Records of the Western Allstralian Museum 22: 1-7 (2003).

The 3-dimensional anatomy of the North-Western Marsupial Mole(Notoryctes caurinus Thomas 1920) using computed tomography,

X-ray and magnetic resonance imaging

Natalie Warburtont, Christopher Wood2, Christopher Lloyd2

, Swithin Song2 and Philip Withers!

1 Department of Zoology, University of Western AustraliaCrawley, Western Australia, 6009. E-mail: nwarbo@cyllene.uwa.edu.au

2 Department of Radiology, Royal Perth HospitalWellington Street, Perth, Western Australia, 6000

Abstract - The 3-D skeletal images obtained from reconstruction of CT scansand X-rays, and soft-tissue images produced by MRI, provide invaluableinformation of the internal and gross anatomy of the north-western marsupialmole (Notoryetes caurinus).

The conical skull, which is quite thin-walled dorsally and anteriorly butstrong in the basicranial region, has little evidence of the orbit or zygomaticarch, and the smoothly-curved posterior region has no sagittal or occipitalcrests. The vertebral column is remarkably strengthened, and in lateral viewhas an unusual flat-shape. The cervical vertebrae appear to be greatlycompressed; 4 or 5 are completely fused (which is unique among marsupials).The thoracic vertebrae are fairly robust with large neural spines. The lumbarvertebrae are distinct, becoming large posteriorly towards the pelvis. Thesacral vertebrae are greatly expanded in size and are fused with the pelvis.Particularly in the middle of the tail, the caudal vertebrae are greatlydeveloped, with large transverse processes and chevron bones. The pectoralgirdle is very anterior, with the shoulder articulation level with the anteriorcervical vertebrae just behind the skull, and low on the side of the body. Thehumerus is robust, and the radius and ulna are very short. The bones of thepelvis are highly derived, and fused to sacral vertebrae. The epipubic bonesare small and not ossified. An ossified patella is present and it has an unusuallarge triangular keel.

The most apparent soft-tissue structure by MRI is a large amount ofsubcutaneous fat, particularly around the ventral surface of the pelvis but alsodorsal to the pelvis and anteriorly around the shoulders. The major musclegroups are visible, but distinction beh,veen individual muscles is not possibleexcept for the very large muscles of the thigh, upper arm and base of the tail.The muscles of the tail are strongly developed, more so ventrally thandorsally.

INTRODUCTION

Computed tomography (CT scan) and magneticresonance imaging (MRI) are used extensively inmedical diagnostics and research and thetechniques have also been incorporated into theveterinary discipline in clinical situations.However, the zoological use of such technologyin more basic anatomical research has been ratherlimited. The few anatomical investigationsincorporating CT and MRI technology includedescriptions of the internal anatomy of the felineabdomen (Samii et al. 1999), and the brain andcoelomic cavity of the domestic pigeon(Romagnano et al. 1996). The techniques have alsobeen used in paleaontological studies, forexample the measurements of brain size inAllosaurus (Rogers 1999). This research uses CT

scan and MRI techniques to investigate theanatomy of the marsupial mole.

The anatomy of the North-western marsupialmole, Notoryctes cauril1US (Marsupialia:Notoryctidae), has not been studied in any detailsince its original description by Thomas (1920). Infact the only published research for this remarkablefossorial marsupial since then is the physiologicalstudy of Withers et al. (2000) and a natural historyaccount by Thompson et al. (2000-2001). Very fewpreserved specimens of N. caurinus are available, soit is important to glean as much anatomicalinformation as non-destructively as possible fromthe few specimens that are available. The purposeof this paper is to provide an account of theusefulness of CT scan, X-ray, and MRl to describeN. caurinus, a small (mass ~ 30 - 35 g) and highlyspecialized marsupial.

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METHODS

The primary specimen of N. caurinus which weexamined was from the Western AustralianMuseum (specimen number M 44175). Capturelocality was Cotton Creek (Newman-Canning StockRoute road; 4/3/1999; 22°59' 122°23'). The specimenwas formalin fixed and preserved in 70% ethanol.The specimen was male, with a total body length of10.5 cm and mass of approximately 35 g.

CT and MRI scans were performed at Royal PerthHospital, Perth, Western Australia. The CT scanswere made with on a GE Cti scanner (GeneralElectric Medical Systems, Milwaukee, USA) using ahelical technique with a slice collimation of 1 mm,and a pitch of 1:1. Images were reconstructed at aninterval of 0.3 mm using soft tissue and bonealgorithms. 3-D volume rendered images were alsoobtained using the AW 3.0 workstation.

MRI scans were performed using a Marconi LOTMR scanner (Marconi Medical Systems, HighlandHeights, Ohio, USA) using a 3-D RF FAST sequencewith 1.4 mm slices, flip angle of 35 degrees, TR of24 ms, TE of 6 ms, field of view of 12 cm, 192x256matrix, one signal average. Images were acquiredin the coronal, transverse and sagittal planes. Thespecimen was placed in a dedicated (human) kneecoil.

X-rays were taken by Or Ken Aplin of theWestern Australian Museum, using standard x-rayfilm and processing techniques. The fine details ofthe images were verified by dissection.

RESULTS AND DISCUSSION

Computed tomography was most useful forexamining hard, bony structures of the body (Figure1). Such 3-D images of the skeleton of N. caurinusare useful because no fully-articulated specimen ofthis species is locally available, and because thefunctional form of its skeleton cannot necessarily beinferred from simple reconstruction ofdisarticulated bones because of its unique fossoriallifestyle among marsupials. Thus, 3-D computerreconstructions of CT scans provide invaluableinformation concerning the normal in situ skeletalarrangement. X-rays were necessary to provide finedetail of skeletal structures. MRI was useful inviewing the internal arrangement of soft tissuestructures.

3-D Skeletal ReconstructionThe axial skeleton, comprising the skull, vertebral

column, sternum and ribs, and tail of the north­western marsupial mole is highly adapted for itsfossorial lifestyle (Figure 1). From the lateral viewof the skeletal image, the skull is conical in shape(Figure lC). The skull of N. caurinus appears to bequite thin-walled dorsally and anteriorly, but strong

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N. Warburton, C. Wood, C. Lloyd, S. Song, P.c. Withers

in the basicranial region. Teeth are not apparent onthe CT scans because they are too small to beresolved by a scan of 1.0 mm slice size. The skull,including the major contours and teeth, is finelyimaged by X-ray (Figure 2). The rostrum curvesventrally slightly, anterior to the molar teeth. Thereis little evidence of the orbit or zygomatic arch. Theposterior region of the skull is smoothly curvedwith no sagittal or occipital crests.

The vertebral column is remarkably strengthened,and in lateral view has an unusually flat shape(Figure lC). Overall, the form of the vertebralcolumn appears highly specialised for burrowing.While CT resolution is not quite adequate for thefine detail of vertebrae, it is apparent that they arevery robust. The cervical vertebrae are obscured bythe pectoral girdle, but appear to be greatlycompressed and follow a deeply concave path tothe base of the skull. This unusual form of thecervical vertebrae is best seen from the lateral viewof the skeletal images (Figures lC, 28). Fusion ofthe cervical vertebrae is extremely unusual formammals, and is unique among marsupials for themarsupial mole. The thoracic vertebrae are not seenas separate entities in the 3-D skeletal images. Theshape of the vertebral column in the thoracic regionfollows a sharp incline behind the pectoral girdleand then flattens out considerably. The vertebralcolumn of more generalised quadrupeds follows awider curve than that apparent for N. caurinus(Slijper 1946).

The lumbar region is best seen from dorsal view(Figures lA, 2A). The lumbar vertebrae are distinct,becoming large posteriorly towards the pelvis.Their transverse processes can just be discerned onCT images. Like the posterior thoracic vertebrae, thelumbar vertebral region is very flat in shape. Thesacral vertebrae are greatly expanded in size andare fused with the pelvis. The wide metapophysesare broadened to form a shield-shaped arrangementwhen viewed from the dorsal aspect (Stirling, 1891;Chapman, 1919).

An unusual feature of the vertebral column fromthe dorsal view is the arrangement of vertebrae inthe tail (Figure 1). Particularly in the middle regionof the tail, these caudal vertebrae are greatlydeveloped; large transverse processes and chevronbones (apparently fused to the vertebral bodies) areevident on the CT scans. From the lateral aspect,the caudal vertebrae are almost as large as thethoracic and lumbar vertebrae, which is mostunusual. Generally, caudal vertebrae of mammalsare smaller and simpler in form than the thoracic orlumbar vertebrae. The large size of the marsupialmole's caudal vertebrae, the development oftransverse processes for muscle attachment, and thelarge size of the chevron bones, suggest that the tailis highly mobile, muscular and probably involvedin burrowing locomotion (see Withers et al., 2000).

3-D anatomy of the North-Western Marsupial Mole 3

Figure 1 Computed tomography images of the three-dimensional anatomy of the North-Western Marsupial Mole,Notoryctes caurinus; a dorsal, b ventral, and c lateral views. Caudal vertebrae (CaV), cranium (Cr), femur(Fern), humerus (Hm), mandible (Ma), manubrium sterni (MS), patella (Pat), pelvis (Pel), ribs (r), scapula(Se), sternum (St), stomach (St), tibia (Tib), vertebral column (VC).

4 N. Warburton, C. Wood, C. Lloyd, S. Song, P.c. Withers

Figure 2 X-ray of the North-Western Marsupial Mole, Notoryctes caurinus (WAM M41482); lateral view. Actual bodylength (snout-tail) = 102 mm. Legend: caudal vertebrae (Cau), chevron bones (Chev), clavicle (Clav), cranium(Cran), femur (Fern), manus (Man), patella (Pat), pelvis (Pel), pes (Pes), radius (Rad), scapula (Sea), tibia(Tib).

The ribs and sternum can be clearly seen in theCT skeletal images. The rib cage is in the form of awide cone, being very narrow anteriorly andexpanding quite widely posteriorly (Figure lA, B).The ribs appear to become less dense posteriorly.The costal cartilages, which join the ribs to thesternum, cannot be seen. The anterior part of thesternum is characterised by two large lateralprocesses of the manubrium.

The appendicular skeleton includes the pectoraland pelvic limbs and their girdles. The highlymodified morphology of these skeletal componentsin the marsupial mole can be seen in the 3-Dskeletal reconstruction (Figure 1). Most noticeably,the pectoral girdle is quite anteriorly placed. Theposition of the shoulder articulation, of the glenoidregion of the scapula and the humerus, appearslevel with the anterior cervical vertebrae, justbehind the skull, rather than the more commonposition of level with the anterior thoracic vertebrae(Hildebrand, 1988). In the lateral view, it isapparent that the shoulder articulation is placedquite low on the side of the body. This antero­ventral position of the pectoral girdle ischaracteristic of the position for other smallfossorial mammals (Gambaryan and Keilan­Jaworowska,1997).

The individual components of the pectoral girdlecan be seen with reasonable clarity (Figure 1). It isapparent that the scapula is substantially modified,being narrow for the most part and fanning outposteriorly. The orientation of the scapula is alsomodified from the usual dorsal arrangement of thescapula in mammals (Hildebrand, 1988), occupying

a more laterally-facing position (Figure 1C). Therobust humerus can be seen most clearly in thedorsal views of the skeletal image; however thismay include parts of the ulna included with thedistal end of the humerus. Unfortunately theantebrachium (lower arm) and the manus were notrepresented in the skeletal images. This is quiteunexpected as the bones of this region are relativelymore robust than, for instance, the ribs, which wereseen clearly in the skeletal reconstruction images.

The pelvic girdle is perhaps the most clearcomponent of the skeleton. The bones of the pelvisare obviously highly derived and are fused to anumber of the sacral vertebrae. The typically (formammals) elongate innominate bones are broadand fused with expanded elements of the sacralvertebrae (Hildebrand, 1988). The fusion betweenthe pelvis and the sacral vertebrae of marsupialmoles is more extreme than in other placentalburrowers, such as armadillos and pocket gophers(Chapman, 1919). It is evident that the pubicsymphysis is either unfused or formed by very thinbone. The epipubic bones of marsupial moles arevestigial and are not seen in the CT scan images.

The hind limbs of N. caurinus extend almostlaterally from the body, rather than ventrally, andthe tibiae lie flat against the ground. Mammalsgenerally have a parasaggital arrangement of thelimbs under the body (Hildebrand, 1988); theabducted posture of the hindlimbs in marsupialmoles is reminiscent of a more reptilian stance. Thelong bones of the limb are relatively robust, but nofusion between the elements has taken place. Anunusual feature is the large triangular keel of the

3-D anatomy of the North-Western Marsupial Mole 5

Figure 3 Magnetic resonance image of the three-dimensional anatomy of the North-Western Marsupial Mole,Notoryctes caurinus; sections through coronal view, dorsal to ventral. Organs visible: anal gland (A), brain(B), fat (F), femur (Fe) heart (H), hip musculature (Hp), humerus (Hu), intestine (I) liver (Li), lungs (Lu), noseshield (N), oral cavity (0), rib cage (R), shoulder musculature (Sh), stomach (St), tail musculature (Ta), testes(T), thigh musculature (Th), and vertebral column (V).

patella at the knee joint of the hind limbs (FigureIC). Few other marsupials (only bandicoots) havean ossified patella, and none have one resemblingthe relatively large, irregular patella of Notoryctes.The tibia and fibula are pictured for the right legonly, but are clearly quite robust for an mammal ofthis size. Unfortunately, like the forelimb, the distal

parts of the hind limb are not pictured in the 3-Dskeletal image. Again this is unexpected as theyhave relatively robust bones. The unusualmorphology of the hind limb, with the largetriangular patella and expanded tibia is moreclearly visible in the X-ray than the eT scan images.Like the carpus of the forelimb, the small bones of

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the pes are highly modified and difficult to identifyeven from X-ray images. However, it is interestingto note that the clawed digits of the foot aresplayed, and held in a somewhat abducted andmedially rotated posture, presumably to achieve themaximum surface area of the sole of the foot whenburrowing.

Soft Tissue StructureFrom all MRI views (Figure 3), the most apparent

soft-structure is a large amount of subcutaneous faton this specimen, particularly around the ventralsurface of the pelvis but also dorsal to the pelvisand anteriorly around the shoulders. This very largeamount of fat appears as a high signal intensity onthe magnetic resonance images. Upon dissection,there was approximately four grams ofsubcutaneous fat - more than 10% of a 35 gramanimal.

The largest organs of the digestive system, theliver, stomach and intestine, are easilydistinguished. The abdominal cavity appears asvery low signal intensity, oval-shaped structure inthe mid-region of the animal, due to this spacebeing air-filled as a result of an incision made in theskin for the penetration of formalin/alcohol forfixation of the internal organs. The liver is the mostanterior organ of the abdomen, and multi-lobed instructure. The stomach appears quite dark on theleft side of the abdominal cavity in the coronalsections. Also seen from CT scans, the stomachcontains a large amount of sand resulting in the lowsignal intensity in MRI. The intestine fills the rest ofthe abdominal cavity and has a higher signalintensity than the stomach and liver. The smallerorgans of the abdominal cavity, spleen, gall bladder,ete are not distinguishable. The oral cavity is alsovisible in the relevant coronal sections (Figure 3).

In the thoracic cavity, the heart is the only portionof the circulatory system that is clearly apparent inMRl. At particular levels of contrast it is possible todiscern the chambers of the heart. Of the respiratorysystem the lungs are visible. Their triangular shape isbest observed from the coronal sections (Figure 3).

The urinogenital system is poorly represented inthe MRI due to the small size of these organs; onlythe testes are clear (in all views) and the kidneys insagittal view. Posteriorly, lying in the mass of fatunder the pelvis, the testes are seen very clearly,with good contrast between gonadal tissue and fat.H is difficult to assess the sex of marsupial molesexternally as males have abdominal testes, so MRIprovides a non-intrusive method for confirmingthat this specimen is male.

The brain is the only obvious component of thenervous system in the MRI images (Figure 3). Withthe MRl having a slice size of 1.4 mm, it is possiblethat the spinal cord was missed altogether incoronal and sagittal sections.

N. Warburton, C. Wood, e. Lloyd, S. Song, r.e. Withers

The hard, bony structures of the skeletal systemappear dark on the MRl. Clearly visible are sectionsthrough the skull, vertebral column, ribs, humeriand femora. The large size of the vertebrae isobvious and it is easier to distinguish adjacentvertebrae in MRI than CT scans. Surrounding thelong bones of the limbs, the major muscle groupsare visible, but distinction between individualmuscles is not possible except for the very largemuscles of the thigh, upper arm and base of the tail.In these areas the muscle bulk is enormous, muchgreater than for non-fossorial mammals ofcomparable size (personal observations). Theextensive muscles attaching to the tail are quiteunusual and would seem to be related to thestrengthening of the caudal vertebrae as noted fromthe CT skeletal images. The muscles of the tail alsoappear to be more strongly developed ventrally (forflexion) than dorsally (for extension).

In summary, traditional X-rays provide detail ofskeletal elements but are sometimes difficult tointerpret because a three dimensional structure hasbeen projected onto a two dimensional image.Reconstruction of CT scans results in a threedimensional image that reduces the problem ofoverlap. Also, with the CT skeletal reconstruction, itis possible to rotate the image to obtain the desiredview, an impossible manipulation with X-rayimages. MRl provides information of the anatomyof soft tissue structures by recording the differentialcontrast of the various soft tissues. Due to the smallsize of the marsupial mole, detail in sections was atthe resolution of the MR scanner. Nevertheless,some valuable information concerning the 3-Darrangement of the soft anatomy is apparent.

ACKNOWLEDGEMENTS

We are grateful to Ms Norah Cooper and Dr KenAplin of the Western Australian Museum for theloan of the specimen, and the X-ray images.

REFERENCES

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Thomas, O. (1920). Notoryctes in north-west Australia.Annals and Magazine of Natllral History (9) 6: 111-113.

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Manuscript received 6 November 2002; accepted 9 June 2003