towards the negative end of this axis. Similar patterns emerge for the length of the lower first molar (large molars plot positively and smaller molars plot negatively along this axis) ( Fig. 10A View Figure 10 ) and the angle of the incisor (proodont incisors plot positively and opisthodont incisors plot negatively along this axis) ( Fig. 10B View Figure 10 ). With regard to relative molar size, herbivorous and omnivorous murines with larger jaws tend to have proportionally large molars while carnivorous murines with small jaws tend to have small molars with some genera showing an unusually small molar size relative to centroid size (e.g., Pseudohydromys , Rhynchomys , and Echiothrix ) ( Fig. 10A View Figure 10 ). Lawomys stands apart by having a large jaw size with proportionately small molars (also see Fig. 8 View Figure 8 ). Similar patterns emerge with incisor angle ( Fig. 10B View Figure 10 ). Omnivorous murids range in body sizes but retain more opisthodont incisors (except for Paulamys naso ) while herbivorous murids that tend to be smaller in body size tend to have opisthodont incisors and the larger sized taxa have a range of incisor angles ( Fig. 10B View Figure 10 ). Conversely, the incisor angle in smaller carnivorous murids is proodont while the incisor angle in larger carnivorous murids range widely. Again, Lawomys separates itself with a large body size and more proodont incisors. Overall, Lawomys is an outlier as compared to omnivorous and herbivorous murid species and is morphologically similar to Hyorhinomys , Rhynchomys , Echiothrix and Pseudohydromys by having a small lower molar relative to its jaw centroid size. Considering the Flores murines, all of these taxa are scattered across each axis ( Fig. 10A,B View Figure 10 ). Rattus hainaldi and Komodomys rintjanus cluster both within the overlapping omnivorous ( Sundamys maxi and S. infraluteus ) and herbivorous ( Bullimus bagobus and B. luzonicus ) clusters in the middle of the morphospace, while Papagomys armandvillei clusters with other large herbivorous species from Sulawesi ( Eropeplus canus , Lenomys meyeri , Taeromys celebensis ). Lastly, Paulamys naso , an omnivore based on stomach contents of captured specimens ( Kitchener et al., 1998), is clearly positioned among carnivorous species from the Indo-Pacific region.

The PCA of log-shape ratio of in-lever and out-lever distances reveals significant ecomorphological differences among dietary categories ( Fig. 10C,D View Figure 10 ) ( Table 3) (F = 5.75; P <0.0001, SI Table 6). Explaining 70.0% of the variance, PC1 separated murids with relatively longer distances between the coronoid and condylar processes (0.90), shorter distance between anterior part of the masseteric ridge and condylar process (–0.20) and those with relatively longer jaw molar and incisor out-levers (–0.26 and –0.26, respectively [numbers indicate the shape variable’s correlation, or loading, with the principal component]). This shape difference along this axis is particularly salient among carnivorous murids, such as moss mice ( Pseudohydromys ) and worm-eating shrew rats ( Hyorhinomys and Echiothrix ) plotting along the positive end, which have shorter log shape ratios of the lower incisor and molar out-levers (–0.26 and –0.26, respectively) and longer in-lever distance ratios for the anterior deep masseter (–0.20), compared to vermivorous Rhynchomys spp. and Tateomys rhinogradoides , which plot on the negative end of this axis ( Fig. 10C,D View Figure 10 ). Lawomys rokusi clusters on the positive end of PC1 together with the New Guinean moss mice ( Pseudohydromys and Microhydromys ) and two Sulawesi shrew rats ( Hyorhinomys and Echiothrix ) as well as one omnivorous species ( Lorentzimys nouhuysi ) showing an elongation on the posterior end of the mandible compared to the anterior region. PC2 explains 23.4% of the variance and distinguishes some carnivorous murids with relatively longer incisor and molar out-levers (–0.31 and –0.48, respectively) as well as longer in-lever distance ratios of the anterior deep masseter (–0.13) and superficial masseter (–0.14). In comparison, most herbivorous and omnivorous species display longer ratios of lateral temporalis muscle (0.70) and posterior deep masseter in-levers (0.37). On this axis, Lawomys rokusi clusters again with one omnivorous ( Lorentzimys nouhuysi ) and some carnivorous murids ( Pseudohydromys species), but has a lever-arm pattern that is shared by both herbivorous, omnivorous, and some carnivorous murids (e.g., Hydromys chrysogaster and Crunomys species) reflecting a relatively short mandible and an overall larger and taller ramus.

Lastly, a comparison was made showing the trade-off between molar length and incisor size relative to centroid size ( Fig. 11 View Figure 11 ). These scatterplots show that the relative size of the lower first molar and lower incisor are more uniform among herbivorous and omnivorous murids while carnivorous murids show a range of adaptations, including worm eating rats with small molars and small lower incisors (e.g., Rhynchomys ) or with small molars and large lower incisors (e.g., Chrotomys , Echiothrix , Pseudohydromys , and Hyorhinomys ), water-rats with large molars and small lower incisors, or some animals with both large molars and incisors (e.g., Paraleptomys ). Lawomys plots outside of the herbivorous and omnivorous cluster due to a combination of a large lower incisor and a small lower first molar, similar to Hyorhinomys , Pseudohydromys , Chrotomys , and Echiothrix ( Fig. 11 View Figure 11 ).

Dental morphology

The cusp pattern on both molars is extremely simple with a relatively wide and tube-like dentine wear shape. The most striking difference between Lawomys and all other murines from the Indo-Pacific is that the outer enamel of both molars forms a continuous outer surface similar to that in Chrotomys , yet three distinct laminae are maintained on the first molar as in Hyorhinomys ( Fig. 12 View Figure 12 ). The first molar’s wear pattern is also generally concentrated towards the midline of the tooth but with heavy wear on the anterolabial cusp compared to the anterolingual cusp ( Fig. 12 View Figure 12 ). While the cusps in the first and second lamina coalesce, the dentine maintains separation and appears more transverse, unlike other shrew rats such as Pseudohydromys , Chrotomys , Crunomys , Echiothrix , and Tateomys , while the third lamina has a similar “bow-tie” wear pattern as in Echiothrix and Hyorhinomys ( Musser & Durden, 2014; Esselstyn et al., 2015). The second and third laminae have a very shallow separation between rows compared to all other shrew rats but resemble the laminae configuration in Hyorhinomys ( Esselstyn et al., 2015) . Moreover, the first and second molars lack the posterior cusp and auxiliary cusplets, creating a simple occlusal pattern overall with little similarity with those of other shrew rat genera from the Philippines, Sulawesi, and New Guinea.

The enamel on the anterior aspect of the second molar in the holotype is broken, obscuring the cusp pattern on the first lamina, but the hypoconid and entoconid in the second lamina maintain separation and are barely worn. The second lamina is not as thick as in Chrotomys or Echiothrix and is more transversely oriented compared to Pseudohydromys ( Fig. 12 View Figure 12 ).

Judging from the relatively tiny and simple morphologies of the first and second molars, the occlusal traits of the third molar in Lawomys are likely simpler than those of other shrew rat taxa in which the third molar is known (e.g., Archboldomys , Melasmothrix , Tateomys ), and is presumably reduced to a tiny and very simple peg-like structure ( Musser, 1969; Musser & Durden, 2014). As observed in other murines where the third molar is similarly reduced, such as in Leptomys and Chrotomys ( Rickart et al., 2005; Musser et al., 2008), it is normally present but occasionally congenitally absent (Charles et al., 2011; Catzeflis et al., 2017).

Compared to the Flores murines, Lawomys has the simplest occlusal pattern, both in terms of additional cusps, auxiliary cusplets, and cusp shape ( Fig. 13 View Figure 13 ). The thick, tube-like dentine wear pattern shown on the first molar of Lawomys shows some resemblance to the dentine wear shape on the first molar of Paulamys , but otherwise, Lawomys remains distinct in all other comparisons. Strikingly, the Lawomys mandible is of similar size to those of Papagomys armandvillei and Spelaeomys florensis , which have the largest mandibles of the Flores murines, yet the breadths of the first and second molars in Lawomys are similar in size to those of the smallest Komodomys and small Rattus species, respectively ( Fig. 13 View Figure 13 ). Overall, additional molars with other degrees of wear are needed to determine how the occlusal morphology of Lawomys compares with that of other shrew rats and murines from the Indo-Pacific, but the features preserved in the holotype suggest that Lawomys maintained an extremely simple occlusal pattern for its size compared to other shrew rat taxa in the region and all other Flores murines. Moreover, the dissimilarity between Lawomys and the other Flores murines suggests that it occupied a different niche, possibly consuming earthworms or similar foods that do not require occlusal complexity.

Body size estimates

Compared to Flores taxa, the mandible of Lawomys overlaps in size with Papagomys armandvillei and Spelaeomys florensis suggesting a similar body size ( Fig. 14 View Figure 14 ). Indeed, regression analyses used to test if mandibular centroid size reasonably predicts the body masses of murid and shrew rat taxa suggest that Lawomys weighed ca. 623 g (R 2 = 0.94; F(1,126) = 1950; p <0.001), making it larger than any shrew rats endemic to the Indo-Pacific region ( Table 4). When considering diet, and assuming Lawomys was carnivorous, Lawomys is predicted to range between ca. 1245–1594 g ( Table 4). Most terrestrial shrew rats are typically smaller in body size with the largest living species ( Echiothrix leucura ) weighing ca. 310 g ( Musser & Durden, 2014). Some species of water rats, such as Hydromys chrysogaster , can reach a similar body size (e.g., AMNH 154358, ca. 580 g) but water rats tend to have relatively smaller jaws, smaller incisors, and larger first and second molars. On Flores, the body mass estimate for Lawomys is comparable to those for Papagomys theodorverhoeveni and Spelaeomys florensis but larger than Hooijeromys nusatenggara and the other smaller endemics (i.e., Paulamys naso , Komodomys rintjanus , Rattus hainaldi ).